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Conformational interchange of a carbohydrate by mechanical compression at the air–water interface† Keita Sakakibara,*abc Takuya Fujisawa,ad Jonathan P. Hillab and Katsuhiko Ariga*ab Methyl xylopyranoside containing three 4-(pyrene-1-yl)benzoyl groups (PyXy) undergoes conformational interchange within a Langmuir monolayer upon mechanical compression. This xylose-type molecular machine PyXy was immobilized within two different matrix lipids, methyl stearate and methyl 2,3,4-triO-stearoyl-b-D-xylopyranoside, which respectively form rigid and soft monolayers. Structural properties of the monolayer were characterized by assessing the compressibility, compression modulus, and ideal limiting molecular area of PyXy, all of which were estimated from the p–A isotherm measurements. Only the rigid monolayer exhibited a transition to the condensed phase with a limiting molecular area of PyXy smaller than that of the cross-sectional area of the xylopyranose ring in its C1 chair conformation. This suggests conformational interchange of PyXy from the most stable 4C1 (C1) form to the metastable 1
C4 (1C) form. Surface-reflective fluorescence spectroscopy of the monolayer was applied to detect
excimer emission resulting from the face-to-face dimerization of pyrenes attached at the O-2 and O-4 Received 2nd December 2013, Accepted 6th February 2014 DOI: 10.1039/c3cp55078h
positions of xylose. Fluorescence intensity of the excimer increased abruptly in the condensed region only when the rigid monolayer was applied. These results indicate that the rigidity of the matrix monolayer is a critical aspect of the precise manipulation of molecular machines at interfaces. Consequently, this study demonstrates that including a molecular machine into a rigid lipid matrix is a promising means for the preparation of a novel nanoassembly with dynamic functionalities variable
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depending on a mechanical stimulus.
Introduction Many different assemblies of functionalized nanoscale structural units including self-assembled nanostructures of p-conjugated and/or amphiphilic molecules, bio(macro)molecular systems, well-defined polymers and metal/inorganic nanostructures have been so far achieved, leading to a variety of electronic, photonic, magnetic, ecological, sensing, catalytic, or medical properties because of the nanoassembly concerted effects.1 Some of the ultimate nanoscale structural units are molecular machines which consist of a single molecule possessing an active mechanical
a
World Premier International (WPI) Centre for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan. E-mail: [email protected] b JST, CREST, 1-1 Namiki, Tsukuba 305-0044, Japan c Institute for Chemical Research (ICR), Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. E-mail: [email protected] d Department of Pure and Applied Chemistry, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan † Electronic supplementary information (ESI) available: 1H-NMR spectra of compound 2 and StXy, 2D [1H, 1H] COSY NMR spectra of PyXy, Scheme S1, and Fig. S1. See DOI: 10.1039/c3cp55078h
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motion such as rotation (like a motor) or pinching (like tweezers).2 For example, intramolecular rotation and/or deformation of molecular machines can be stimulated by applying photonic or chemical inputs. In those cases, observations are usually made in solution and therefore cannot be accessed in order to exploit their properties.3 This necessitates their immobilization at a particular surface/interface for their effective utilization and detection. For example, sophisticated assembly systems of molecular machines are common in Nature immobilized in biological membranes. Biological membranes, which are essential media for membrane trafficking and signal transduction in cells, can be modulated by mechanical forces ranging from osmotic force to touch, vibration and texture, due to their fluid and flexible responses to applied stimuli. Hence, their highly dynamic nature is crucial to their mechanosensitivity, mechanotransduction, and/or adaptations to mechanical stresses. In general, conformational interchange of proteins upon application of pressure leads to mechanical senses.4 Mechanosensitive channels and rhodopsin are excellent examples of such proteins.5 In another example, the presence of anesthetic molecules causes a redistribution of lateral pressures in a membrane, in turn shifting the
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conformation of membrane proteins such as ligand-gated ion channels.6 The elegance with which biological systems perform pressure-driven functions has inspired us and other groups to manipulate molecular machines by applying macroscopic mechanical stimuli.3,7 For this purpose, a dynamic interface can be utilized as a medium for controlling and manipulating nanosystems through macroscopic mechanical motions.8–11 In our recent studies, macrocyclic molecules including cyclophanes and cyclens have been used as fundamental components for conformational interchange by compression because of their constrained cyclic structures. Such conformational interchange is transmitted by changes in surface pressure, which is a possible mechanism for molecular signal transduction.12 Another promising candidate for pressure transduction would be carbohydrates. Force-induced elastic deformation of the pyranose ring in polysaccharides such as dextran, amylose, or pullulan has been demonstrated by single molecule force spectroscopy.13 Metal ion-triggered conformational ring flip of monosaccharides has also been demonstrated, and is applicable for signal transduction.14 However, the use of macroscopic mechanical stimuli rather than local mechanical (AFM tips) or chemical (ions) forces has not been investigated so far, despite carbohydrates in dynamic biological membranes playing a pivotal role such as cell–cell recognition and signalling events. These aspects led us to investigate the behaviour of carbohydrates embedded in an artificial biological membrane where their dynamics are regulated by mechanical force. Here, we experimentally demonstrate that the conformation of a carbohydrate molecule immobilized in a dynamic monolayer (a Langmuir monolayer) can be directly regulated by varying the applied surface pressure (Fig. 1a). To determine how force regulates the conformational change, a xylose derivative bearing fluorescent moieties at O-2, O-3, and O-4 positions was used. This molecule exhibits excimer emission when its conformation changes from 4C1 (C1) to 1C4 (1C) within a
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Fig. 2 Chemical structures of PyXy and the two matrix molecules MS and StXy.
Langmuir monolayer (Fig. 1b). For this study, we designed and synthesized a pyrene-labelled xylose (PyXy) and carried out in situ fluorescence spectroscopy on the water surface. PyXy has three pyrene moieties as a probe of the conformational change with p-phenylene spacers (Fig. 2). In order to mimic the hydrophobic and organized environment of naturally occurring biological membranes, this compound was immobilized separately with two types of matrix lipids, methyl stearate (MS) and methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy), respectively, forming rigid and soft monolayers (Fig. 2). Conformational interchange from the most stable C1 form to the metastable 1C form could be detected by excimer emission originating from the dimerization of pyrenes attached at the O-2 and O-4 positions of the xylopyranoside. We demonstrate that external compression of monolayers in lateral directions can alter carbohydrate conformation depending sensitively on the softness/hardness of the mixed monolayers.
Experimental General
Fig. 1 (a) Schematic image of mechanically controlled molecular conformational interchange and (b) the molecular design for this purpose.
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Solvents and materials were purchased from Nacalai Tesque, Inc. (Kyoto, Japan); Wako Pure Chemical Industries, Ltd. (Osaka, Japan); or TCI (Tokyo, Japan). 1H-NMR spectra were obtained from CDCl3 solutions of the compounds using a JEOL AL300BX spectrometer with tetramethylsilane (TMS) as an internal standard. Chemical shifts are given as d (ppm) and
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coupling constants in Hz. Ultraviolet-visible (UV-vis) and fluorescence spectra of solutions were recorded on a UV-3600 spectrophotometer (Shimadzu, Kyoto, Japan) in a quartz cuvette (d = 1 cm) and FP-6500 (JASCO, Tokyo, Japan) at 20 1C, respectively. Water used for the subphase was distilled using an Autostill WG220 (Yamato, Tokyo, Japan) and deionized using a Milli-Q Lab (Millipore, MA, USA). Its specific resistance was greater than 18 MO cm. Spectroscopic grade chloroform (Kanto Chemical Co., Tokyo, Japan) was used as the spreading solvent. Monolayer measurements All p–A isotherm and in situ fluorescence measurements were conducted in a darkened room. p–A isotherms of monolayers were measured at 20.0 1C using an FSD-300 computercontrolled film balance (USI System, Fukuoka, Japan). Fluctuation of the subphase temperature was within 0.2 1C. The surface pressure was measured by the Wilhelmy method. A diluted solution was spread onto the water subphase in a Teflon-coated Langmuir trough. After spreading of the sample, the solvent was allowed to evaporate for 15 min. Compression was commenced at a rate of 0.2 mm s1. Compressibility and compression modulus For evaluation of phase separation of p–A isotherms precisely, compressibility was calculated on the basis of eqn (1). Cs ¼
1 dA A dp
(1)
The neighboring nine points in a p–A isotherm were fitted to the second-order equation using the Savitzky–Golay method, and the mathematically obtained slope of the isotherms was converted to compressibility ((dA/dp)/A).15 The minima in the plots of Cs versus p correspond to phase transitions. Similarly, one can define compression modulus (Cs1), which is the reciprocal of the monolayer compressibility, as Cs1 ¼ A
dp dA
(2)
Estimation of p–A isotherms for PyXy alone The PyXy component in the p–A isotherm can be estimated according to the following procedure.16 When the two components in a mixed Langmuir monolayer are sufficiently phase separated, the total area, A, occupied by a mixed monolayer should be a simple sum of the areas of PyXy and the matrix lipid as shown in eqn (3), where Am, Nm, AP, and NP are the molecular area of the matrix lipid, the number of matrix molecules, the molecular area of PyXy and the number of PyXy molecules, respectively. A = AmNm + ApNp
(3)
Then, eqn (4) is derived from eqn (3), A Np ¼ Am þ Ap Nm Nm
(4)
Fig. S1 (ESI†) represents the plot of A/Nm against Np/Nm for each mixed monolayer, indicating rather well linearity at
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all pressures. Then, the Ap values were calculated from the slope by a least-squares method at 0.5 mN m1 intervals. Hypothetical p–A isotherms for PyXy alone were estimated by plotting Ap data points at the corresponding surface pressures, as shown in Fig. 8. In situ fluorescence measurements for monolayers In situ fluorescence spectra of Langmuir monolayers at the air– water interface were measured using an optical fibre detector at the top of the trough. This was connected to a high sensitivity spectro-multi-channel photodetector (Photal MCPD-7000, Otsuka Electronics, Osaka, Japan). A Y-shaped optical fibre with an area of 0.25 cm2 was placed 1 mm above the surface of the subphase. Excitation light of 346 nm was incident to the Langmuir monolayers through the optical fibre from the xenon light source (incident angle of 7.51). Light emitted from the monolayer was returned to the detector through the same optical fibre. Transmitted light was reflected by a mirror set in the subphase to eliminate reflected light from the measuring system. Fluorescence was negligible when a monolayer was absent. Synthesis Methyl 2,3,4-tri-O-[4-(pyrene-1-yl)benzoyl]-b-D-xylopyranoside (PyXy) and methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy) were synthesized from D-xylose according to the reaction route shown in Scheme S1 (ESI†). Methyl b-D-xylopyranoside (1) was obtained according to the method of McGeary et al.17 1H-NMR spectra of 2 and StXy, and gCOSY spectra of PyXy are shown in the ESI.† Methyl 2,3,4-tri-O-(4-bromobenzoyl)-b-D-xylopyranoside (2). 4-Bromobenzoyl chloride (0.79 g, 3.6 mmol) and Et3N (0.70 mL, 5.0 mmol) were added to a solution of methyl b-D-xylopyranoside (1) (0.16 g, 1.0 mmol) in dichloromethane (5 mL) at 0 1C under stirring. The mixture was stirred at room temperature overnight under a nitrogen atmosphere. The reaction mixture was diluted with ethyl acetate, washed with distilled water and brine, then dried over anhydrous Na2SO4, followed by removal of solvents under reduced pressure. The crude product was purified on a silica gel column eluting with ethyl acetate–hexane (1 : 4, v/v) to afford 2 as colorless oil (0.58 g) in 81% yield. 1H NMR (300 MHz, CDCl3) d: 3.52 (s, 3H, –OCH3), 3.67 (dd, 1H, J4,5a = 7.7 Hz, J5a,5b = 11.9 Hz, H-5a), 4.40 (dd, 1H, J4,5b = 4.6 Hz, J5a,5b = 11.9 Hz, H-5b), 4.70 (d, 1H, J1,2 = 6.1 Hz, H-1), 5.28–5.38 (m, 2H, H-2, H-4), 5.73 (t, 1H, J2,3 = J3,4 = 8.1 Hz, H-3), 7.50 (dt, 2H, o-C6H4), 7.79 (t, 2H, m-C6H4). Methyl 2,3,4-tri-O-[4-(pyrene-1-yl)benzoyl]-b-D-xylopyranoside (PyXy). Aqueous Na2CO3 solution (2 M, 2.5 mL), Pd(PPh3)4 (0.0035 g, 3 mmol), and 1-pyreneboronic acid (0.096 g, 0.39 mmol) were added to a solution of methyl 2,3,4-tri-O-(4-bromobenzoyl)b-D-xylopyranoside (2) (0.07 g, 0.10 mmol) in tetrahydrofuran (2.5 mL) with stirring under a nitrogen atmosphere. The reaction mixture was refluxed for 15 h, then diluted with ethyl acetate, washed with distilled water and brine, and dried over anhydrous Na2SO4, then concentrated in vacuo. The crude product was first purified on a silica gel column eluted with CH2Cl2, then purified by recycling preparative HPLC
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(LC-9104, Japan Analytical Industry Co., Ltd, Tokyo) with CHCl3 as eluent to afford PyXy as yellowish oil (0.04 g) in a 37% yield. 1 H NMR (300 MHz, C6D6) d: 3.30 (s, 3H, –OCH3), 3.64 (dd, 1H, J4,5a = 6.5 Hz, J5a,5b = 11.9 Hz, H-5a), 4.44 (dd, 1H, J4,5b = 4.5 Hz, J5a,5b = 12.4 Hz, H-5b), 4.76 (d, 1H, J1,2 = 5.4 Hz, H-1), 5.65 (m, 1H, H-4), 5.97 (dd, 1H, J2,3 = 7.6 Hz, H-2), 6.32 (t, 1H, J2,3 = 7.1 Hz, H-3), 6.89–8.42 (39H, aromatic-H). 1H NMR (300 MHz, CDCl3) d: 3.64 (s, 3H, –OCH3), 3.91 (dd, 1H, J4,5a = 6.2 Hz, J5a,5b = 12.5 Hz, H-5a), 4.59 (dd, 1H, J4,5b = 3.7 Hz, J5a,5b = 12.3 Hz, H-5b), 4.93 (d, 1H, J1,2 = 4.9 Hz, H-1), 5.48 (m, 1H, H-4), 5.53 (t, 1H, J2,3 = 6.4 Hz, H-2), 7.65–8.31 (39 H, aromatic-H). Methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy). Stearic acid (1.1 g, 4.5 mmol), DCC (1.2 g, 6.0 mmol), and DMAP (0.98 g, 8.0 mmol) were added to a solution of methyl b-Dxylopyranoside (1) (0.16 g, 1.0 mmol) in CH2Cl2 (5 mL) at 0 1C under stirring. The mixture was stirred at room temperature overnight under a nitrogen atmosphere. After removal of the by-product, dicyclohexylurea (DCU) by filtration, the crude waxy solid was purified on a silica gel column eluted with CH2Cl2–methanol (1 : 2, v/v) and concentrated in vacuo to give StXy as a white solid (1.0 g) in a quantitative yield. 1H NMR (300 MHz, CDCl3) d: 0.88 (t, 9H, J = 7.2 Hz, stearoyl –CH3), 1.26 (s, 42H, stearyol CH2), 1.56 (broad s, 6H, stearoyl CO–CH2– CH2–), 2.27 (m, 6H, stearoyl CO–CH2–), 3.35 (dd, 1H, J4,5a = 9.3 Hz, J5a,5b = 12.0 Hz, H-5a), 3.46 (s, 3H, –OCH3), 4.12 (dd, 1H, J4,5b = 5.1 Hz, J5a,5b = 11.7 Hz, H-5b), 4.38 (d, 1H, J1,2 = 7.5 Hz, H-1), 4.94, 4.95 (2 dd, 2H, H-2, H-4), 5.19 (t, 1H, J2,3 = J3,4 = 8.7 Hz, H-3).
Results and discussion Synthesis and optical properties in solution The pyrene-labeled xylose derivative (PyXy) was prepared by 4-bromobenzoylation of methyl b-D-xylopyranoside (1), followed by Suzuki cross-coupling with pyrene-1-boronic acid in the presence of catalytic amounts of palladium(0) and sodium carbonate (Scheme S1, ESI†). First, a 1H NMR study of the effect of solvent identity on PyXy was conducted. The 1H NMR spectrum of PyXy in C6D6 suggests that the conformational equilibrium is shifted toward the 4C1 conformation,18 with almost all substitutents in equatorial orientation: J1,2 = 5.4 Hz, J2,3 = 7.6 Hz, and J3,4 = 7.1 Hz (Fig. 3). On the other hand, the 1H NMR spectrum of PyXy in CDCl3 exhibits upfield shifts for peaks due to H-2, H-3, and H-4 and downfield shifts for peaks due to H-1, H-5a,b, and OMe. This is presumably due to the p-stacking interaction between aromatic rings and the resulting shielding of protons due to anisotropy of the ring current effect. These results suggest that pyrene moieties attached to the xylose molecule can dynamically interact depending on their environment. Coupling constants J1,2 in C6D6 or CDCl3 are similar, suggesting that the equatorial C1 conformation is dominant in solution, and is confirmed by the fluorescence spectroscopy described below.
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Fig. 3
1
H NMR spectra of PyXy in C6D6 (upper) and CDCl3 (lower).
Fig. 4 Absorption and fluorescence spectra of PyXy in CHCl3 or benzene (concentration, 20 mM for absorption, 1 nM for fluorescence; excited at l = 346 nm).
PyXy was then subjected to absorption and fluorescence measurements to determine its optical properties in solution. Absorption spectra in CHCl3 and benzene show a single intense band with a maximum absorbance at around 346 nm and values around 4.8–4.9 for log e (Fig. 4). This absorption is assigned to be the p–p* transition due to intramolecular charge transfer between pyrene and benzoyl moieties of the 4-(pyrene-1-yl)benzoyl moieties.19 Fig. 4 also shows fluorescence spectra with excitation at l = 346 nm for 1 nM solutions of PyXy in CHCl3 or benzene. Solvents used did not affect the shape of the fluorescence spectra although 1H-NMR measurements indicated solvent effect chemical shifts. The emission peak due to the monomer emission of 1-phenylpyrene was observed at around l = 410 nm, corresponding to that of an unsubstituted pyrene (lmax = 393 nm). Interestingly, the fluorescence spectra contain two other peaks at lmax = 431 and 455 nm corresponding to intramolecular charge transfer of a planar conformation between pyrene and benzoyl moieties and the excimer emission due to the pyrene dimer. The latter excimer emission is associated with the conformational preference for the axial 1C form over the equatorial C1 form due to the stacking effect of pyrene moieties attached at the O-2 and O-4 positions.
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Monolayer formation Initially, we prepared a dynamic floating monolayer of PyXy alone on a water surface. However, its p–A isotherm showed a very small molecular area, indicating formation of collapsed PyXy multilayers. Subsequently, we selected two compounds as matrix lipids [methyl stearate (MS), and methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy)] for embedding of PyXy for formation of a stable monolayer at the air–water interface (Fig. 2). MS is a nonionic substance with long hydrophobic chains, which forms a stable and rigid monolayer. StXy was designed and newly synthesized from the viewpoint of compatibility with PyXy and forms rather soft monolayers as described below. StXy was also expected to undergo conformational interchange upon application of surface pressure along with PyXy. However, surface pressure (p)–area (A) isotherm measurements demonstrated no transition from C1 to 1C and we concluded that there was no concerted dynamic motion in the mixed monolayer of PyXy and StXy upon compression. For preparation of a mixed monolayer of matrix molecules/ PyXy at the air–water interface, PyXy was first diluted with the matrix in a stock solution and then spread on water, leading to the stable monolayers. p–A isotherm measurements were then conducted for PyXy/each matrix with the mixing molar ratios of 1 : 20 and 1 : 5 (corresponding to PyXy mole fraction wPyXy = 0.048 and 0.17, respectively). Fig. 5 summarizes the p–A isotherms for pure MS, StXy, and their mixtures with PyXy in the mole fractions wPyXy = 0.048 and 0.17. In the profiles, the x-axis represents the molecular area per each matrix molecule. As expected, the PyXy–matrix mixtures formed a stable monolayer at the air–water interface. The limiting molecular areas for pure MS and StXy were estimated to be 0.20 and 0.68 nm2 per molecule, which respectively correspond to the expected areas for an ideal close-packed state of individual alkyl chains (ca. 0.2 nm2) and three alkyl chains (ca. 0.6 nm2) with a methoxyl group. The limiting area of pure StXy is larger than the cross-sectional area of the xylopyranose ring in the C1 chair, ca. 0.3–0.4 nm2, which is due to the bulkiness of three alkyl chains. For the mixed monolayers, the molecular area increased as the content of PyXy increased. Physical properties of monolayers The phase transition can be estimated in a more precise way by considering the compressibility (Cs) isotherms in Fig. 6. A transition point was determined from the local minimum close to inflection points. For the MS mixed monolayers (Fig. 6a), there are two local minima, suggesting a transition from expanded to condensed phases. In addition, the isotherm for PyXy–MS with wPyXy = 0.17 exhibits the coexistence of liquidexpanded and liquid-condensed phases around p = ca. 10 mN m1. Overall, it is important to mention that the MS mixtures form a condensed hard monolayer even in the presence of PyXy. In contrast, the StXy mixed monolayers show only one detectable local minimum with their isotherms exhibiting a gradual increase in surface pressure. These phenomena demonstrate that the PyXy–StXy mixtures form the expanded phase without any transition to the condensed phase upon compression at the
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Fig. 5 Surface pressure–area (p–A) isotherms of monolayers of PyXy and lipid matrices (a: MS and b: StXy) in pure water at 20 1C; x-axis represents area per each matrix molecule. The mixed molar ratios of PyXy and the matrix molecule are indicated in the graph.
air–water interface. This indicates that the PyXy–StXy mixtures form a liquid-type soft monolayer. To investigate the monolayer rigidity more deeply, the compression modulus (Cs1 = A (dp/dA)) for pure MS, StXy, and their mixtures with PyXy were plotted as a function of surface pressure (Fig. 7). The Cs1–p curve for pure MS shows a pronounced maximum, with a Cs1 value greater than 1000 mN m1. This value is characteristic of solid-type monolayers according to Davies and Rideal.20 The mixed monolayers of PyXy–MS also exhibit high maximal values (Cs,max1) of around 500 mN m1 corresponding to solid character (Table 1). The Cs,max1 values for pure StXy and the mixtures with PyXy are lower (200–300 mN m1) than those of MS systems, though these values are still high corresponding to the defined values for solid-type (Cs,max1 4 250 mN m1) and for liquid-condensed (Cs,max1 = 100–250 mN m1) character. Most importantly from these results, the rigidity of the monolayers was quantitatively evaluated. Consequently, PyXy–MS mixed monolayers are hard, whereas those of PyXy–StXy are soft. p–A isotherms of PyXy alone The ideal p–A isotherms of PyXy were deduced according to the previously published procedure16 (see experimental section).
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Fig. 6 Compressibility isotherm (Cs–A) of the mixed monolayer of PyXy with a matrix of (a) MS and (b) StXy; PyXy mole fraction wPyXy = 0 (black circles), 0.048 (red circles) and 0.17 (blue circles) for each profile. The corresponding p–A isotherms are also shown as lines (the same as in Fig. 5). In panel (a), E-C represents coexistence of liquid-expanded (E) and liquidcondensed (C) phases.
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Fig. 7 Cs1–p curves for the mixed monolayer of PyXy with a matrix of (a) MS and (b) StXy; PyXy mole fraction wPyXy = 0 (black circles), 0.048 (red triangles) and 0.17 (blue squares) for each profile.
Table 1 Characteristics of the mixed monolayers of PyXy with matrix molecules
Fig. 8 shows the p–A isotherms for PyXy alone in the matrix molecules MS and StXy. The isotherm of the MS matrix shows a gradual increase in surface pressure starting at p = ca. 0.4 mN m1, and the presence of both expanded and condensed phases with collapse pressure at p = ca. 33 mN m1. In the StXy matrix, on the contrary, the isotherm shows a gradual increase, thereby the presence of the expanded phase. The limiting molecular areas of PyXy in the expanded phase of MS and StXy are 0.33 and 0.45 nm2. As described above, the cross-sectional area of the xylopyranose ring is estimated to be ca. 0.3–0.4 nm2. Thus, it is reasonable to say that the conformation of PyXy in the expanded phase is C1 chair (Fig. 1). Furthermore, the limiting molecular area of PyXy in the condensed phase of MS (Alim = 0.22 mN m1) is smaller than that of the crosssectional area of the xylopyranose ring, clearly suggesting 1C chair conformation. In situ fluorescence measurements The conformational interchange of PyXy by mechanical compression was investigated by in situ surface fluorescence spectroscopy. As shown in Fig. 9, the fluorescence intensity increased upon monolayer compression. Here, the emission at l = 470 nm (I470) corresponds to the excimer emission due to
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Matrix
wPyXya
Cs,max1 b (mN m1)
MS
0 0.048 0.17
1326 508 481
StXy
0 0.048 0.17
295 273 228
a
PyXy mole fraction in the mixed monolayer. in the Cs1–p profiles shown in Fig. 7.
b
Maximal value of Cs1
the pyrene dimer excited at l = 346 nm, as described above. The fluorescence intensities detected at 470 nm are plotted as a function of the molecular area of MS (Fig. 9a and b). Intensity remained low for molecular areas greater than 0.24 nm2 for wPyXy of 0.17 (Fig. 9a) and 0.23 nm2 for that of 0.048 (Fig. 9b), and abruptly elevated at these molecular areas. The observed profiles of the fluorescence intensity are closely related to the p–A isotherm of the corresponding monolayer. Namely, in the region of the condensed phases, the fluorescence intensity increased abruptly. This indicates that the conformational interchange from C1 to 1C is certainly induced upon mechanical compression.
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The surface fluorescence profiles for the PyXy–StXy mixed monolayers show different phenomena (Fig. 10). In the case of wPyXy of 0.17 (Fig. 10a), the intensities in the regions below the surface area of 0.85 nm2 are constant upon compression, though a gradual increase above p = 0.85 nm2 was observed. This suggests that mechanical compression does not induce the dimerization of pyrene moieties and does not affect conformational interchange. It is probable that the increase in I470 for molecular areas greater than 0.85 nm2 is due to the increase in concentration of PyXy in the measurement area, which is supported by the fact that I470 slightly increased below the surface pressure of 0.24 nm2 in the case of PySt–MS (Fig. 9a). In a similar way, any notable increment in fluorescence intensity cannot be observed when wPyXy was 0.048 (Fig. 10b). Fig. 8 p–A isotherms for PyXy alone in the matrix molecules MS and StXy. The curves are calculated from p–A isotherms of the mixed monolayer of PyXy and the matrix molecules according to the method described in the experimental section. E: expanded phase; E-C: coexistence of expanded and condensed phases; C: condensed phase.
Fig. 9 Fluorescence intensity at l = 470 nm (I470) observed from the PyXy– MS mixed monolayer with wPyXy of (a) 0.17 and (b) 0.048 against molecular area per MS (circles). The corresponding p–A isotherms are also shown as lines (the same as in Fig. 5). The insets show fluorescence spectra observed from the PyXy–MS mixed monolayer with wPyXy of 0.17 (a) and 0.048 (b), at a surface pressure from 0 to 40 mN m1 excited at l = 346 nm.
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Mechanism for conformational interchange upon compression In this study, two types of matrix lipids were used: MS as a hard matrix and StXy as a soft matrix. The monolayer rigidity was demonstrated quantitatively by the compression modulus studies, indicating the existence of a liquid-condensed phase for the PyXy–MS monolayers. In addition, the ideal p–A isotherms for PyXy alone revealed that the limiting molecular area was
Fig. 10 Fluorescence intensity at l = 470 nm (I470) observed from the PyXy–StXy mixed monolayer with wPyXy of (a) 0.17 and (b) 0.048 against molecular area per StXy (circles); excited at l = 346 nm. The corresponding p–A isotherms are also shown as lines (the same as in Fig. 5).
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smaller than that of the cross-sectional area of the xylopyranose ring in the condensed region, indicative of the conformational interchange from C1 to 1C when MS was used as a matrix. The reason for a smaller molecular area may be related to the rigidity of the mixed monolayers since mechanical compression can directly affect molecular conformation. On the other hand, a soft environment can lead to molecular motion such as rotation and diffusion without affecting the molecular conformation. Consequently, the conformational interchange of the molecular machine by mechanical compression can be only achieved when the molecules are embedded in the hard matrix.
Conclusions This study opens up the novel concept of conformational interchange of a carbohydrate by mechanical compression at the air– water interface. A xylose derivative bearing pyrene moieties (PyXy) was newly synthesized as a fluorescent molecular machine. 1 H NMR and fluorescent spectroscopy suggested that the conformation of PyXy in solution is equilibrated between C1 and 1C chairs; however, C1 is dominant. PyXy could form a stable monolayer with two types of matrix lipids, methyl stearate (MS) and methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy), forming rigid and soft monolayers, respectively. The p–A isotherm measurements revealed the following things: (i) the mixture of PyXy–MS show a transition from liquid-expanded to liquid-condensed phases in their isotherms; (ii) from the compression moduli, the PyXy–MS mixed monolayers are hard, whereas those of PyXy–StXy are soft; and (iii) by estimating the ideal p–A isotherms for PyXy alone, the limiting molecular area of PyXy in the PyXy–MS monolayers was smaller than that of the cross-sectional area of the xylopyranose ring in C1 chair, suggesting the conformational interchange of PyXy from the most stable C1 to metastable 1C. The conformational interchange could be detected by observing the excimer emission originating from the dimerization of pyrenes attached at the O-2 and O-4 positions of xylopyranoside. Most importantly, in the region of the condensed phases, the fluorescence intensity increased abruptly, suggesting that the rigidity of the mixed monolayers is crucial for the current purpose. This new concept of this study is simple but important since the molecular conformation can be changed without application of conventional stimuli such as heat, light, electric field, addition of ions and guest molecules and so on.
Acknowledgements
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This work was partly supported by WPI Initiative, MEXT, Japan, and CREST program of JST, Japan.
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Conformational interchange of a carbohydrate by mechanical compression at the air–water interface† Keita Sakakibara,*abc Takuya Fujisawa,ad Jonathan P. Hillab and Katsuhiko Ariga*ab Methyl xylopyranoside containing three 4-(pyrene-1-yl)benzoyl groups (PyXy) undergoes conformational interchange within a Langmuir monolayer upon mechanical compression. This xylose-type molecular machine PyXy was immobilized within two different matrix lipids, methyl stearate and methyl 2,3,4-triO-stearoyl-b-D-xylopyranoside, which respectively form rigid and soft monolayers. Structural properties of the monolayer were characterized by assessing the compressibility, compression modulus, and ideal limiting molecular area of PyXy, all of which were estimated from the p–A isotherm measurements. Only the rigid monolayer exhibited a transition to the condensed phase with a limiting molecular area of PyXy smaller than that of the cross-sectional area of the xylopyranose ring in its C1 chair conformation. This suggests conformational interchange of PyXy from the most stable 4C1 (C1) form to the metastable 1
C4 (1C) form. Surface-reflective fluorescence spectroscopy of the monolayer was applied to detect
excimer emission resulting from the face-to-face dimerization of pyrenes attached at the O-2 and O-4 Received 2nd December 2013, Accepted 6th February 2014 DOI: 10.1039/c3cp55078h
positions of xylose. Fluorescence intensity of the excimer increased abruptly in the condensed region only when the rigid monolayer was applied. These results indicate that the rigidity of the matrix monolayer is a critical aspect of the precise manipulation of molecular machines at interfaces. Consequently, this study demonstrates that including a molecular machine into a rigid lipid matrix is a promising means for the preparation of a novel nanoassembly with dynamic functionalities variable
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depending on a mechanical stimulus.
Introduction Many different assemblies of functionalized nanoscale structural units including self-assembled nanostructures of p-conjugated and/or amphiphilic molecules, bio(macro)molecular systems, well-defined polymers and metal/inorganic nanostructures have been so far achieved, leading to a variety of electronic, photonic, magnetic, ecological, sensing, catalytic, or medical properties because of the nanoassembly concerted effects.1 Some of the ultimate nanoscale structural units are molecular machines which consist of a single molecule possessing an active mechanical
a
World Premier International (WPI) Centre for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan. E-mail: [email protected] b JST, CREST, 1-1 Namiki, Tsukuba 305-0044, Japan c Institute for Chemical Research (ICR), Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. E-mail: [email protected] d Department of Pure and Applied Chemistry, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan † Electronic supplementary information (ESI) available: 1H-NMR spectra of compound 2 and StXy, 2D [1H, 1H] COSY NMR spectra of PyXy, Scheme S1, and Fig. S1. See DOI: 10.1039/c3cp55078h
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motion such as rotation (like a motor) or pinching (like tweezers).2 For example, intramolecular rotation and/or deformation of molecular machines can be stimulated by applying photonic or chemical inputs. In those cases, observations are usually made in solution and therefore cannot be accessed in order to exploit their properties.3 This necessitates their immobilization at a particular surface/interface for their effective utilization and detection. For example, sophisticated assembly systems of molecular machines are common in Nature immobilized in biological membranes. Biological membranes, which are essential media for membrane trafficking and signal transduction in cells, can be modulated by mechanical forces ranging from osmotic force to touch, vibration and texture, due to their fluid and flexible responses to applied stimuli. Hence, their highly dynamic nature is crucial to their mechanosensitivity, mechanotransduction, and/or adaptations to mechanical stresses. In general, conformational interchange of proteins upon application of pressure leads to mechanical senses.4 Mechanosensitive channels and rhodopsin are excellent examples of such proteins.5 In another example, the presence of anesthetic molecules causes a redistribution of lateral pressures in a membrane, in turn shifting the
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conformation of membrane proteins such as ligand-gated ion channels.6 The elegance with which biological systems perform pressure-driven functions has inspired us and other groups to manipulate molecular machines by applying macroscopic mechanical stimuli.3,7 For this purpose, a dynamic interface can be utilized as a medium for controlling and manipulating nanosystems through macroscopic mechanical motions.8–11 In our recent studies, macrocyclic molecules including cyclophanes and cyclens have been used as fundamental components for conformational interchange by compression because of their constrained cyclic structures. Such conformational interchange is transmitted by changes in surface pressure, which is a possible mechanism for molecular signal transduction.12 Another promising candidate for pressure transduction would be carbohydrates. Force-induced elastic deformation of the pyranose ring in polysaccharides such as dextran, amylose, or pullulan has been demonstrated by single molecule force spectroscopy.13 Metal ion-triggered conformational ring flip of monosaccharides has also been demonstrated, and is applicable for signal transduction.14 However, the use of macroscopic mechanical stimuli rather than local mechanical (AFM tips) or chemical (ions) forces has not been investigated so far, despite carbohydrates in dynamic biological membranes playing a pivotal role such as cell–cell recognition and signalling events. These aspects led us to investigate the behaviour of carbohydrates embedded in an artificial biological membrane where their dynamics are regulated by mechanical force. Here, we experimentally demonstrate that the conformation of a carbohydrate molecule immobilized in a dynamic monolayer (a Langmuir monolayer) can be directly regulated by varying the applied surface pressure (Fig. 1a). To determine how force regulates the conformational change, a xylose derivative bearing fluorescent moieties at O-2, O-3, and O-4 positions was used. This molecule exhibits excimer emission when its conformation changes from 4C1 (C1) to 1C4 (1C) within a
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Fig. 2 Chemical structures of PyXy and the two matrix molecules MS and StXy.
Langmuir monolayer (Fig. 1b). For this study, we designed and synthesized a pyrene-labelled xylose (PyXy) and carried out in situ fluorescence spectroscopy on the water surface. PyXy has three pyrene moieties as a probe of the conformational change with p-phenylene spacers (Fig. 2). In order to mimic the hydrophobic and organized environment of naturally occurring biological membranes, this compound was immobilized separately with two types of matrix lipids, methyl stearate (MS) and methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy), respectively, forming rigid and soft monolayers (Fig. 2). Conformational interchange from the most stable C1 form to the metastable 1C form could be detected by excimer emission originating from the dimerization of pyrenes attached at the O-2 and O-4 positions of the xylopyranoside. We demonstrate that external compression of monolayers in lateral directions can alter carbohydrate conformation depending sensitively on the softness/hardness of the mixed monolayers.
Experimental General
Fig. 1 (a) Schematic image of mechanically controlled molecular conformational interchange and (b) the molecular design for this purpose.
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Solvents and materials were purchased from Nacalai Tesque, Inc. (Kyoto, Japan); Wako Pure Chemical Industries, Ltd. (Osaka, Japan); or TCI (Tokyo, Japan). 1H-NMR spectra were obtained from CDCl3 solutions of the compounds using a JEOL AL300BX spectrometer with tetramethylsilane (TMS) as an internal standard. Chemical shifts are given as d (ppm) and
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coupling constants in Hz. Ultraviolet-visible (UV-vis) and fluorescence spectra of solutions were recorded on a UV-3600 spectrophotometer (Shimadzu, Kyoto, Japan) in a quartz cuvette (d = 1 cm) and FP-6500 (JASCO, Tokyo, Japan) at 20 1C, respectively. Water used for the subphase was distilled using an Autostill WG220 (Yamato, Tokyo, Japan) and deionized using a Milli-Q Lab (Millipore, MA, USA). Its specific resistance was greater than 18 MO cm. Spectroscopic grade chloroform (Kanto Chemical Co., Tokyo, Japan) was used as the spreading solvent. Monolayer measurements All p–A isotherm and in situ fluorescence measurements were conducted in a darkened room. p–A isotherms of monolayers were measured at 20.0 1C using an FSD-300 computercontrolled film balance (USI System, Fukuoka, Japan). Fluctuation of the subphase temperature was within 0.2 1C. The surface pressure was measured by the Wilhelmy method. A diluted solution was spread onto the water subphase in a Teflon-coated Langmuir trough. After spreading of the sample, the solvent was allowed to evaporate for 15 min. Compression was commenced at a rate of 0.2 mm s1. Compressibility and compression modulus For evaluation of phase separation of p–A isotherms precisely, compressibility was calculated on the basis of eqn (1). Cs ¼
1 dA A dp
(1)
The neighboring nine points in a p–A isotherm were fitted to the second-order equation using the Savitzky–Golay method, and the mathematically obtained slope of the isotherms was converted to compressibility ((dA/dp)/A).15 The minima in the plots of Cs versus p correspond to phase transitions. Similarly, one can define compression modulus (Cs1), which is the reciprocal of the monolayer compressibility, as Cs1 ¼ A
dp dA
(2)
Estimation of p–A isotherms for PyXy alone The PyXy component in the p–A isotherm can be estimated according to the following procedure.16 When the two components in a mixed Langmuir monolayer are sufficiently phase separated, the total area, A, occupied by a mixed monolayer should be a simple sum of the areas of PyXy and the matrix lipid as shown in eqn (3), where Am, Nm, AP, and NP are the molecular area of the matrix lipid, the number of matrix molecules, the molecular area of PyXy and the number of PyXy molecules, respectively. A = AmNm + ApNp
(3)
Then, eqn (4) is derived from eqn (3), A Np ¼ Am þ Ap Nm Nm
(4)
Fig. S1 (ESI†) represents the plot of A/Nm against Np/Nm for each mixed monolayer, indicating rather well linearity at
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all pressures. Then, the Ap values were calculated from the slope by a least-squares method at 0.5 mN m1 intervals. Hypothetical p–A isotherms for PyXy alone were estimated by plotting Ap data points at the corresponding surface pressures, as shown in Fig. 8. In situ fluorescence measurements for monolayers In situ fluorescence spectra of Langmuir monolayers at the air– water interface were measured using an optical fibre detector at the top of the trough. This was connected to a high sensitivity spectro-multi-channel photodetector (Photal MCPD-7000, Otsuka Electronics, Osaka, Japan). A Y-shaped optical fibre with an area of 0.25 cm2 was placed 1 mm above the surface of the subphase. Excitation light of 346 nm was incident to the Langmuir monolayers through the optical fibre from the xenon light source (incident angle of 7.51). Light emitted from the monolayer was returned to the detector through the same optical fibre. Transmitted light was reflected by a mirror set in the subphase to eliminate reflected light from the measuring system. Fluorescence was negligible when a monolayer was absent. Synthesis Methyl 2,3,4-tri-O-[4-(pyrene-1-yl)benzoyl]-b-D-xylopyranoside (PyXy) and methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy) were synthesized from D-xylose according to the reaction route shown in Scheme S1 (ESI†). Methyl b-D-xylopyranoside (1) was obtained according to the method of McGeary et al.17 1H-NMR spectra of 2 and StXy, and gCOSY spectra of PyXy are shown in the ESI.† Methyl 2,3,4-tri-O-(4-bromobenzoyl)-b-D-xylopyranoside (2). 4-Bromobenzoyl chloride (0.79 g, 3.6 mmol) and Et3N (0.70 mL, 5.0 mmol) were added to a solution of methyl b-D-xylopyranoside (1) (0.16 g, 1.0 mmol) in dichloromethane (5 mL) at 0 1C under stirring. The mixture was stirred at room temperature overnight under a nitrogen atmosphere. The reaction mixture was diluted with ethyl acetate, washed with distilled water and brine, then dried over anhydrous Na2SO4, followed by removal of solvents under reduced pressure. The crude product was purified on a silica gel column eluting with ethyl acetate–hexane (1 : 4, v/v) to afford 2 as colorless oil (0.58 g) in 81% yield. 1H NMR (300 MHz, CDCl3) d: 3.52 (s, 3H, –OCH3), 3.67 (dd, 1H, J4,5a = 7.7 Hz, J5a,5b = 11.9 Hz, H-5a), 4.40 (dd, 1H, J4,5b = 4.6 Hz, J5a,5b = 11.9 Hz, H-5b), 4.70 (d, 1H, J1,2 = 6.1 Hz, H-1), 5.28–5.38 (m, 2H, H-2, H-4), 5.73 (t, 1H, J2,3 = J3,4 = 8.1 Hz, H-3), 7.50 (dt, 2H, o-C6H4), 7.79 (t, 2H, m-C6H4). Methyl 2,3,4-tri-O-[4-(pyrene-1-yl)benzoyl]-b-D-xylopyranoside (PyXy). Aqueous Na2CO3 solution (2 M, 2.5 mL), Pd(PPh3)4 (0.0035 g, 3 mmol), and 1-pyreneboronic acid (0.096 g, 0.39 mmol) were added to a solution of methyl 2,3,4-tri-O-(4-bromobenzoyl)b-D-xylopyranoside (2) (0.07 g, 0.10 mmol) in tetrahydrofuran (2.5 mL) with stirring under a nitrogen atmosphere. The reaction mixture was refluxed for 15 h, then diluted with ethyl acetate, washed with distilled water and brine, and dried over anhydrous Na2SO4, then concentrated in vacuo. The crude product was first purified on a silica gel column eluted with CH2Cl2, then purified by recycling preparative HPLC
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(LC-9104, Japan Analytical Industry Co., Ltd, Tokyo) with CHCl3 as eluent to afford PyXy as yellowish oil (0.04 g) in a 37% yield. 1 H NMR (300 MHz, C6D6) d: 3.30 (s, 3H, –OCH3), 3.64 (dd, 1H, J4,5a = 6.5 Hz, J5a,5b = 11.9 Hz, H-5a), 4.44 (dd, 1H, J4,5b = 4.5 Hz, J5a,5b = 12.4 Hz, H-5b), 4.76 (d, 1H, J1,2 = 5.4 Hz, H-1), 5.65 (m, 1H, H-4), 5.97 (dd, 1H, J2,3 = 7.6 Hz, H-2), 6.32 (t, 1H, J2,3 = 7.1 Hz, H-3), 6.89–8.42 (39H, aromatic-H). 1H NMR (300 MHz, CDCl3) d: 3.64 (s, 3H, –OCH3), 3.91 (dd, 1H, J4,5a = 6.2 Hz, J5a,5b = 12.5 Hz, H-5a), 4.59 (dd, 1H, J4,5b = 3.7 Hz, J5a,5b = 12.3 Hz, H-5b), 4.93 (d, 1H, J1,2 = 4.9 Hz, H-1), 5.48 (m, 1H, H-4), 5.53 (t, 1H, J2,3 = 6.4 Hz, H-2), 7.65–8.31 (39 H, aromatic-H). Methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy). Stearic acid (1.1 g, 4.5 mmol), DCC (1.2 g, 6.0 mmol), and DMAP (0.98 g, 8.0 mmol) were added to a solution of methyl b-Dxylopyranoside (1) (0.16 g, 1.0 mmol) in CH2Cl2 (5 mL) at 0 1C under stirring. The mixture was stirred at room temperature overnight under a nitrogen atmosphere. After removal of the by-product, dicyclohexylurea (DCU) by filtration, the crude waxy solid was purified on a silica gel column eluted with CH2Cl2–methanol (1 : 2, v/v) and concentrated in vacuo to give StXy as a white solid (1.0 g) in a quantitative yield. 1H NMR (300 MHz, CDCl3) d: 0.88 (t, 9H, J = 7.2 Hz, stearoyl –CH3), 1.26 (s, 42H, stearyol CH2), 1.56 (broad s, 6H, stearoyl CO–CH2– CH2–), 2.27 (m, 6H, stearoyl CO–CH2–), 3.35 (dd, 1H, J4,5a = 9.3 Hz, J5a,5b = 12.0 Hz, H-5a), 3.46 (s, 3H, –OCH3), 4.12 (dd, 1H, J4,5b = 5.1 Hz, J5a,5b = 11.7 Hz, H-5b), 4.38 (d, 1H, J1,2 = 7.5 Hz, H-1), 4.94, 4.95 (2 dd, 2H, H-2, H-4), 5.19 (t, 1H, J2,3 = J3,4 = 8.7 Hz, H-3).
Results and discussion Synthesis and optical properties in solution The pyrene-labeled xylose derivative (PyXy) was prepared by 4-bromobenzoylation of methyl b-D-xylopyranoside (1), followed by Suzuki cross-coupling with pyrene-1-boronic acid in the presence of catalytic amounts of palladium(0) and sodium carbonate (Scheme S1, ESI†). First, a 1H NMR study of the effect of solvent identity on PyXy was conducted. The 1H NMR spectrum of PyXy in C6D6 suggests that the conformational equilibrium is shifted toward the 4C1 conformation,18 with almost all substitutents in equatorial orientation: J1,2 = 5.4 Hz, J2,3 = 7.6 Hz, and J3,4 = 7.1 Hz (Fig. 3). On the other hand, the 1H NMR spectrum of PyXy in CDCl3 exhibits upfield shifts for peaks due to H-2, H-3, and H-4 and downfield shifts for peaks due to H-1, H-5a,b, and OMe. This is presumably due to the p-stacking interaction between aromatic rings and the resulting shielding of protons due to anisotropy of the ring current effect. These results suggest that pyrene moieties attached to the xylose molecule can dynamically interact depending on their environment. Coupling constants J1,2 in C6D6 or CDCl3 are similar, suggesting that the equatorial C1 conformation is dominant in solution, and is confirmed by the fluorescence spectroscopy described below.
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Fig. 3
1
H NMR spectra of PyXy in C6D6 (upper) and CDCl3 (lower).
Fig. 4 Absorption and fluorescence spectra of PyXy in CHCl3 or benzene (concentration, 20 mM for absorption, 1 nM for fluorescence; excited at l = 346 nm).
PyXy was then subjected to absorption and fluorescence measurements to determine its optical properties in solution. Absorption spectra in CHCl3 and benzene show a single intense band with a maximum absorbance at around 346 nm and values around 4.8–4.9 for log e (Fig. 4). This absorption is assigned to be the p–p* transition due to intramolecular charge transfer between pyrene and benzoyl moieties of the 4-(pyrene-1-yl)benzoyl moieties.19 Fig. 4 also shows fluorescence spectra with excitation at l = 346 nm for 1 nM solutions of PyXy in CHCl3 or benzene. Solvents used did not affect the shape of the fluorescence spectra although 1H-NMR measurements indicated solvent effect chemical shifts. The emission peak due to the monomer emission of 1-phenylpyrene was observed at around l = 410 nm, corresponding to that of an unsubstituted pyrene (lmax = 393 nm). Interestingly, the fluorescence spectra contain two other peaks at lmax = 431 and 455 nm corresponding to intramolecular charge transfer of a planar conformation between pyrene and benzoyl moieties and the excimer emission due to the pyrene dimer. The latter excimer emission is associated with the conformational preference for the axial 1C form over the equatorial C1 form due to the stacking effect of pyrene moieties attached at the O-2 and O-4 positions.
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Monolayer formation Initially, we prepared a dynamic floating monolayer of PyXy alone on a water surface. However, its p–A isotherm showed a very small molecular area, indicating formation of collapsed PyXy multilayers. Subsequently, we selected two compounds as matrix lipids [methyl stearate (MS), and methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy)] for embedding of PyXy for formation of a stable monolayer at the air–water interface (Fig. 2). MS is a nonionic substance with long hydrophobic chains, which forms a stable and rigid monolayer. StXy was designed and newly synthesized from the viewpoint of compatibility with PyXy and forms rather soft monolayers as described below. StXy was also expected to undergo conformational interchange upon application of surface pressure along with PyXy. However, surface pressure (p)–area (A) isotherm measurements demonstrated no transition from C1 to 1C and we concluded that there was no concerted dynamic motion in the mixed monolayer of PyXy and StXy upon compression. For preparation of a mixed monolayer of matrix molecules/ PyXy at the air–water interface, PyXy was first diluted with the matrix in a stock solution and then spread on water, leading to the stable monolayers. p–A isotherm measurements were then conducted for PyXy/each matrix with the mixing molar ratios of 1 : 20 and 1 : 5 (corresponding to PyXy mole fraction wPyXy = 0.048 and 0.17, respectively). Fig. 5 summarizes the p–A isotherms for pure MS, StXy, and their mixtures with PyXy in the mole fractions wPyXy = 0.048 and 0.17. In the profiles, the x-axis represents the molecular area per each matrix molecule. As expected, the PyXy–matrix mixtures formed a stable monolayer at the air–water interface. The limiting molecular areas for pure MS and StXy were estimated to be 0.20 and 0.68 nm2 per molecule, which respectively correspond to the expected areas for an ideal close-packed state of individual alkyl chains (ca. 0.2 nm2) and three alkyl chains (ca. 0.6 nm2) with a methoxyl group. The limiting area of pure StXy is larger than the cross-sectional area of the xylopyranose ring in the C1 chair, ca. 0.3–0.4 nm2, which is due to the bulkiness of three alkyl chains. For the mixed monolayers, the molecular area increased as the content of PyXy increased. Physical properties of monolayers The phase transition can be estimated in a more precise way by considering the compressibility (Cs) isotherms in Fig. 6. A transition point was determined from the local minimum close to inflection points. For the MS mixed monolayers (Fig. 6a), there are two local minima, suggesting a transition from expanded to condensed phases. In addition, the isotherm for PyXy–MS with wPyXy = 0.17 exhibits the coexistence of liquidexpanded and liquid-condensed phases around p = ca. 10 mN m1. Overall, it is important to mention that the MS mixtures form a condensed hard monolayer even in the presence of PyXy. In contrast, the StXy mixed monolayers show only one detectable local minimum with their isotherms exhibiting a gradual increase in surface pressure. These phenomena demonstrate that the PyXy–StXy mixtures form the expanded phase without any transition to the condensed phase upon compression at the
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Fig. 5 Surface pressure–area (p–A) isotherms of monolayers of PyXy and lipid matrices (a: MS and b: StXy) in pure water at 20 1C; x-axis represents area per each matrix molecule. The mixed molar ratios of PyXy and the matrix molecule are indicated in the graph.
air–water interface. This indicates that the PyXy–StXy mixtures form a liquid-type soft monolayer. To investigate the monolayer rigidity more deeply, the compression modulus (Cs1 = A (dp/dA)) for pure MS, StXy, and their mixtures with PyXy were plotted as a function of surface pressure (Fig. 7). The Cs1–p curve for pure MS shows a pronounced maximum, with a Cs1 value greater than 1000 mN m1. This value is characteristic of solid-type monolayers according to Davies and Rideal.20 The mixed monolayers of PyXy–MS also exhibit high maximal values (Cs,max1) of around 500 mN m1 corresponding to solid character (Table 1). The Cs,max1 values for pure StXy and the mixtures with PyXy are lower (200–300 mN m1) than those of MS systems, though these values are still high corresponding to the defined values for solid-type (Cs,max1 4 250 mN m1) and for liquid-condensed (Cs,max1 = 100–250 mN m1) character. Most importantly from these results, the rigidity of the monolayers was quantitatively evaluated. Consequently, PyXy–MS mixed monolayers are hard, whereas those of PyXy–StXy are soft. p–A isotherms of PyXy alone The ideal p–A isotherms of PyXy were deduced according to the previously published procedure16 (see experimental section).
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Fig. 6 Compressibility isotherm (Cs–A) of the mixed monolayer of PyXy with a matrix of (a) MS and (b) StXy; PyXy mole fraction wPyXy = 0 (black circles), 0.048 (red circles) and 0.17 (blue circles) for each profile. The corresponding p–A isotherms are also shown as lines (the same as in Fig. 5). In panel (a), E-C represents coexistence of liquid-expanded (E) and liquidcondensed (C) phases.
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Fig. 7 Cs1–p curves for the mixed monolayer of PyXy with a matrix of (a) MS and (b) StXy; PyXy mole fraction wPyXy = 0 (black circles), 0.048 (red triangles) and 0.17 (blue squares) for each profile.
Table 1 Characteristics of the mixed monolayers of PyXy with matrix molecules
Fig. 8 shows the p–A isotherms for PyXy alone in the matrix molecules MS and StXy. The isotherm of the MS matrix shows a gradual increase in surface pressure starting at p = ca. 0.4 mN m1, and the presence of both expanded and condensed phases with collapse pressure at p = ca. 33 mN m1. In the StXy matrix, on the contrary, the isotherm shows a gradual increase, thereby the presence of the expanded phase. The limiting molecular areas of PyXy in the expanded phase of MS and StXy are 0.33 and 0.45 nm2. As described above, the cross-sectional area of the xylopyranose ring is estimated to be ca. 0.3–0.4 nm2. Thus, it is reasonable to say that the conformation of PyXy in the expanded phase is C1 chair (Fig. 1). Furthermore, the limiting molecular area of PyXy in the condensed phase of MS (Alim = 0.22 mN m1) is smaller than that of the crosssectional area of the xylopyranose ring, clearly suggesting 1C chair conformation. In situ fluorescence measurements The conformational interchange of PyXy by mechanical compression was investigated by in situ surface fluorescence spectroscopy. As shown in Fig. 9, the fluorescence intensity increased upon monolayer compression. Here, the emission at l = 470 nm (I470) corresponds to the excimer emission due to
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Matrix
wPyXya
Cs,max1 b (mN m1)
MS
0 0.048 0.17
1326 508 481
StXy
0 0.048 0.17
295 273 228
a
PyXy mole fraction in the mixed monolayer. in the Cs1–p profiles shown in Fig. 7.
b
Maximal value of Cs1
the pyrene dimer excited at l = 346 nm, as described above. The fluorescence intensities detected at 470 nm are plotted as a function of the molecular area of MS (Fig. 9a and b). Intensity remained low for molecular areas greater than 0.24 nm2 for wPyXy of 0.17 (Fig. 9a) and 0.23 nm2 for that of 0.048 (Fig. 9b), and abruptly elevated at these molecular areas. The observed profiles of the fluorescence intensity are closely related to the p–A isotherm of the corresponding monolayer. Namely, in the region of the condensed phases, the fluorescence intensity increased abruptly. This indicates that the conformational interchange from C1 to 1C is certainly induced upon mechanical compression.
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The surface fluorescence profiles for the PyXy–StXy mixed monolayers show different phenomena (Fig. 10). In the case of wPyXy of 0.17 (Fig. 10a), the intensities in the regions below the surface area of 0.85 nm2 are constant upon compression, though a gradual increase above p = 0.85 nm2 was observed. This suggests that mechanical compression does not induce the dimerization of pyrene moieties and does not affect conformational interchange. It is probable that the increase in I470 for molecular areas greater than 0.85 nm2 is due to the increase in concentration of PyXy in the measurement area, which is supported by the fact that I470 slightly increased below the surface pressure of 0.24 nm2 in the case of PySt–MS (Fig. 9a). In a similar way, any notable increment in fluorescence intensity cannot be observed when wPyXy was 0.048 (Fig. 10b). Fig. 8 p–A isotherms for PyXy alone in the matrix molecules MS and StXy. The curves are calculated from p–A isotherms of the mixed monolayer of PyXy and the matrix molecules according to the method described in the experimental section. E: expanded phase; E-C: coexistence of expanded and condensed phases; C: condensed phase.
Fig. 9 Fluorescence intensity at l = 470 nm (I470) observed from the PyXy– MS mixed monolayer with wPyXy of (a) 0.17 and (b) 0.048 against molecular area per MS (circles). The corresponding p–A isotherms are also shown as lines (the same as in Fig. 5). The insets show fluorescence spectra observed from the PyXy–MS mixed monolayer with wPyXy of 0.17 (a) and 0.048 (b), at a surface pressure from 0 to 40 mN m1 excited at l = 346 nm.
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Mechanism for conformational interchange upon compression In this study, two types of matrix lipids were used: MS as a hard matrix and StXy as a soft matrix. The monolayer rigidity was demonstrated quantitatively by the compression modulus studies, indicating the existence of a liquid-condensed phase for the PyXy–MS monolayers. In addition, the ideal p–A isotherms for PyXy alone revealed that the limiting molecular area was
Fig. 10 Fluorescence intensity at l = 470 nm (I470) observed from the PyXy–StXy mixed monolayer with wPyXy of (a) 0.17 and (b) 0.048 against molecular area per StXy (circles); excited at l = 346 nm. The corresponding p–A isotherms are also shown as lines (the same as in Fig. 5).
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smaller than that of the cross-sectional area of the xylopyranose ring in the condensed region, indicative of the conformational interchange from C1 to 1C when MS was used as a matrix. The reason for a smaller molecular area may be related to the rigidity of the mixed monolayers since mechanical compression can directly affect molecular conformation. On the other hand, a soft environment can lead to molecular motion such as rotation and diffusion without affecting the molecular conformation. Consequently, the conformational interchange of the molecular machine by mechanical compression can be only achieved when the molecules are embedded in the hard matrix.
Conclusions This study opens up the novel concept of conformational interchange of a carbohydrate by mechanical compression at the air– water interface. A xylose derivative bearing pyrene moieties (PyXy) was newly synthesized as a fluorescent molecular machine. 1 H NMR and fluorescent spectroscopy suggested that the conformation of PyXy in solution is equilibrated between C1 and 1C chairs; however, C1 is dominant. PyXy could form a stable monolayer with two types of matrix lipids, methyl stearate (MS) and methyl 2,3,4-tri-O-stearoyl-b-D-xylopyranoside (StXy), forming rigid and soft monolayers, respectively. The p–A isotherm measurements revealed the following things: (i) the mixture of PyXy–MS show a transition from liquid-expanded to liquid-condensed phases in their isotherms; (ii) from the compression moduli, the PyXy–MS mixed monolayers are hard, whereas those of PyXy–StXy are soft; and (iii) by estimating the ideal p–A isotherms for PyXy alone, the limiting molecular area of PyXy in the PyXy–MS monolayers was smaller than that of the cross-sectional area of the xylopyranose ring in C1 chair, suggesting the conformational interchange of PyXy from the most stable C1 to metastable 1C. The conformational interchange could be detected by observing the excimer emission originating from the dimerization of pyrenes attached at the O-2 and O-4 positions of xylopyranoside. Most importantly, in the region of the condensed phases, the fluorescence intensity increased abruptly, suggesting that the rigidity of the mixed monolayers is crucial for the current purpose. This new concept of this study is simple but important since the molecular conformation can be changed without application of conventional stimuli such as heat, light, electric field, addition of ions and guest molecules and so on.
Acknowledgements
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This work was partly supported by WPI Initiative, MEXT, Japan, and CREST program of JST, Japan.
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