H NMR Spectroscopy of the Adsorption and Decomposition of Glycine

Article pubs.acs.org/JPCB

Study by Use of 1H NMR Spectroscopy of the Adsorption and Decomposition of Glycine, Leucine, and Derivatives in TiO2 Photocatalysis Atsuko Y. Nosaka,* Goro Tanaka, and Yoshio Nosaka* Department of Materials Science and Technology, Nagaoka University of Technology, Nagaoka 940-2188, Japan ABSTRACT: The photocatalytic decomposition and adsorption of glycine (Gly), Gly-Gly, and Gly-Gly-Gly, and leucine (Leu), Leu-Gly, Gly-Leu, and Leu-Gly-Gly, in TiO2 (100% anatase crystal form) aqueous suspension were investigated by 1H NMR spectroscopy. The side chain of Leu, the carboxylic group, and the peptide bond were recognized as the adsorptive sites of the peptides on the surface of TiO2. For Gly-Leu and Leu-Gly-Gly, the photocatalytic decomposition that took place under UV irradiation resulted from the preferable adsorption of the hydrophobic side chain of Leu on the TiO2 surface, while for Gly-Gly and Gly-Gly-Gly, the photodecomposition proceeded by weak adsorption of the peptide bonds on the surface of TiO2.

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INTRODUCTION Among the various semiconductor materials, titanium dioxide is one of the most widely studied metal oxides. Organic molecules incorporated into TiO2 photocatalytic systems are known to be decomposed by photogenerated active oxygen species.1−4 TiO2 is widely applied not only to the environmental cleanup5 but also to solar cells,6 antifogging, and self-cleaning of mirrors and glasses.7 The application to biological fields such as the antibacterial effect and medical treatments for diseases including cancer has been also extensively proceeding.8−11 It is believed that the active oxygen species that generate on the photocatalysts such as H2O2, OH radicals, and singlet oxygen are involved in the attack to bring about extinction of several kinds of viruses and bacteria. However, the mechanism underlying the photobiological activity of TiO2 is not yet well understood. One of the important factors affecting the photocatalysis is the adsorption amount and modes of reactant molecules on the TiO2 surface.2,12 Because the photocatalytic process is expected to occur at the interface between the TiO2 semiconductor and the liquid medium, the interface between protein molecules and inorganic materials has recently received much attention. Besides the photocatalysis, the adhesion of proteins to TiO2 is also of essential interest for tissue growth on implants.13 Adsorption of amino acids on different titanium dioxide surfaces plays a key role in the biocompatibility of titanium-based implant materials, which are passivated by oxide layers.1 Proteins and peptides are composed of various kinds of amino acids. For the proper understanding of the adsorptive and photocatalytic interactions between the TiO2 surface and proteins/peptides, fundamental knowledge of the adsorption and photocatalytic reactivity of individual constituent amino acids would be inevitable. Although some efforts have been © 2014 American Chemical Society

devoted to the studies of amino acids on metal single-crystal surfaces, even for the simplest amino acid glycine (2aminoacetic acid), the adsorbability on the TiO2 surface is still controversial.14−20 The apparent discrepancy of the adsorbability on the TiO2 surfaces seems to arise from the difference of the surface characteristics of the individual TiO2’s employed. The surface of titanium oxide is amphiphilic, which consists of hydrophobic and hydrophilic parts.21 The hydrophilic parts involve two kinds of hydroxyl group, that is, the acidic bridged hydroxyl group and the basic terminal hydroxyl group. Both hydrophobic and hydrophilic parts can be adsorptive and/or photocatalytic active sites, depending on the kinds of titanium dioxides, which are characterized by different particle size, surface area, and crystal forms such as anatase, rutile, and brookite. Previously, we have investigated the photocatalytic decomposition of acetic acid and benzoic acid for several TiO2’s prepared in different ways. For several TiO2’s of anatase crystal forms, the active sites were found to be hydrophobic parts of the surface, while for the others, which contain rutile crystal forms, the photocatalytic active sites were assigned to the hydrophilic parts.22 Thus, each photocatalyst with different characteristic surfaces can show different adsorbability and photocatalytic activity.23−26 In the present study, we employed the simplest amino acid glycine, whose adsorbability on the TiO2 surface is still controversial, and its homopeptides (Gly-Gly and Gly-GlyGly) to ensure the adsorption and decomposition sites of peptides. As TiO2 photocatalysts, we employed ST-01 (100% anatase crystal form, Ishihara Sangyo Ltd.). As reported Received: April 10, 2014 Revised: May 24, 2014 Published: June 16, 2014 7561

dx.doi.org/10.1021/jp503546z | J. Phys. Chem. B 2014, 118, 7561−7567

The Journal of Physical Chemistry B

Article

previously, the surface hydroxyl groups of several TiO2’s in 100% of the anatase crystal form are easily removed upon calcinations at 973 K without changing the crystal form, and a highly hydrophobic surface is created. For the case of ST-01, the removed hydroxyl groups did not recover even after 1 month.22 The decomposition of benzoic acid, which consists of an aromatic ring and a carboxyl group, was significantly enhanced on the calcinated surface as compared to that for acetic acid, although the surface area was substantially decreased. However, the decomposition rate was retarded with the recovery of the hydroxyl groups. These results suggested that the photocatalytic active site of ST-01 should be the hydrophobic part.22 In the present study, by exploiting the characteristic surface properties of the calcined ST-01 (designated as HT-TiO2 hereafter), we attempted to elucidate the adsorption and decomposition sites for Gly, Gly-Gly (2-[(2-aminoacetyl)amino]acetic acid), and Gly-Gly-Gly ([(aminoacetyl)amino]acetyl)amino)acetic acid) by performing NMR measurements with HT-TiO2 and comparing the results with those obtained for noncalcined ST-01. As stated above, the photocatalytic active site of ST-01 was assigned to the hydrophobic part. For these Gly-related peptides, the side chain of the glycine residue is neutral, and the peptide bond is hydrophobic; if any hydrophobic side chain is incorporated into these simple peptide sequences, the hydrophobic side chain might compete with the peptide bond in terms of the adsorption and photodecomposition on the hydrophobic surface of TiO2. Hence, to ensure the effect of the introduction of the hydrophobic side chain, we employed the peptides in which a hydrophobic residue of Leu (2-amino-4-methylpentanoic acid) is incorporated into the glycine-related peptides, that is, GlyLeu (2-[(2-aminoacetyl)amino]-4-methylpentanoic acid), LeuGly (2-(2-amino-4-methylpentanoylamino)acetic acid), and Leu-Gly-Gly (2-[2-(2-amino-4-methylpentanoylamino)acetylamino]acetic acid), and investigated the adsorption and decomposition behaviors on TiO2.

reactions. 1H NMR spectra of amino acids and peptides were measured with a JEOL EX-400 spectrometer at 400 MHz. The amount of the samples adsorbed on the TiO2 surface was estimated from the difference of NMR signal intensities measured before and 12 h after the addition of TiO2. For the photodecomposition measurements, the sample was photoirradiated with three 4 W black light bulbs surrounding the NMR sample tube under aerobic conditions. The sample tube was rotated during the irradiation to maintain the powder suspension. The incident light with the wavelength range of 320−380 nm was about 1 mW cm−2 for each light bulb. To measure the concentration change of the reactant, the NMR spectra were recorded at intermissions in the total irradiation time of 3 h. The concentration of the samples in the solution was estimated by taking the relative peak area to that of the external standard of DSS in a glass capillary. The photodegradation rates were estimated according to the procedure described in the previous report.25

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RESULTS AND DISCUSSION Gly, Gly-Gly, and Gly-Gly-Gly. Figure 1 shows the 1H NMR spectra of Gly-Gly measured in (a) D2O and in (b) the

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Figure 1. 1H NMR spectra of 10 mM Gly-Gly (a) in D2O, (b) in aqueous suspension of TiO2 (5 mg of TiO2/0.5 mL of D2O), and (c) 20 and (d) 120 min after UV irradiation of the sample (b), measured at 297 K.

EXPERIMENTAL SECTION Photocatalyst powders used were TiO2 (ST-01; 100% anatase crystal form with a BET surface area of 320 m2 g−1 and a particle size of 9 nm22) and HT-TiO2 (the calcined ST-01). ST01 TiO2 was a generous gift from Ishihara Sangyo Ltd. and used as received. The HT-TiO2 powders were prepared by the calcinations in an electric furnace for 3 h at 973 K under air. After the calcinations, the BET surface area and particle size became 21 m2 g−1 and 53 nm, respectively. The crystal phase was 100% anatase even after the calcinations at 973 K, which is consistent with our previous measurements.22 Gly and Leu were purchased from Wako Pure Chemical. Glycolic acid was purchased from Nacalai Tesque Co Ltd. Gly-Gly, Gly-Gly-Gly, Leu-Gly, Gly-Leu, and Leu-Gly-Gly were purchased from Peptide Inst. Inc. (Osaka). All samples were A.R. grade. Adsorption and photocatalytic decomposition were measured in an NMR sample tube of 5 mm diameter. TiO2 powder of 5 mg was dispersed in 0.5 mL of a D2O (99.9% Isotec Inc.) solution of 10 mM (M = mol dm−3) amino acids and peptides. The initial concentration of the samples in the dispersion was fixed at 10 mM. The accuracy of the amino acid concentration of the samples was about 10%. Chemical shifts were measured relative to DSS (sodium 2,2-dimethyl-2-silapentane-5-sulfonate). The final pH was in the range of 6−7.5. The pH was not adjusted to avoid the effect of counterions on the photocatalytic

suspension of TiO2 in D2O and (c,d) those measured after UV irradiation of 20 and 120 min, respectively. As shown in Figure 1a and b, the peak intensities decreased after the addition of TiO2 because of the adsorption of the molecule on the TiO2 surface. With UV irradiation, the peak intensities decreased (Figure 1c and d) with increasing irradiation time due to the decomposition of the molecule. In Figure 2A and B, the time profiles of the peak intensity for Gly, Gly-Gly, and Gly-Gly-Gly in the TiO2 and HT-TiO2 suspensions against UV irradiation times are shown, respectively. The decrease in the intensity at time 0 corresponds to the amount of adsorption. For HT-TiO2, the amounts of adsorption were significantly reduced as compared to those for the noncalcined TiO2. In Figure 3, the amount of adsorption of Gly, Gly-Gly, and Gly-Gly-Gly on the TiO2 (A) and HT-TiO2 surface (C) before UV irradiation and the decomposition rates upon UV irradiation ((B) and (D)) are summarized. For untreated TiO2, the amount of adsorption decreased in the order Gly > Gly-Gly > Gly-Gly-Gly (Figure 3A). On the other hand, for HT-TiO2 (Figure 3C), as compared to those for the untreated TiO2, the amounts of the adsorption are significantly reduced 7562



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Figure 2. Time profiles of the concentrations of Gly ◆, Gly-Gly ■, and Gly-Gly-Gly ▲ in aqueous suspensions of TiO2 (5 mg of TiO2/ 0.5 mL of D2O); (A) TiO2 and (B) HT-TiO2 (TiO2 calcined at 973 K) under UV irradiation measured at 297 K.

Figure 4. Adsorption (A) and photocatalytic decomposition rates (B) of Gly and glycolic acid, measured in an aqueous suspension of TiO2 and HT-TiO2.

thermal desorption spectroscopy (TDS), it was reported that in wet systems, H2O on TiO2 is replaced by glycine, which forms a multilayer composed of intact and dissociated molecues.27 In contrast to these studies, Ojame et al. did not find evidence for glycine adsorption from aqueous solution onto TiO 2 powders.18 The present results clearly provide evidence for glycine adsorption onto TiO2 powders in aqueous solution. The apparent discrepancy of the adsorbability of glycine on the TiO2 surfaces may arise from the difference of the surface characteristics of the individual TiO2’s employed. It is notable that despite the large difference in the adsorption between untreated TiO2 and HT-TiO2, the decomposition rates are almost the same and increase in the same order (Figure 3B and D). Thus, the photocatalytic activities on the untreated TiO2 with the amphiphilic surface and those on the HT-TiO2, whose surface is predominantly hydrophobic, are almost the same. This observation indicates that the photocatalytic active sites for untreated TiO2 are the hydrophobic parts of the surface. After the calcinations, the porosity, which can affect the adsorption and decomposition of the samples, may change. However, taking into account that the decomposition rate before and after the calcinations is almost the same, the effect of the porosity change would be small, if any. As shown in Figure 2, the amount of peptide adsorption is very small. The amount of adsorption that influences the photodegradation rates would be the amount of peptide that adsorbs at the active site. Although the amount of adsorbed peptide is significantly decreased for calcined TiO2(Figure 3A and C), the decomposition rate is not influenced (Figure 3B and D), indicating that the amount of the peptide adsorbed on the TiO2 surface other than at the active site should not affect the photodegradation rate. Glycine is the simplest amino acid that consists of a carboxyl group, an amino group, and a side chain of H. The plausible adsorption sites would be the carboxyl group at the C terminal. In addition, for homopeptides Gly-Gly and Gly-Gly-Gly, the peptide bonds formed between the amino group and the carboxyl group at the α-position can also be the binding sites, although they have not been recognized as candidates of the adsorption site. On the basis of the present results, the hydrophobic parts on the TiO2 surface could be assigned as the photocatalytic active

Figure 3. Adsorption and the decomposition rates under UV irradiation at 297 K for Gly, Gly-Gly, and Gly-Gly-Gly in aqueous suspensions of TiO2, (A,B) for untreated TiO2 and (C,D) for HTTiO2 (TiO2 calcined at 973 K).

and increased in the order Gly < Gly-Gly < Gly-Gly-Gly, as opposed to the case for the untreated TiO2. The significantly larger amounts of adsorption observed for the untreated TiO2 indicate that the peptides adsorbed on the hydrophilic parts on the surface because the surface hydroxyl groups on the HT-TiO2 were eliminated after heat treatments. Carboxyl and amino groups are of major importance for the adsorption process, which occurs at the terminals of each strand. Hence, the principal adsorption sites for the untreated TiO2 would be the carboxyl group at the C terminal and the amino group at the N terminal, which adsorb on the basic terminal hydroxyl group and the acidic bridged hydroxyl group on the TiO2 surface, respectively. Glycolic acid is a molecule in which the amino group of glycine is replaced by OH, as illustrated in the bottom of Figure 4; the adsorption and photodecomposition were significantly enhanced by the replacement, suggesting that the adsorption of the amino group on TiO2 should be weak. Theoretical studies by Car− Parrinello simulations of aqueous solutions demonstrated that the backbone COO− oxygen of peptides forms a coordinative bond to a five-fold-coordinated surface Ti, without interference by adsorbed water and independently of pH value. Only a small interaction of amino groups with the stoichiometric surface was indicated.17 Experimentally, on the basis of the studies by 7563



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and the importance of the hydrophobic part of TiO2 as a photocatalytic active site has not been well recognized. Leu and Leu-Containing Peptides. Next, we investigated the effect of the introduction of a hydrophobic side chain Leu on the adsorption and decomposition for TiO2 (ST-01) and HT-TiO2 (heat-treated ST-01). As Leu-containing peptides, we employed Leu, Leu-Gly, Gly-Leu, and Leu-Gly-Gly. Figure 6 shows the 1H NMR spectra of Leu-Gly-Gly measured (a) in D2O and (b) in the suspension of TiO2 in

site. Then, the preferable adsorptive site of the peptide on the hydrophobic parts of TiO2 could be the hydrophobic part of the peptide. Then, the photocatalytic decomposition would proceed through the interaction with the hydrophobic parts of the peptides on the hydrophobic parts of TiO2. As shown in Figure 3B and D, the decomposition rate increases with an increase of the number of peptide bonds. In addition, for HTTiO2, whose surface is predominantly hydrophobic, the amounts of the adsorption increase also with the increase of the number of peptide bonds (Gly < Gly-Gly < Gly-Gly-Gly). Taking account of these facts, the most plausible hydrophobic adsorptive site of these peptides is considered to be the peptide bonds. Although Gly possesses no peptide bond, small amounts of adsorption and decomposition on the HT-TiO2 were observed. Zwitterions of amino acid monomers with a deprotonated carboxyl and a protonated amino group have strong polar backbones, independent of the side chain.17 The UPS spectra of shallow core and valence levels by Soria et al. suggested that glycine takes on a zwitterionic form when adsorbed on the TiO2 surface.28 Thus, taking into account that the Gly are stabilized by taking a zwitterionic form, the molecule could be adsorbed on the photocatalytically active hydrophobic parts of the TiO2 surface to be decomposed. The TiO2 powders prepared with different procedures showed specific photocatalytic activities.22 The photocatalyst with a different characteristic surface can show different adsorbability and photocatalytic activity. Therefore, the photocatalytic active sites can be either the hydrophobic part or the hydrophilic parts of the surface of photocatalysts, depending on the specific TiO2 employed. One should take account of such surface characteristics of individual TiO2’s on the applications. For the case of the simplest amino acid Gly and its homopeptides such as Gly-Gly and Gly-Gly-Gly (illustrated in Figure 5 as an example), they would adsorb by the C-

Figure 6. 1H NMR spectra of 10 mM Leu-Gly-Gly measured at 297 K (a) in D2O, (b) in aqueous suspension of TiO2 (5 mg of TiO2/0.5 mL of D2O), and (c) 20 and (d) 120 min after UV irradiation of the sample (b).

D2O, and (c) and (d) show those measured after UV irradiation for 20 and 120 min of sample (b), respectively. As shown in Figure 6a and b, the peak intensities decreased after the addition of TiO2 due to the adsorption of the molecule on the TiO2 surface. Upon UV irradiation, the peak intensities decreased further with the irradiation time due to the decomposition of the molecule. In Figure 7A and B, the time profiles of the peak intensity for Leu, Leu-Gly, Gly-Leu, and Leu-Gly-Gly against UV irradiation

Figure 7. Time profiles of the concentrations of Leu ◆, Leu-Gly ■, Gly-Leu ▲, and Leu-Gly-Gly ● in aqueous suspensions of (A) TiO2 (5 mg of TiO2/0.5 mL of D2O) and (B) HT-TiO2 (TiO2 calcined at 973 K) under UV irradiation at 297 K.

Figure 5. Schematic presentation of the adsorption of the peptide (Gly-Gly-Gly) on the hydrophilic and hydrophobic surface of TiO2 (ST-01).

times for TiO2 and HT-TiO2 were shown, respectively. The decrease in the intensity at time 0 corresponds to the amount of adsorption. In Figure 8A and C, the amount of adsorption of Leu, LeuGly, Gly-Leu, and Leu-Gly-Gly on the untreated TiO2 and the HT-TiO2 surface before UV irradiation is shown, respectively. The corresponding decomposition rates upon UV irradiation are summarized in Figure 8B and D, respectively. It is noted that both the amount of adsorption and photocatalytic

terminal carboxyl group most probably with the terminal hydroxyl group at the five-coordinated Ti of TiO2, as is generally believed. However, upon employing ST-01 as a TiO2 photocatalyst, the photocatalytic decomposition takes place preferably by adsorbing of peptide bonds on the hydrophobic parts of the TiO2 surface, although the adsorption of a peptide bond on the TiO2 surface has not been paid attention so far, 7564



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for Leu, and Gly-Leu are significantly reduced and increased in the order Leu < Leu-Gly, Gly-Leu < Leu-Gly-Gly, different from the case for the noncalcined TiO2 (Leu-Gly < Leu < LeuGly-Gly < Gly-Leu). On the other hand, the decomposition rates are almost the same as those observed for the noncalcined TiO2 except for Leu-Gly (Figure 8B and D). The fact that the photocatalytic activities on the noncalcined TiO2 with the amphiphilic surface and those on the HT-TiO2, whose surface is predominantly hydrophobic, are almost the same indicates clearly that the photocatalytic active sites for noncalcined TiO2 are the hydrophobic parts of the surface, as discussed above. Besides the peptide bonds, another plausible adsorption site of the peptide interacting with the hydrophobic part of the TiO2 would be the hydrophobic leucyl side chain. For the noncalcined TiO2 (Figure 8A), the larger amounts of the adsorption were observed for Leu and Gly-Leu, suggesting that these peptides should adsorb on the hydrophilic parts on the surface. Hence, the principal adsorption site might be the carboxyl group at the C terminal, which would adsorb the basic terminal hydroxyl group on the TiO2 surface, as discussed above, and the hydrophobic leucyl side chain and the peptide bonds could also weakly adsorb on the hydrophobic surface. On the other hand, for HT-TiO2, the adsorption of these Leu-containing peptides increased with the increase of the number of peptide bonds, that is, Leu < Leu-Gly, Gly-Leu < Leu-Gly-Gly (Figure 8C). However, the decomposition rates are almost the same. These facts suggest that both the peptide bond and leucyl side chain could adsorb on the hydrophobic surface of HT-TiO2, but photocatalytic decomposition should take place through the adsorption of the leucyl side chain, which would adsorb preferably on the photocatalytic active part of the hydrophobic HT-TiO2 surface. As stated above, it is noted that the adsorption and decomposition of Leu-Gly for untreated TiO2 are notably lower as compared with those for Gly-Leu. As shown in Figure 9, Gly-Leu and Leu-Gly present quite different NMR spectral features. For Gly-Leu, the rotations around C−N are not restricted and present equivalent proton signals. On the other hand, it has been reported that Leu-Gly takes a specific

Figure 8. Adsorption and decomposition rates under the UV irradiation measured at 297 K for Leu, Leu-Gly, Gly-Leu, and LeuGly-Gly in aqueous suspensions of TiO2, (A,B) for untreated TiO2 and (C,D) for HT-TiO2 (TiO2 calcined at 973 K).

decomposition of Leu-Gly are significantly low as compared to those of the other molecules. On the calcined surface (HT-TiO2), the amount of adsorption increases in the order Leu < Leu-Gly, Gly-Leu < Leu-Gly-Gly (Figure 8C). Thus, the amount of adsorption increased with the number of the peptide bonds on HT-TiO2, the surface of which is dominantly hydrophobic, suggesting that the peptide bonds should adsorb on the hydrophobic surface of TiO2. For the untreated TiO2, whose surface possesses hydroxyl groups, additional adsorption would take place. The hydrophilic carboxyl group could adsorb the hydroxyl groups of the untreated TiO2, as is the case for Gly, Gly-Gly, and GlyGly-Gly. For HT-TiO2 (Figure 8C), the amounts of adsorption

Figure 9. 1HNMR spectra of 10 mM (A) Gly-Leu and (B) Leu-Gly measured at 297 K (a) in D2O, (b) in aqueous suspension of TiO2 (5 mg of TiO2/0.5 mL of D2O), and (c) 120 min after UV irradiation of the sample (b). 7565



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peptides on the surface of TiO2. The photodecomposition took place by weak adsorption of the peptide bonds on the surface of TiO2. On the other hand, when a hydrophobic side chain Leu was incorporated, in addition to the carboxylic group and the peptide bond, Leu was recognized as the adsorptive site. Thus, for Gly-Leu and Leu-Gly-Gly, the photocatalytic decomposition proceeded under UV irradiation by the preferable adsorption of the hydrophobic side chain of Leu over the TiO2 surface. However, it is notable that Leu-Gly showed remarkably low adsorbability and decomposition as compared to Gly-Leu because of the specific conformation. Thus, when the peptide or proteins take a specific conformation, photocatalysis does not work effectively. On the application to design a TiO2 effective to diminish a specific virus, disease, or environmental hazardous materials, it would be inevitable to acquire information on the surface conformation of the proteins to access the surface of the photocatalysts.

conformation in which the positively charged amino group and the negatively charged carboxyl group interact strongly by the electrostatic interaction, as illustrated at the top of the spectrum.29 Therefore, these protons are not equivalent to split, as shown in Figure 9. Because of the specific conformation within the molecule, the adsorption of Leu-Gly on the TiO2 surface would be prevented, leading the low photocatalytic decomposition. Thus, when peptides or proteins take specific conformations, photocatalysis does not work effectively. It is inevitable to get information on the surface conformation of the proteins to access the photocatalysts. To combine the information about the surface conformation of the peptide and active sites of TiO2 (hydrophobic or hydrophilic), we may design the TiO2 to be effective to the specific virus, disease, or environmental hazardous materials. In Figure 10, for the case of Leu-Gly-Gly, the adsorption and the decomposition features on TiO2 (ST-01) are illustrated; the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +81-258-479315. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed under the management of the Project to Create Photocatalyst Industry for Recycling-Oriented Society, supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan.

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Figure 10. Schematic presentation of the plausible adsorption of the peptide (Leu-Gly-Gly) on the hydrophilic and hydrophobic surface of TiO2 (ST-01).

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peptide would adsorb by the C-terminal carboxyl group most probably with the terminal hydroxyl group at the fivecoordinated Ti of TiO2, as is generally believed. In addition, adsorption of peptide bonds and the hydrophobic leucyl residue on the hydrophobic parts of the TiO2 surface would take place. However, in this case, the leucyl residue would adsorb preferably on the active site of the hydrophobic part of TiO2, and photocatalysis would proceed.

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CONCLUSION The photocatalytic decomposition and adsorption of Gly, GlyGly, and Gly-Gly-Gly and Leu, Leu-Gly, Gly-Leu, and Leu-GlyGly in TiO2 aqueous suspensions were investigated by 1H NMR spectroscopy. Among the various TiO2 photocatalysts, we selected ST-01 (100% anatase crystal form), whose surface characteristics were investigated previously in detail.22 The photocatalytic active site for ST-01 was assigned to be the hydrophobic surface of the photocatalyst. After the calcinations at 973 K, the hydrophilic parts of the surface can be eliminated, and a highly hydrophobic surface is created without changing the crystal form. By employing these characteristics, we studied the adsorption and decomposition of the simplest amino acid Gly and Gly-containing amino acids, Gly-Gly and Gly-Gly-Gly. For Gly-Gly and Gly-Gly-Gly, the carboxylic group and the peptide bond were recognized as the adsorptive sites of the 7566



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H NMR Spectroscopy of the Adsorption and Decomposition of Glycine

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