Accepted Manuscript Silver(I) Complexes of 2,4-dihydroxybenzaldehyde-Amino Acid Schiff Bases - Novel Noncompetitive α-Glucosidase Inhibitors Jingwei Zheng, Lin Ma PII: DOI: Reference:

S0960-894X(15)00294-2 http://dx.doi.org/10.1016/j.bmcl.2015.03.078 BMCL 22566

To appear in:

Bioorganic & Medicinal Chemistry Letters

Received Date: Revised Date: Accepted Date:

8 January 2015 3 March 2015 24 March 2015

Please cite this article as: Zheng, J., Ma, L., Silver(I) Complexes of 2,4-dihydroxybenzaldehyde-Amino Acid Schiff Bases - Novel Noncompetitive α-Glucosidase Inhibitors, Bioorganic & Medicinal Chemistry Letters (2015), doi: http://dx.doi.org/10.1016/j.bmcl.2015.03.078

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Silver(I) Complexes of 2,4-dihydroxybenzaldehyde-Amino Acid Schiff Bases - Novel Noncompetitive α-Glucosidase Inhibitors ＊

Jingwei Zheng , Lin Ma School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PRC

＊

Corresponding author. Jingwei Zheng: tel: +86 (020) 8411 3690 E-mail: [email protected] 1

Abstract A series of silver(I) complexes of 2,4-dihydroxybenzaldehyde-amino acid Schiff bases were designed and tested for α-glucosidase inhibition. Our results indicate that all the silver complexes (4a-18a) possessed strong inhibitory activity at µmol· L-1 level, especially glutamine (12a) and histidine (18a) Schiff base silver(I) complexes exhibited an IC50 value of less than 0.01 µmol· L-1. This series of compounds exhibited noncompetitive inhibition characteristics in kinetic studies. In addition, we investigated the mechanism of inhibition and the structure-activity relationships of the amino acid Schiff base silver complexes. Our results reveal that Schiff base silver complexes may be explored for their therapeutic potential as alternatives of α-glucosidase inhibitors.

Keywords：α-glucosidase; amino acid Schiff bases; silver(I) complexes; noncompetitive inhibitors

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Alpha-glucosidase (EC 3.2.1.20, α-D-glucoside glucohydrolases) is significant for forming the oligosaccharide moiety of glycoproteins, metabolizing polysaccharides and glycoconjugates, and for other essential biological recognition processes. Recent studies have shown that controlling or regulating α-glucosidase activity can prevent diseases caused by metabolic disorders, immune responses, differentiation of nerve cells, tumor metastases, and viral infections1-4, such as diabetes mellitus, cancer, and HIV5-9. However, most of the well established glycosidase inhibitors are sugar mimics. The action of OAD (Oral Antidiabetic Drugs) for noninsulin-dependent diabetes mellitus (NIDDM, Type 2 diabetes mellitus) is based on the inhibition of α-glucosidase activity. Preliminary studies have demonstrated that aminochalcones, sulfonamide chalcones,10 and phenylsulfonamide chalcone derivatives11 , representing the chalcones, are strong inhibitors of α-amylase and β-amylase. Flavones that possess the chalcone substructure that is incorporated into the benzopyran backbone, like Baicalein12, Oroxylin A12, Chrysin13 and chromenone derivatives14, also manifest an α-glucosidase inhibitory activity. Xanthones and its derivatives15 with non-coplanar and flexible structures have been shown to exhibit potent inhibitory activities. Stilbenoids, for example resveratrol 116, the major active compound isolated from Syagrus romanzoffiana, were shown to be potent α-glucosidase inhibitors. In addition, studies on polyhydroxy Schiff bases 217, where the C=C bond is substituted for the C=N bond, demonstrated that they structurally constrain analogs of stilbenoids and possess a high inhibitory potential against α-glucosidase. In fact, all the compounds mentioned above have a comparable structure and studies on the inhibitory mechanism indicate that they behave as noncompetitive inhibitors. Previous reports found transition metals and their complexes strongly inhibited α- glucosidase activity, and their effects were approximately 1000 times higher than Acarbose (a type 2 diabetes

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mellitus drug)18. Some drugs have greater activity when complexed with transition metals19 . Thus, Schiff base transition metal complexes may be an uptapped resource of α-glucosidase inhibitors. Since amino acid-based Schiff bases are very effective metal chelators containing >C=O, >C=N groups and S, O, N atoms with lone pair electrons, the synthesis of Schiff base 3 silver(I) complexes were derived from amino acids. Refer to the coordination mode of Azo-linked Schiff base Cu(II)20, we hypothesized the common structure of complexes is 3. Chemical elements Cu and Ag are in the same group of the periodic table. Since the Cu(II) in the reference was quadridentate, we suspected the Ag(I) was terdentate. This series of silver(I) complexes of 2,4-dihydroxybenzaldehyde-amino acid Schiff bases that have a structure similar to flavones and xanthones (Figure 1.), act as a novel class of highly specific α-glucosidase inhibitors that can be obtained with economically simple methods. Moreover, the additional reactive functional groups on the amino acid side chain may lead to unexpected binding with the enzyme21. Here, we evaluated the inhibitory effect on α-glucosidase compounds and investigated their enzyme kinetics. The structure-activity relationship was also discussed. To our knowledge, our study is the first to analyze the inhibitory effect of silver complexes of salicylaldehyde-amino acid Schiff bases. In this paper, we applied mechanochemistry (grinding) to synthesize amino acid Schiff bases and their corresponding silver complexes (Table 1) in the absence of solvent under microwave irradiation (Scheme 1). The in vitro inhibitory activity of the target compounds were evaluated using the Bio-Rad 680 microplate reader at 405 nm for S. cerevisiae α- glucosidase, instead of the UV-VIS spectrophotometer. The increment of absorption at 405nm is based on the hydrolysis of PNPG.

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Several metal complexes of 2,4-dihydroxybenzaldehyde-histidine Schiff base, including Bi3+, Ce3+, Ni2+, Sn4+, Al3+, Zn2+, Mn2+ , Fe2+, and Ag+ , were screened for inhibitory activities in vitro before silver was chosen as the target coordinated metal ion. The silver complexes demonstrated high activity, while others displayed IC50 values that were greater than 50 µmol· L-1. As shown in Table 1, most of the silver complexes of the 2,4-dihydroxybenzaldehyde-amino acid Schiff bases (4a-18a) demonstrated potent α-glucosidase inhibitory activity, with IC50 values ranging from 0.00922 µmol· L-1 to 0.246 µmol· L-1. However, most of the amino acid Schiff bases (4-18) showed little inhibition, except tyrosine Schiff base 16. We assume that the hydroxyl group attached to the benzene ring in the tyrosine residues correspond with the hydroxyl groups in the sugar rings of the substrate, forming hydrogen bonds that strengthen the binding between the inhibitor and enzyme. All in all, the silver complexes are more reactive with respect to their corresponding metal-free Schiff base amino acid ligands. On comparing the silver complexes of the 2,4-dihydroxybenzaldehyde amino acid Schiff bases (4a-18a), we observed that the aliphatic amino acid silver complexes (10a, 11a, 13a, 15a) showed better activities. Our experimental results suggested that 12a and 18a showed the best activity with IC50 values that were less than 0.01 µmol· L-1 . A number of amines were reported previously as glycosidase inhibitors, such as Tris (Ki = 1.024 mmol/L) and especially alkaline analogues, due to the hydrogen bonds existing between the amino groups and the active site. Previous experimental results have projected the crucial role of the NH-group in the binding of sugar/non-sugar derivatives to the active site22. In addition, the reactant silver acetate 19 and the reference silver nitrate 20 displayed inhibitory activities with IC50 values of 0.0273 and 0.0197 µmol· L-1, respectively, suggesting that the inhibition of Schiff base metal complexes depend not

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only on the ions but also the ligand. Yutaka, Y. and coworkers18 previously observed the in vitro and in vivo inhibitory effects of insulinomimetic metal ions on α-glucosidase activity, suggesting one of the action mechanisms of the anti-diabetic metal ions and complexes is related to α-glucosidase inhibition. Metal ions may take part in transferring electrons, atoms, or functional groups during enzyme-catalyzed reactions in order to control the reactions that occur in stages. Moreover, the positive ionic charge in the metal can shield or neutralize the negative charge in some part of the substrate, or change the charge distribution of the enzyme. They are bound to the active sites of the enzyme or the altered regions to break the conformational integrity of the enzyme, which affects the catalytic activity. Therefore, we conclude that the silver ion may have the most profound effect on the activity of the enzyme. Among the tested compounds, compounds 7a, 16a and 18a were chosen to characterize the type of inhibition. As mentioned above, 18a showed very strong inhibitory activity with hydroxy, nitro, and bromine substituent groups, respectively but they are all histidine Schiff bases. Thus, compounds 7a and 16a with L-Try and L-Tyr residues, were examined as a reference. We hypothesized that compounds with similar structures should have the same mechanism of α-glucosidase inhibition. The Lineweaver-Burk plot of α-glucosidase kinetics is shown in Figure 2. The linear regression and the extrapolation of data gave a series of lines crossing the X axis, with increasing concentration of the inhibitor. The kinetic result illustrates that the mechanism of α-glucosidase inhibition of this series of compounds was noncompetitive. These results indicate that inhibitors may reduce the catalytic activity by modulating the conformation of the enzyme, and the binding

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site is distinct from the active domains of the enzyme. This is an expected outcome because as briefly mentioned above, silver(I) complexes have a similar structure compared to flavones and xanthones, and exhibit potent inhibitory activities towards α-glucosidase via a noncompetitive mechanism. The metal ion may act as a noncompetitive inhibitor against yeast α-glucosidase, as reported previously18, and the mechanism of α-glucosidase inhibition of this series of compounds was also noncompetitive. We hypothesized that since the silver ion did not affect the affinity between enzymes and substrates, the enzyme and the substrate, p-nitrophenol-a-D-glucopyranoside (PNPG), form the intermediate first followed by the silver ion in the compound that combines with the substrate in the previously formed intermediate. The silver ion does not bind directly with the enzyme as the substrate acts as a ligand that complexes with the metal (Enzyme-Substrate-Metal-Ligand pattern). It is worth mentioning that the silver ion can also be combined with the -SH groups of cysteine residues in the active site of an enzyme. It is possible that there is another combination between silver ions and the active sites of enzymes in other types of complexes. Another possible combination is shown in Figure 3. We illustrated the predicted binding mode by using compound 18a as an example. Previous research has shown that from the sequence alignment of templates and target proteins, Asp68, Asp214, Phe177 and Phe300 are the key active site residues conserved in α-glucosidase22-23, and some other important amino acids Arg212, Asp349, Ser244 and His348 are also present around the active site. We speculate that the oxygen atoms that coordinate with the silver atoms are offered by both the carboxyl and the hydroxyl groups of the amino acid Asp349, and are not offered by the amino acid Schiff base. What is more,

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the amino acid residue and the hydroxyl or other groups in the benzene derivatives correspond to the area near the active site of α-glucosidase forming hydrogen bonds or salt bonds. We infer that the amino acid residues near the active site, such as Arg212, Asp349, Ser244, and His348, may have π-stacking, hydrophobic effects with compounds and may bind selectively with the -NH, -OH groups in the compounds due to structural flexibility. Forming hydrogen bonds with hydroxyl groups may have a crucial role both in catalytic and in substrate binding. These interactions can be predominant factors that modulate the binding mode and the inhibitory ability. In addition, when compounds interact with enzymes, the silver ion could possibly become penta-coordinated. Although the metal ion shows a strong α-glucosidase inhibition activity, the inhibitory effect does not have a good specificity. Furthermore, high concentrations of metal ions are toxic. However, metal complexes have many advantages, such as high selective combination with the active site of the enzyme and modulating the conformation of the enzyme by binding the residue near the active site of the enzyme. In summary, a new class of salicylaldehyde-amino acid Schiff bases (4-18) and the corresponding silver(I) complexes (4a-18a) were synthesized and evaluated as α-glucosidase inhibitors. These compounds were prepared in a simple procedure with a good yield from minimally expensive and commercially available materials and apparatus. The results demonstrate that all the silver(I) complexes exhibit good inhibitory activities, with IC50 values less than 0.5 µmol· L-1. Compounds 12a and 18a were identified as the main compounds showing significant α-glucosidase inhibition, with an IC50 value that is less than 0.01 µmol· L-1. Moreover, our experiment indicates that the silver(I) ion and the -NH group seemed to be crucial for inhibitory activity. In addition, the inhibition kinetics analyzed by Lineweaver-Burk plots revealed that this

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series of silver complexes showed noncompetitive inhibition. To date, there has been no previous report on the inhibitory effect of silver complexes of 2,4-hydroxybenzaldehyde-amino acid Schiff bases. Due to their relative important inhibitory activity and hydrophobicity, they may serve as model compounds for research studies on the design and the development of drugs to treat diabetes mellitus, hyperglycemia, cancer, viral infections, and HIV infections.

References 1. Zhu, Y. P.; Yin, L. J.; Cheng, Y. Q.; Yamaki, K.; Mon, Y.; Su, Y. C.; Li, L. T. Food Chem. 2008, 109, 737. 2. Naowaboot, J.; Pannangpetch, P.; Kukongviriyapan, V.; Kukongviriyapan, U.; Nakmareong, S.; Itharat, A. Nur. Res. 2009, 29, 602. 3. Raju, B. C.; Ashok, A. K.; Kumar, J. A.; Ali, A. Z.; Agawane, S. B.; Saidachary, G.; Madhusudana, K. Bioorg. Med. Chem. 2010, 18, 358. 4. Yao, Y.; Sang, W.; Zhou, M.; Ren, G. J. Agric. Food Chem. 2010, 58, 770. 5. Bertozzi, C. R.; Kiessling, L. L. Chemical glycobiology. Science 2001, 291, 2357. 6. Horii, S.; Fukase, H. J.Med.Chem. 1986, 29, 1038. 7. Floris, A. L.; Peter, L. L.; Reinier, P.; Eloy, H. L.; Guy, E. R.; Chris, W. Diabetes Care. 2005, 28, 154. 8. Fernandes, B.; Sagman, U.; Auger,N.; Demetrio, M.; Dennis, J.W. Cancer Res. 1991, 51, 718. 9. Seiichiro, O.; Ayako, M.;Takashi, O.; Hideya, Y.; Hironobu, H. Eur. J. Org. Chem. 2001, 5, 967. 10. Seo, W. D.; Kim, J. H.; Kang, J. E.; Ryu, H. W.; Curtis-Long M. J.; Lee, H. S.; Yang, M. S.; Park, K. H. Bioorg. Med. Chem. Lett. 2005, 15, 5514.

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11. Wang, S. J.; Yan, J. F.; Wang, X. Y.; Yang, Z.; Lin, F. W.; Zhang, T. J. Eur. J. Med. Chem. 2010, 45, 1250. 12. Gao. H.; Kawabata. J. Bioorg. Med. Chem. 2005, 13, 1661. 13. Schulze, M. B.; Liu, S.; Rimm, E. B.; Manson, J. E.; Willett, W. C.; Hu, F. B. Am. J. Clin. Nutr. 2004, 80, 348. 14. Raju, B. C.; Tiwari, A. K.; Kumar, J. A.; Ali, A. Z.; Agawane, S. B.; Saidachary, G.; Madhusudana, K. Bioorg. Med. Chem. 2010, 18, 358. 15. Li, G. L.; He, J. Y.; Zhang, A. Q.; Wan, Y. Q.; Wang, B.; Chen, W. H. Eur. J. Med. Chem. 2011, 46, 4050. 16. Kerem, Z.; Bilkis, I,; Flasishman, M.A.; Sivan, L. J. Agric. Food Chem. 2006, 54, 1243. 17. Zheng, T.; Huang, S. M.; Zhou, B.; Liang, X. F.; Huang, M. X.; Yao D. S.; Yan, S. J.; Ma, L. Chin. J. Med. Chem. 2011, 21, 370. 18. Yutaka, Y.; Ryoko, H.; Hiroyuki, Y.; Hiromu, S. Biochimie 2009, 91, 1339. 19. You, Z. L.; Shi, D. -H.; Xu, C.; Zhang, Q.; Zhu, H. L. Eur. J. Org. Chem. 2008, 43, 862. 20. Moamen, S. R.; Ibrahim, M. E. D.; Hassan, K. I.; Samir, E. G. Spectrochimica Acta A. 2006, 65, 1208-1220. 21. Bibhesh, K. S.; Hemant K. R.; Anant P. Spectrochimica Acta. 2012, 94, 143. 22. Bharatham, K.; Bharatham, N.; Park, K. H.; Lee, K. W. J. Mol. Graph. Model. 2008, 26, 1202. 23. Garlapati, R.; Pottabathini, N.; Gurram, V.; Kasani, K. S.; Gundla, R.; Thulluri, C.; Machiraju, K. S.; Gundla, R.; Thulluri, C.; Machiraju, P.K.; Chaudhary, A. B.; Addepally, U.; Dayam, R.; Chunduri, V. R.; Patro, B. Org. Biomol. Chem. 2013, 11 ,4778. 24. Spectroscopic Data: 6a, Gray-green powder. IR (KBr) σ/cm-1 : 3433.29 (ν-OH), 1631.37 (νC=N), 1582.23 (νasCOO), 1479.37 (νAr-C=C), 1354.20 (δsCOO), 1236.05 (δAr- C-H), 1174.51 (δAr-C-H), 10

1174.51 (δAr-OH), 843.89 (τAr-C-H), 790.65 (τAr-C-H), 538.42 (νAg-N), 473.05 (νAg-O). 1H-NMR (DMSO-d6, 300MHz) δ: 9.88 (s, 1H, OH), 8.31 (1s, 1H, N=CH), 7.53 -7.45 (d, 1H, Ar-H), 6.39 6.32 (d, 1H, Ar-H), 6.36 (s, 1H, Ar-H), 2.43 - 2.39 (m, 1H, CH), 2.10 - 1.96 (m, 4H, 2-CH2), 1.90 (s, 3H, -CH3). 11a, Black powder. IR (KBr) σ/cm-1: 3395.93(ν-OH), 2957.47 (ν-CH), 2870.10 (ν-CH), 1633.66 (νC=N), 1592.86 (νasCOO), 1481.07 (νAr-C=C), 1362.17 (δsCOO), 1312.8, 1278.05 (δAr-C-H), 1195.09 (δAr-OH), 1122.04 (δAr-C-H), 846.37 (τAr-C-H), 532.11 (νAg-N), 485.66 (νAg-O). 1 H-NMR (DMSO-d6 , 300MHz) δ: 9.88 (s, 1H, OH), 8.37 (s, 1H, N=CH), 7.55 - 7.45 (m, 1H, Ar-H), 7.17 - 7.08 (m, 1H, Ar-H), 6.29 (s, 1H, Ar-H), 3.94 - 3.83 (m, 1H, -CH), 2.70 - 2.72 (m, 2H, -CH2), 2.26 - 2.24 (m, 1H, -CH), 1.84 - 1.43 (m, 6H, 2-CH3) 25. By making comparison between the 1H NMR data and the most important IR spectral bounds of some more ligands and its Ag(I) complexes the mode of coordination was clarified (Table 2-3) but could not fully identify of the prepared complexes. 1H NMR spectrum of the Schiff base ligand showed the peaks at around 13.77 ppm is attributed to the OH-proton of carboxylic acid. The disappearance of this peak noted for the silver complexes confirms the loss of the -COOH proton of amino acid moiety due to complexion. Also we could observed that there was only one signal of 4-OH-proton in the benzene ring at 9.88 ppm. The azomethine proton -CH=N signals in the spectra of the silver complexes are shifted to high field (around 8.30 ppm) compared to the free ligands (around 8.00 ppm), suggesting shielding of azomethine group due to the coordination with metal ion. Meanwhile, the peaks attributed to aromatic protons had similar variation trend. The comparison of the IR spectra of the complexes with the above Schiff bases indicates the coordination of the imine linkage nitrogen in chelation with the metal ion. The involvement of nitrogen to the metal ion could be expected to withdrawing the electrons causing a shift in the

ν(C=N) group. Moreover, the strong absorptions at 1580-1610cm-1 and 1450-1510cm-1 were observed for νas(COO) and δs(COO). These bands were shifted by certain pattern, which reveals the organic ligand is involved in the coordination through the carboxyl group. The separation between anti-symmetric and symmetric stretching is ~300cm-1, suggesting the covalent nature of metal oxygen bond in complexes and monodenticity of carboxylate. The azo-linked Schiff base ligands showed bands around 1240-1230 cm-1 assigned of the metal complexes, the C-O band shifted to the lower region around 1200cm-1. One more further support evidence was the appearance of new bands in the spectra of all metal complexes appearing in the low frequency regions at below 650 cm-1 and 500 cm-1 characteristic to ν(Ag-N) and ν(Ag-O) stretching vibrations respectively, that are not observed in the spectra of free ligands. The stretching vibrations of -OH, -C=N, -COO and Ar-C-O bonds of complexes are different because the backbones are influenced by the silver ion and the structure of amino acid residue. 25. The general procedure to synthesize amino acid schiff bases 4-18: 1 mmol amino acid, aldehydes and KOH were grinded until finely blended in an agate mortar. Deprotonation of ligands becomes easier for the addition of KOH. The reactants were then placed in a microwave oven (500–800 W) for 5 min (grind 15 s per min). The reaction products were obtained, recrystallized, filtered under reduced pressure, washed with ethanol and finally dried over by the infrared light, yielding 70-90%. 26. The general procedure to synthesize silver(I) complexes 4a-18a: the newly synthesized schiff base (0.1mmol) and silver acetate (0.1mmol) were mixed also in an agate mortar. 2- 3 drops of ethyl alcohol were added during the grinding. The reactants were then placed in a microwave oven (800 W) for 3 min (grind 30 s per min). The target complexes were obtained, recrystallized,

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filtered under reduced pressure, washed with 30%- 50% aqueous ethanolic solution and finally dried over by the infrared light, yielding 60-80%. 27. The enzyme and the substrate solution was prepared by dissolving Saccharomyces cerevisiae α-glucosidase in 0.01M potassium phosphate buffer (pH 7). Diluted enzyme solution (10µL), test samples (0.016-2µL, in DMSO) and buffer solution (90µL) were mixed in each well of a 96-well microtiter plate. After pre-incubated for 20min at 37℃ , PNPG (10µL, 1.5mg/mL) was added to start the enzymatic reaction measured by a microtiter plate reader immediately. The increment of absorption at 405nm is based on the hydrolysis of PNPG. Controls without enzyme or without substrate were included. The resveratrol was used as reference and averages of three replicates were presented. The inhibition percentage (%) was calculated by the equation: [Abssample/Absblank]×100(%), where Absblank represents the absorbance of the blank with the same volume DMSO.

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Legends Table 1. Reaction of Silver Complexes of Salicylaldehyde-Amino Acid Schiff Bases Scheme 1. Synthetic route for compounds 4-26 and 4a-26a Figure 1. Schematic diagram depicting the procedure for the design of the target compounds Figure 2. Lineweaver-Burk plot analyses of the inhibition kinetics of α-glucosidase by compounds (A) 7a, (B) 16a, (C) 18a

Figure 3. Predicted binding mode of 20a - 2D representation. Hydrogen bonding, hydrophobic, or other interactions are shown in dotted or bent lines.

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Figure 1. Schematic diagram depicting the procedure for the design of the target compounds O O O O O

Flavones

Xanthones

Chalcones

HO

HO OH

R

HO N

HO

HO 1

N

OH HO

2

14

O 3

Ag

O O

Figure 2. Lineweaver-Burk plot analyses of the inhibition kinetics of α-glucosidase by compounds (A) 7a, (B) 16a, (C) 18a

A Compound 7a

15

B Compound 16a

16

C Compound 18a

17

Figure 3. Predicted binding mode of 18a - 2D representation. Hydrogen bonding, hydrophobic, or other interactions are shown in dotted or bent lines. (Dotted lines represent hydrogen bond, Solid lines represent π-stacking, hydrophobic effects) O H N

O

H

N

HO Ser244

O

HO

N H

O

N

Asp68

O

His348

H2N

N O H

H2N N

H N

CH2

NH2

NH2

Ag

H O

O

O

OH

NH2

NH2

Phe177

O

Arg212

18a O

Asp349

O

HO

H2N

O OH

Asp214 H2N

H Phe300

O

18

OH

NH2 OH

Scheme 1. Synthetic route for compounds 4-18 and 4a-18a First step: Synthesis of Schiff Base Amino Acid Ligands

Second step: Synthesis of the Silver(I) Complexes

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Table 1. Reaction and the IC50 Values of Silver Complexes of 2,4-dihydroxybenzaldehyde-Amino Acid Schiff Bases and the Reference Inhibitors

IC50 a (µmol· L-1)

No.

IC50 a (µmol· L-1)

L-Ser

NIb

4a

0.121

CH2SH

L-Cys

NIb

5a

0.246

CH2CH2SCH3

L-Met

NIb

6a

0.0221

L-Try

NIb

7a

0.179

8

L-Phe

NIb

8a

0.0815

9

L-Asp

NIb

9a

0.0163

Gly

NIb

10a

0.0168

11

L-Leu

NIb

11a

0.0202

12

L-Gln

NIb

12a

0.00973

13

L-Val

NIb

13a

0.0455

14

L-Ile

NIb

14a

0.138

D-Ala

2.16

15a

0.0769

L-Tyr

0.0391

16a

0.132

No.

R

4

CH2OH

5 6

CH2 7

HN

10

15

16

H

CH3

20

17

L-Arg

NIb

17a

0.127

L-His

NIb

18a

0.00922

CH2 NH 18

N

19

Silver acetate

0.0273

20

Silver nitrate

0.0197

1

resveratrol

12.7

a

IC50 values: the concentration of the inhibitor required to produce 50% inhibition of α-glucosidase

b

NI, no inhibition. IC50 > 50 µM, when tested at a maximum concentration of 1000 µM and produced

no inhibitory activity.

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Table 2. 1H NMR data for the Salicylaldehyde-Amino acid Schiff Base Ligands and Their Complexes (ppm) Compound 6 6a 11 11a 13 13a 14 14a

-COOH

-OH

N=CH

Ar-H

Ar-H

Ar-H

13.81 (s) — 13.77 (s) — 13.74 (s) — 13.77 (s) —

— 9.88 (s) — 9.88 (s) — 9.89 (s) — 9.89 (s)

8.04 (s) 8.31 (s) 8.04 (s) 8.37 (s) 7.95 (s) 8.27 (s) 7.94 (s) 8.29 (s)

7.00 - 6.97 (d) 7.53 - 7.45 (m) 7.00 - 6.93 (d) 7.55 - 7.45 (m) 7.02 - 6.89 (d) 7.55 - 7.44 (d) 7.11 - 6.83 (d) 7.56 - 7.44 (d)

6.04 - 6.01 (d) 6.39 - 6.32 (m) 6.06 - 5.97 (d) 7.17 - 7.08 (m) 6.05 - 5.94 (d) 6.44 - 6.32 (d) 6.05 - 5.94 (d) 6.46 - 6.32 (d)

5.91 (s) 6.36 (s) 5.91 (s) 6.29 (s) 5.90 (s) 6.31 (s) 5.88 (s) 6.29 (s)

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Table 3. Infrared Spectral Data for the Salicylaldehyde-Amino Acid Schiff Base Ligands and Their Complexes (cm-1) No. 6 6a 11 11a 13 13a 14 14a

ν (OH)

ν(C=N)

νas(COO)

ν(C=C)

δs(COO)

ν(Ar- C-O)

ν(Ag-N)

ν (Ag-O)

3423.09 3433.29

1630.17 1631.37

1589.25 1582.23

1501.70 1479.37

1351.49 1354.20

1242.03 1174.51

— 538.42

— 473.05

3428.66 3395.95 3412.57 3428.82 3427.55 3428.90

1632.21 1633.66 1622.25 1633.19 1632.76 1633.34

1596.68 1592.86 1607.28 1602.98 1596.05 1585.96

1501.46 1481.07 1503.67 1482.27 1503.67 1452.95

1357.17 1362.17 1355.80 1360.97 1348.36 1452.95

1242.03 1195.09 1230.07 1176.16 1234.16 1177.90

— 532.11

— 485.66

— 549.94 — 648.96

— 478.03 — 481.33

23

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