Colloids and Surfaces B: Biointerfaces 117 (2014) 248–251
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Short Communication
Controlled influence of quantum dots on purple membranes at interfaces Sergei Yu. Zaitsev ∗ , Eugeni P. Lukashev, Daria O. Solovyeva, Alexander A. Chistyakov, Vladimir A. Oleinikov Laboratory of Nano-Bioengineering, National Research Nuclear University “Moscow Engineering Physics Institute”, 115409 Moscow, Russian Federation
a r t i c l e
i n f o
Article history: Received 16 December 2013 Received in revised form 29 January 2014 Accepted 19 February 2014 Available online 28 February 2014 Keywords: Biointerfaces Quantum dots Purple membranes Bionanomaterials
a b s t r a c t The development of bio-sensitized nanofilms engineered from biomembrane components and inorganic nanoparticles is a promising field of colloid and interface science and technologies. Recent nanobioengineering approaches employing quantum dots (QDs) permit the enhancement of the purple membrane (PM) “light-harvesting capacity” compared to native PMs. The influence of QDs on the PM properties, especially the bacteriorhodopsin (bR) photocycle, has been found that has both fundamental (mechanisms of photoreception) and applied implications (including the fabrication of hybrid bionanomaterials). Samples of PM–QD complexes capable of energy transfer and characterized by increased rates of M-intermediate formation and decay have been obtained. The modified bR photocycle kinetic parameters may be explained by changes in the PM interface upon QD adsorption. The increase and decrease in absorption at 410 nm (or photopotential) for PM–QD complexes are, on average, several times more rapid than for PM suspensions or PM dry films. These results provide a strong impetus for the development of nanomaterials with advanced properties. © 2014 Published by Elsevier B.V.
1. Introduction The development and study of bio–nano hybrid systems engineered from membrane proteins and inorganic nanoparticles constitute a fascinating modern field at the interface of chemistry, biology, physics, and nano-biotechnology [1–4]. A number of photosensitive proteins have been explored in terms of sensoring, bioelectronic, and optical applications, but bacteriorhodopsin (bR), a photosensitive membrane protein from purple membranes (PMs) of Halobacterium salinarium, has attracted the most attention [5–9]. bR possesses unique physicochemical properties and has three main molecular functions: photoelectric, photochromic, and proton-transporting [8–11]. Native PMs (without light-harvesting complexes) can utilize only 0.5% of the solar light. Recent nanobioengineering approaches employing quantum dots (QDs) and bR permit the enhancement of the PM “light-harvesting capacity,” thus providing a strong impetus for the development of novel nanomaterials [4]. It is known [1–4,12,13] that illumination of QDs in the region of their absorption causes some specific effects, including Förster resonance energy transfer (FRET) in the presence of an
acceptor, such as a photosensitive protein. This effect in a hybrid bio-nanosystem depends on the properties of the PM–QD system and is used in numerous applications [2–4,11,14]. One of the first PM–QD hybrid systems was studied by Li et al. [15]. Although some of their experimental results were inconclusive, the authors developed a theoretical model explaining how QDs could act as “nanoscaled light sources” (embedded in PMs) to promote the generation of a stationary photocurrent [15]. In general, the main advantage of this system is the possibility of FRET, which improves the bR function in a hybrid material consisting of PMs and QDs [2–4]. However, inorganic nanocrystals, being in direct contact with PMs, may have some effects on bR that are not directly related to FRET. We expected that QDs as additional parts of the composition (which have a specific charge and an intrinsic dipole moment) would significantly influence the bR photocycle. Here, we studied the QD effect on PMs, in particular, on the bR photocycle. This effect may be of both fundamental and applied importance, especially for the development of new bio–nano hybrid materials. 2. Experimental
∗ Corresponding author. Tel.: +7 4953779539; fax: +7 4953774939. E-mail addresses: [email protected], [email protected] (S.Yu. Zaitsev). http://dx.doi.org/10.1016/j.colsurfb.2014.02.033 0927-7765/© 2014 Published by Elsevier B.V.
PMs were isolated from H. salinarum by the standard procedure [3]. The concentration of the PM dispersion was 6 mg/ml as
S.Yu. Zaitsev et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 248–251
249
Fig. 1. Spectra of (1) absorption and (2) fluorescence of the quantum dots used (QD570 ) and (3) the absorption spectrum of the PM suspension.
measured by light absorbance at 570 nm (the molar extinction coefficient was 63,000 M−1 cm−1 ). PMs are generally used more often than bR, because the isolation of bR from PMs considerably decreases both the biochemical and thermodynamic stabilities of the protein. The changes in the PM light absorbance were measured using a laboratory flash-photolysis setup with the probing beam passing a double monochromator after excitation of the sample with a NdYAG laser (532 nm, 8 ns, 15 mJ). Films based on pure PM fragments or their mixtures with QD570 were obtained on a transparent conductive electrode (PET-ITO) by means of electrophoretic deposition (EPD) and measured according to the procedure described earlier [16]. Ni foil was used for the second (top) electrode, which could be tightly pressed against the smooth surface of the oriented PM films. CdSe/ZnS quantum dots emitting at a wavelength of 570 nm were synthesized. These QDs were covered with trioctylphosphine oxide (TOPO) as an amphiphilic ligand for particle stabilization [17]. The TOPO molecules were replaced with poly(ethylene glycol) at the QD surface as described earlier [3,17]. The fluorescence intensity and lifetime of the QDs were measured by means of a FLUOROLOG-3 spectrofluorimeter (Horiba Jobin Yvon) using the TCSPC option. All measurements were carried out in a 100 mM sodium phosphate buffer solution containing 100 mM KCl (pH 7.5). The lifetimes (, obtained from fluorescence and photopotential measurements) were evaluated by multiexponential fits and presented as average lifetimes ( avr ) in all cases. The avr value was calculated as the sum of the products of each value by the respective amplitude divided by the sum of all amplitudes. All standard chemicals of analytical grade were purchased from Sigma–Aldrich.
Fig. 2. (ɑ) The change in the fluorescence intensity of a QD570 solution (1 M) after the addition of a PM suspension: curve 1 (top), QD570 without PMs; curve 11 (bottom), the same QD570 in the presence of PMs at the maximum concentration (about 6 M of bR). (b) Kinetics of the photoluminescence intensity: curve 1, QD570 feed solution ( avr = 18.5 ± 0.5 ns); curve 2, QD570 films ( avr = 7.8 ± 0.3 ns); curve 3, EPD films from a suspension with a QD to BR molar ratio of 2:9 ( avr = 5.7 ± 0.1 ns); curve 4, EPD films from a suspension with a QD to BR molar ratio of 1.4:9 ( avr = 3.7 ± 0.1 ns); curve 5, EPD films from a suspension with a QD to BR molar ratio of 1:9 ( avr = 2.2 ± 0.1 ns).
3. Results and discussion QDs are energy converters that absorb light in a wide range of photon energies (Fig. 1, curve 1) and can fluoresce (Fig. 1, curve 2) and transfer the harvested energy to bR (in PMs) with a high efficiency [2–4] because of a wide overlap between the QD fluorescence (Fig. 1, curve 2) and PM absorption (Fig. 1, curve 3) spectra. The spectral properties of all the QD batches synthesized were studied. The overlap between the bR absorption spectrum and the QD fluorescence spectrum was the largest for QD570 (with fluorescence at 570) (Fig. 1). An important result of the experiments with photoactivation of PM–QD570 mixtures is that the energy harvested
Fig. 3. Förster resonance energy transfer (FRET) in a system of purple membranes (PMs) and quantum dots (QDs), where illumination of the donor (QD) in the region of its absorption leads to FRET to the acceptor (PM), which changes the optical properties of photosensitive membrane proteins.
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S.Yu. Zaitsev et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 248–251
Fig. 4. Kinetics of the laser-induced parameters of the PM mixtures: (a) kinetics of the laser-induced absorbance changes at 410 nm (the M-intermediate absorption peak) in (1) the feed PM suspension and (2) a PM–QD570 suspension with a bR to QD570 molar ratio of 4:1. (b) Kinetics of the laser-induced photovoltage generation in (1) an oriented PM film and (2) a similar film obtained from a PM–QD570 mixture. The amplitude of curve 2 is several times higher compared to the reference sample level, since the addition of negatively charged QDs to the PM suspension significantly reduced the degree of PM fragments orientation in the mixed film. The dotted line shows the actual kinetic trace; the solid line is the result of multiexponential fitting.
by QD570 is transferred to bR by the FRET mechanism with a high efficiency (Fig. 2). This is confirmed by significant changes in the fluorescence intensity of a QD570 solution (∼1 M) after the addition of PM suspensions with different concentrations (from 0.25 to 6 M of the bR chromophore) (Fig. 2a). A pronounced decrease in the photoluminescence lifetime of QD570 in the presence of PMs (at QD570 to bR molar ratios of 1:9, 1.4:9, and 2:9) (Fig. 2b) further confirmed the FRET effect (Fig. 3). Although there are numerous published data on the photosensitive parameters of bR in the presence of various substances and under different conditions, our study is the first to demonstrate that QDs cause significant changes in the PM (bR) absorbtion kinetics at 410 nm (Fig. 4a). The average times of the absorption increase are 40 ± 1 s for PM–QD complexes and 105 ± 2 s for PM suspensions; the average times of the absorption decrease are 4.9 ± 0.2 ms and 5.6 ± 0.4 ms, respectively. These changes (Fig. 4a) could be attributed to different rates of the M-intermediate formation and decay via deprotonation and reprotonation of the retinal Schiff base in bR. The effect of QDs on the bR photocycle may be accounted for by several factors. The first one is specific changes (reorganization) in the “microenvironment” of the membrane surface due to small local pH changes caused by adsorption of QD particles. The second one is the changes in the PM membrane potential due to the “field effect” upon QD adsorption at the surface, because QDs have a negative charge (their zeta potential is about −5 mV). This explanation is supported by the well-known changes in the rate of M-intermediate formation and decay upon the adsorption of heavy metal cations on the PM surface and changes in the PM membrane potential or external electric field [18]. In summary, the effects on the bR photocycle kinetic parameters can be explained by changes in the PM structure caused by QD adsorption on the surface; however, the specific mechanism of these changes remains unclear. The average times of the photopotential increase are 20.5 ± 1 s for PM–QD complexes and 84.0 ± 3 s for the PM suspension; the average times of the photopotential decrease are 306 ± 50 ms and 872 ± 35 ms, respectively. The increased rate of the photopotential rise in the case of the PM–QD570 system (Fig. 4b) may be attributed to the acceleration of the M-intermediate formation. These changes are especially pronounced at the stage of photopotential decrease (Fig. 4b), which can be explained as a decay of the M-intermediate caused by protonation of the retinal Schiff base.
4. Conclusion In summary, we have found a pronounced influence of QDs on the PM properties. This has both fundamental implications (for studying the molecular mechanisms of photoreception) and potential practical applications (including the fabrication of bio–nano hybrid materials). The samples of PM–QD570 capable of FRET and characterized by increased rates of M-intermediate formation and decay have been obtained. The effects observed in the PM–QD570 system are significantly stronger compared to combinations of PMs with other QDs synthesized. The pronounced changes in the bR photocycle kinetic parameters may be explained by local reorganization of the membrane structure caused by QD adsorption on the PM surface. The possibility to use the PM–QD570 system as a basis for novel bio–nano hybrid materials with advanced parameters is important in terms of the development of photovoltaic cells and optoelectronic devices. Acknowledgements We are grateful to I.R. Nabiev, the leading scientist of the Laboratory of Nano-Bioengineering (LNBE), for the valuable discussion and to P.S. Samokhvalov, P.S. Linkov, and K. Brazhnik, researchers of LNBE, for the preparation and modification of samples. This study was supported by the Ministry of Education and Science of the Russian Federation (grant no. 11.G34.31.0050). References [1] I. Nabiev, A. Rakovich, A. Sukhanova, E. Lukashev, V. Zagidulin, V. Pachenko, Y.P. Rakovich, J.F. Donegan, A.B. Rubin, A.O. Govorov, Angew. Chem. Int. Ed. 49 (2010) 7217. [2] A. Rakovich, A. Sukhanova, N. Bouchonville, E. Lukashev, V. Oleinikov, M. Artemyev, V. Lesnyak, N. Gaponik, M. Molinari, M. Troyon, Y.P. Rakovich, J.F. Donegan, I. Nabiev, Nano Lett. 10 (2010) 2640. [3] N. Bouchonville, A. Le Cigne, A. Sukhanova, M. Molinari, I. Nabiev, Laser Phys. Lett. 10 (2013) 085901. [4] S.Yu. Zaitsev, D.O. Solovyeva, I. Nabiev, Adv. Colloid Interface Sci. 183–184 (2012) 14. [5] Y.A. Ovchinnikov, E.A. Arystarkhova, N.M. Arzamazova, K.N. Dzhandzhugazyan, R.G. Efremov, I.R. Nabiev, N.N. Modyanov, FEBS Lett. 227 (1988) 235. [6] R.R. Birge, Annu. Rev. Phys. Chem. 41 (1990) 683. [7] K. Sokolov, P. Khodorchenko, A. Petukhov, I. Nabiev, G. Chumanov, T.M. Cotton, Appl. Spectrosc. 47 (1993) 515. [8] N.D. Hampp, Osterhelt in Protein Science Encyclopedia, Wiley Online Library, Berlin, 2008. [9] J.W. Kevin, B.G. Nathan, A.S. Jeffrey, P.K. Mark, R.B. Robert, Trends Biotechnol. 20 (2002) 387.
S.Yu. Zaitsev et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 248–251 [10] S.Yu. Zaitsev, N.M. Kozhevnikov, Yu.O. Barmenkov, M.Yu. Lipovskaya, Photochem. Photobiol. 55 (1992) 851. [11] S.Yu. Zaitsev, Russ. Nanotechnol. 4 (2009) 6–18. [12] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Nat. Mater. 4 (2005) 435. [13] S.Yu. Zaitsev, D.O. Solovyeva, I. Nabiev, Russ. Chem. Rev. 83 (2014) 38–81. [14] A.G. Manoj, K.S. Narayan, Biosens. Bioelectron. 19 (2004) 1067. [15] R. Li, Ch.-M. Li, H. Bao, Q. Bao, Appl. Phys. Lett. 91 (2007) 223901.
251
[16] A.A. Kononenko, E.P. Lukashev, A.V. Maximychev, S.K. Chamorovsky, A.B. Rubin, S.F. Timashev, L.N. Chekulaeva, Biochim. Biophys. Acta 850 (1986) 162. [17] A. Sukhanova, K. Even-Desrumeaux, A. Kisserli, T. Tabary, B. Reveil, J.M. Milot, P. Chames, D. Baty, M. Artemyev, V. Oleinikov, M. Pluot, J.H.M. Cohen, I. Nabiev, Nanomedicine: NBM 8 (2012) 516. [18] A.L. Drachev, L.A. Drachev, A.D. Kaulen, L.V. Khitrina, Eur. J. Biochem. 138 (1984) 349.
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Short Communication
Controlled influence of quantum dots on purple membranes at interfaces Sergei Yu. Zaitsev ∗ , Eugeni P. Lukashev, Daria O. Solovyeva, Alexander A. Chistyakov, Vladimir A. Oleinikov Laboratory of Nano-Bioengineering, National Research Nuclear University “Moscow Engineering Physics Institute”, 115409 Moscow, Russian Federation
a r t i c l e
i n f o
Article history: Received 16 December 2013 Received in revised form 29 January 2014 Accepted 19 February 2014 Available online 28 February 2014 Keywords: Biointerfaces Quantum dots Purple membranes Bionanomaterials
a b s t r a c t The development of bio-sensitized nanofilms engineered from biomembrane components and inorganic nanoparticles is a promising field of colloid and interface science and technologies. Recent nanobioengineering approaches employing quantum dots (QDs) permit the enhancement of the purple membrane (PM) “light-harvesting capacity” compared to native PMs. The influence of QDs on the PM properties, especially the bacteriorhodopsin (bR) photocycle, has been found that has both fundamental (mechanisms of photoreception) and applied implications (including the fabrication of hybrid bionanomaterials). Samples of PM–QD complexes capable of energy transfer and characterized by increased rates of M-intermediate formation and decay have been obtained. The modified bR photocycle kinetic parameters may be explained by changes in the PM interface upon QD adsorption. The increase and decrease in absorption at 410 nm (or photopotential) for PM–QD complexes are, on average, several times more rapid than for PM suspensions or PM dry films. These results provide a strong impetus for the development of nanomaterials with advanced properties. © 2014 Published by Elsevier B.V.
1. Introduction The development and study of bio–nano hybrid systems engineered from membrane proteins and inorganic nanoparticles constitute a fascinating modern field at the interface of chemistry, biology, physics, and nano-biotechnology [1–4]. A number of photosensitive proteins have been explored in terms of sensoring, bioelectronic, and optical applications, but bacteriorhodopsin (bR), a photosensitive membrane protein from purple membranes (PMs) of Halobacterium salinarium, has attracted the most attention [5–9]. bR possesses unique physicochemical properties and has three main molecular functions: photoelectric, photochromic, and proton-transporting [8–11]. Native PMs (without light-harvesting complexes) can utilize only 0.5% of the solar light. Recent nanobioengineering approaches employing quantum dots (QDs) and bR permit the enhancement of the PM “light-harvesting capacity,” thus providing a strong impetus for the development of novel nanomaterials [4]. It is known [1–4,12,13] that illumination of QDs in the region of their absorption causes some specific effects, including Förster resonance energy transfer (FRET) in the presence of an
acceptor, such as a photosensitive protein. This effect in a hybrid bio-nanosystem depends on the properties of the PM–QD system and is used in numerous applications [2–4,11,14]. One of the first PM–QD hybrid systems was studied by Li et al. [15]. Although some of their experimental results were inconclusive, the authors developed a theoretical model explaining how QDs could act as “nanoscaled light sources” (embedded in PMs) to promote the generation of a stationary photocurrent [15]. In general, the main advantage of this system is the possibility of FRET, which improves the bR function in a hybrid material consisting of PMs and QDs [2–4]. However, inorganic nanocrystals, being in direct contact with PMs, may have some effects on bR that are not directly related to FRET. We expected that QDs as additional parts of the composition (which have a specific charge and an intrinsic dipole moment) would significantly influence the bR photocycle. Here, we studied the QD effect on PMs, in particular, on the bR photocycle. This effect may be of both fundamental and applied importance, especially for the development of new bio–nano hybrid materials. 2. Experimental
∗ Corresponding author. Tel.: +7 4953779539; fax: +7 4953774939. E-mail addresses: [email protected], [email protected] (S.Yu. Zaitsev). http://dx.doi.org/10.1016/j.colsurfb.2014.02.033 0927-7765/© 2014 Published by Elsevier B.V.
PMs were isolated from H. salinarum by the standard procedure [3]. The concentration of the PM dispersion was 6 mg/ml as
S.Yu. Zaitsev et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 248–251
249
Fig. 1. Spectra of (1) absorption and (2) fluorescence of the quantum dots used (QD570 ) and (3) the absorption spectrum of the PM suspension.
measured by light absorbance at 570 nm (the molar extinction coefficient was 63,000 M−1 cm−1 ). PMs are generally used more often than bR, because the isolation of bR from PMs considerably decreases both the biochemical and thermodynamic stabilities of the protein. The changes in the PM light absorbance were measured using a laboratory flash-photolysis setup with the probing beam passing a double monochromator after excitation of the sample with a NdYAG laser (532 nm, 8 ns, 15 mJ). Films based on pure PM fragments or their mixtures with QD570 were obtained on a transparent conductive electrode (PET-ITO) by means of electrophoretic deposition (EPD) and measured according to the procedure described earlier [16]. Ni foil was used for the second (top) electrode, which could be tightly pressed against the smooth surface of the oriented PM films. CdSe/ZnS quantum dots emitting at a wavelength of 570 nm were synthesized. These QDs were covered with trioctylphosphine oxide (TOPO) as an amphiphilic ligand for particle stabilization [17]. The TOPO molecules were replaced with poly(ethylene glycol) at the QD surface as described earlier [3,17]. The fluorescence intensity and lifetime of the QDs were measured by means of a FLUOROLOG-3 spectrofluorimeter (Horiba Jobin Yvon) using the TCSPC option. All measurements were carried out in a 100 mM sodium phosphate buffer solution containing 100 mM KCl (pH 7.5). The lifetimes (, obtained from fluorescence and photopotential measurements) were evaluated by multiexponential fits and presented as average lifetimes ( avr ) in all cases. The avr value was calculated as the sum of the products of each value by the respective amplitude divided by the sum of all amplitudes. All standard chemicals of analytical grade were purchased from Sigma–Aldrich.
Fig. 2. (ɑ) The change in the fluorescence intensity of a QD570 solution (1 M) after the addition of a PM suspension: curve 1 (top), QD570 without PMs; curve 11 (bottom), the same QD570 in the presence of PMs at the maximum concentration (about 6 M of bR). (b) Kinetics of the photoluminescence intensity: curve 1, QD570 feed solution ( avr = 18.5 ± 0.5 ns); curve 2, QD570 films ( avr = 7.8 ± 0.3 ns); curve 3, EPD films from a suspension with a QD to BR molar ratio of 2:9 ( avr = 5.7 ± 0.1 ns); curve 4, EPD films from a suspension with a QD to BR molar ratio of 1.4:9 ( avr = 3.7 ± 0.1 ns); curve 5, EPD films from a suspension with a QD to BR molar ratio of 1:9 ( avr = 2.2 ± 0.1 ns).
3. Results and discussion QDs are energy converters that absorb light in a wide range of photon energies (Fig. 1, curve 1) and can fluoresce (Fig. 1, curve 2) and transfer the harvested energy to bR (in PMs) with a high efficiency [2–4] because of a wide overlap between the QD fluorescence (Fig. 1, curve 2) and PM absorption (Fig. 1, curve 3) spectra. The spectral properties of all the QD batches synthesized were studied. The overlap between the bR absorption spectrum and the QD fluorescence spectrum was the largest for QD570 (with fluorescence at 570) (Fig. 1). An important result of the experiments with photoactivation of PM–QD570 mixtures is that the energy harvested
Fig. 3. Förster resonance energy transfer (FRET) in a system of purple membranes (PMs) and quantum dots (QDs), where illumination of the donor (QD) in the region of its absorption leads to FRET to the acceptor (PM), which changes the optical properties of photosensitive membrane proteins.
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S.Yu. Zaitsev et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 248–251
Fig. 4. Kinetics of the laser-induced parameters of the PM mixtures: (a) kinetics of the laser-induced absorbance changes at 410 nm (the M-intermediate absorption peak) in (1) the feed PM suspension and (2) a PM–QD570 suspension with a bR to QD570 molar ratio of 4:1. (b) Kinetics of the laser-induced photovoltage generation in (1) an oriented PM film and (2) a similar film obtained from a PM–QD570 mixture. The amplitude of curve 2 is several times higher compared to the reference sample level, since the addition of negatively charged QDs to the PM suspension significantly reduced the degree of PM fragments orientation in the mixed film. The dotted line shows the actual kinetic trace; the solid line is the result of multiexponential fitting.
by QD570 is transferred to bR by the FRET mechanism with a high efficiency (Fig. 2). This is confirmed by significant changes in the fluorescence intensity of a QD570 solution (∼1 M) after the addition of PM suspensions with different concentrations (from 0.25 to 6 M of the bR chromophore) (Fig. 2a). A pronounced decrease in the photoluminescence lifetime of QD570 in the presence of PMs (at QD570 to bR molar ratios of 1:9, 1.4:9, and 2:9) (Fig. 2b) further confirmed the FRET effect (Fig. 3). Although there are numerous published data on the photosensitive parameters of bR in the presence of various substances and under different conditions, our study is the first to demonstrate that QDs cause significant changes in the PM (bR) absorbtion kinetics at 410 nm (Fig. 4a). The average times of the absorption increase are 40 ± 1 s for PM–QD complexes and 105 ± 2 s for PM suspensions; the average times of the absorption decrease are 4.9 ± 0.2 ms and 5.6 ± 0.4 ms, respectively. These changes (Fig. 4a) could be attributed to different rates of the M-intermediate formation and decay via deprotonation and reprotonation of the retinal Schiff base in bR. The effect of QDs on the bR photocycle may be accounted for by several factors. The first one is specific changes (reorganization) in the “microenvironment” of the membrane surface due to small local pH changes caused by adsorption of QD particles. The second one is the changes in the PM membrane potential due to the “field effect” upon QD adsorption at the surface, because QDs have a negative charge (their zeta potential is about −5 mV). This explanation is supported by the well-known changes in the rate of M-intermediate formation and decay upon the adsorption of heavy metal cations on the PM surface and changes in the PM membrane potential or external electric field [18]. In summary, the effects on the bR photocycle kinetic parameters can be explained by changes in the PM structure caused by QD adsorption on the surface; however, the specific mechanism of these changes remains unclear. The average times of the photopotential increase are 20.5 ± 1 s for PM–QD complexes and 84.0 ± 3 s for the PM suspension; the average times of the photopotential decrease are 306 ± 50 ms and 872 ± 35 ms, respectively. The increased rate of the photopotential rise in the case of the PM–QD570 system (Fig. 4b) may be attributed to the acceleration of the M-intermediate formation. These changes are especially pronounced at the stage of photopotential decrease (Fig. 4b), which can be explained as a decay of the M-intermediate caused by protonation of the retinal Schiff base.
4. Conclusion In summary, we have found a pronounced influence of QDs on the PM properties. This has both fundamental implications (for studying the molecular mechanisms of photoreception) and potential practical applications (including the fabrication of bio–nano hybrid materials). The samples of PM–QD570 capable of FRET and characterized by increased rates of M-intermediate formation and decay have been obtained. The effects observed in the PM–QD570 system are significantly stronger compared to combinations of PMs with other QDs synthesized. The pronounced changes in the bR photocycle kinetic parameters may be explained by local reorganization of the membrane structure caused by QD adsorption on the PM surface. The possibility to use the PM–QD570 system as a basis for novel bio–nano hybrid materials with advanced parameters is important in terms of the development of photovoltaic cells and optoelectronic devices. Acknowledgements We are grateful to I.R. Nabiev, the leading scientist of the Laboratory of Nano-Bioengineering (LNBE), for the valuable discussion and to P.S. Samokhvalov, P.S. Linkov, and K. Brazhnik, researchers of LNBE, for the preparation and modification of samples. This study was supported by the Ministry of Education and Science of the Russian Federation (grant no. 11.G34.31.0050). References [1] I. Nabiev, A. Rakovich, A. Sukhanova, E. Lukashev, V. Zagidulin, V. Pachenko, Y.P. Rakovich, J.F. Donegan, A.B. Rubin, A.O. Govorov, Angew. Chem. Int. Ed. 49 (2010) 7217. [2] A. Rakovich, A. Sukhanova, N. Bouchonville, E. Lukashev, V. Oleinikov, M. Artemyev, V. Lesnyak, N. Gaponik, M. Molinari, M. Troyon, Y.P. Rakovich, J.F. Donegan, I. Nabiev, Nano Lett. 10 (2010) 2640. [3] N. Bouchonville, A. Le Cigne, A. Sukhanova, M. Molinari, I. Nabiev, Laser Phys. Lett. 10 (2013) 085901. [4] S.Yu. Zaitsev, D.O. Solovyeva, I. Nabiev, Adv. Colloid Interface Sci. 183–184 (2012) 14. [5] Y.A. Ovchinnikov, E.A. Arystarkhova, N.M. Arzamazova, K.N. Dzhandzhugazyan, R.G. Efremov, I.R. Nabiev, N.N. Modyanov, FEBS Lett. 227 (1988) 235. [6] R.R. Birge, Annu. Rev. Phys. Chem. 41 (1990) 683. [7] K. Sokolov, P. Khodorchenko, A. Petukhov, I. Nabiev, G. Chumanov, T.M. Cotton, Appl. Spectrosc. 47 (1993) 515. [8] N.D. Hampp, Osterhelt in Protein Science Encyclopedia, Wiley Online Library, Berlin, 2008. [9] J.W. Kevin, B.G. Nathan, A.S. Jeffrey, P.K. Mark, R.B. Robert, Trends Biotechnol. 20 (2002) 387.
S.Yu. Zaitsev et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 248–251 [10] S.Yu. Zaitsev, N.M. Kozhevnikov, Yu.O. Barmenkov, M.Yu. Lipovskaya, Photochem. Photobiol. 55 (1992) 851. [11] S.Yu. Zaitsev, Russ. Nanotechnol. 4 (2009) 6–18. [12] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Nat. Mater. 4 (2005) 435. [13] S.Yu. Zaitsev, D.O. Solovyeva, I. Nabiev, Russ. Chem. Rev. 83 (2014) 38–81. [14] A.G. Manoj, K.S. Narayan, Biosens. Bioelectron. 19 (2004) 1067. [15] R. Li, Ch.-M. Li, H. Bao, Q. Bao, Appl. Phys. Lett. 91 (2007) 223901.
251
[16] A.A. Kononenko, E.P. Lukashev, A.V. Maximychev, S.K. Chamorovsky, A.B. Rubin, S.F. Timashev, L.N. Chekulaeva, Biochim. Biophys. Acta 850 (1986) 162. [17] A. Sukhanova, K. Even-Desrumeaux, A. Kisserli, T. Tabary, B. Reveil, J.M. Milot, P. Chames, D. Baty, M. Artemyev, V. Oleinikov, M. Pluot, J.H.M. Cohen, I. Nabiev, Nanomedicine: NBM 8 (2012) 516. [18] A.L. Drachev, L.A. Drachev, A.D. Kaulen, L.V. Khitrina, Eur. J. Biochem. 138 (1984) 349.
