Building the First Hydration Shell of Deprotonated Glycine by the

ARTICLE pubs.acs.org/JPCB

Building the First Hydration Shell of Deprotonated Glycine by the MCMM and ab Initio Methods Yuheng Yao,† Dong Chen,† Shuai Zhang,‡ Yinli Li,† Pinghui Tu,† Bo Liu,*,† and Mingdong Dong*,‡ † ‡

Institute of Photo-Biophysics, Physics and Electronics Department, Henan University, 475004, Kaifeng, China Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark

bS Supporting Information ABSTRACT: The first hydration shell of the deprotonated glycine is built up by the discrete hydration model. The potential energy surfaces (PESs) of the deprotonated glycine and its hydration complexes with different number of water molecules have been scanned by the Monte Carlo multiple minimum (MCMM) conformational search analysis with the MMFFs force field. Then the energy-minimized structures are predicted using the high-level ab initio calculations/MP2/6311þþG(d,p). The results of the structural parameters and the infrared spectra indicate that the first-shell water molecules around the anion of deprotonated glycine play a more important role in determining the hydration process of deprotonated glycine. The competition between the hydrate site I and the hydrate site II represents a dynamic process of hydrated complexes. The vibrational properties of CdO and N
H are determined to characterize the structure of deprotonated glycine in solution by the discrete hydration model and the conductor-like polarizable continuum model (CPCM) in the gas phase, respectively.

I. INTRODUCTION Noncovalent interaction between water and amino acid aids our understanding of the conformation and the folding process of protein.1
6 Most of the amino acids, which are found in nature, exist in the zwitterionic form. In the zwitterion, the amino group at the N-terminus is usually protonated and the carboxylate group at the C-terminus is deprotonated. In the solution, these polar groups directly interact with the surrounding water molecules. As a result, the hydrogen bonds are possibly formed between the polar groups and the water molecules. The hydrogen bonds, which are one of the most important noncovalent interactions found in the living system, provide a key to determine the structures and properties of macromolecules at the molecular scale.7
11 Consequently, knowledge of the interaction between water and amino acid and the mechanisms of the hydrogen bonds is significant for understanding the biological activity of protein and the structure of polymers, proteins, and nucleic acids. Great attention has been attracted to charged animo acids and small peptides with water complexes both experimentally and theoretically. With elctrospray ionization (ESI) source, sequential hydration of small peptides and noncovalent interactions were examined.12,13 The gas-phase conformations of protonated and deprotonated mononucleotides are determined by ion mobility and theoretical modeling.14 The investigation of hydration of the protonated aromatic amino acids15 has been performed experimentally by employing a mass spectrometer and theoretically by a combination of molecular mechanics and electronic structure calculations on the three amino acid systems r 2011 American Chemical Society

including up to five water molecules, which found that both the ammonium and carboxyl groups offer good water-binding sites. Furthermore, the infrared spectra of alkaline earth metal ion complexation on zwitterionic amino acids and protonated and lithiated valine in the solvation process were studied recently.16,17 Structure information was reconstructed when infrared absorption peaks were assigned by theoretical calculations. Also, the theoretical approaches have been found to be more reasonable to search the energy-minimized conformations of hydrated biomolecules and provide the necessary thermodynamic information. The density-functional electronic structure and molecular dynamic simulations were employed by Degtyarenko et al.18 to study the structural properties of an L-alanine molecule in aqueous solution. Michaux et al.19
22 described the microhydration of protonated amino acid by an ab initio family tree, which can focus on the most interesting structures with the limited computational resources. Although these investigations provide important information in the interactions of water molecules, the first hydration shell of charged amino acids has not been modeled in a systemic theoretical way. In this paper, the first hydration shell of a biomolecule has been successfully established by the subsequent addition of water molecules to the preferential site and optimizing the representative conformations. In addition, we utilized the CPCM model and the discrete hydration model to obtain the vibrational Received: December 9, 2010 Revised: March 24, 2011 Published: April 21, 2011 6213

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properties of [Gly
H]
3 (H2O)8. It is of interest to see that the vibrational property of the amino group from the CPCM model is different from the discrete hydration model. The interactions of hydragen bond are employed to explain this difference.

II. COMPUTATIONAL MODEL The possible initial structures of deprotonated glycine and its hydrated complexes were determined by fully random conformational searches, which were performed through the MCMM method and MMFFs force fields as implemented in the MacroModel 9.5 (Schr€odinger, LLC, New York, 2007). The resulting conformations were optimized by ab initio calculations. The minimized energies and harmonic frequencies of the deprotonated glycine and its hydrated complexes were calculated by MP2/6-311þþG(d,p) using the Gaussian 03 software package. All the simulations were performed in the gas phase. The vibrational frequency of [Gly
H]
3 (H2O)8 was also studied at the MP2/6-311þþG(d,p) level theory by the discrete hydration model and then compared to the CPCM model of deprotonated glycine. The computational details for searching the lowest energies of the [Gly
H]
3 (H2O)n conformer structures are as follows. For each structure of [Gly
H]
3 (H2O)n, a starting geometry is considered by sequentially adding water molecules to [Gly
H]
3 (H2O)n
1. According to the noncovalent interaction between deprotonated glycine and each H2O molecule, the resulting low-energy structures are classified into a few groups. Representative conformers from each family are optimized by energy minimization at the MP2/6-311þþG(d,p) level. For example, 5000 conformations were generated in 5000 fully random conformational searches of [Gly
H]
3 (H2O) without any dihedral and distance constrains. Then, these 5000 structures were categorized into 31 groups by different hydrogen-bonding motifs and dihedral angles. One representative conformation in each category was opimized at the MP2/6-311þþG(d,p) level. Four different conformers remained distinguishable after the calculations converged. For the [Gly
H]
3 (H2O)2, the starting conformations are achieved by adding one water molecule to each of the four resulting conformer of [Gly
H]
3 (H2O), namely a, b, c, and d. 929 structures were generated for conformer a. 881 structures were generated for conformer b. 938 structures were generated for conformer c. 1008 structures were generated for conformer d. These structures were categorized into 6 groups, 9 groups, 9 groups, and 7 groups by hydrogen-bonding motifs, respectively. In order to simplify our calculations, a further classification was made in all 31 groups and finally converged into 29 groups. Briefly, the random conformational search will generate many structures for [Gly
H]
3 (H2O)n=1
8. The harmonic frequencies of low-energy conformations were calculated to obtain the zero-point vibrational energies (ZPVE), while the relative energies of optimized structures is calculated by MP2/6-311þþG(d,p) and corrected by the ZPVE. We have performed test calculations for some conformers of [Gly
H]
3 (H2O), and good agreement between CCSD(T) and MP2 relative energies was found. Therefore, we are convinced that such a method is perfectly suitable for our purpose.23,24 The hydration energies of hydrated deprotonated glycine are defined as follows. The hydration energy of hydrated deprotonated glycine is given by Ehyd = E0 [Gly
H]
3 (H2O)n
E0 [Gly
H]
3 (H2O)n
1
E0 H2O, where E0 [Gly
H]
3 (H2O)n is the

Figure 1. Optimized structure of deprotonated glycine with possible hydration sites.

Figure 2. Low-energy conformers of [Gly
H]
3 (H2O). In each frame, the dashed lines indicate the H-bonds between water molecule and the deprotonated glycine. The values in parentheses are the relative energies (kJ/mol).

energies of the optimized structures of [Gly
H]
3 (H2O)n. In particular, n refers to the number of water molecules. E0 [Gly
H]
3 (H2O)n
1 is calculated by removing one H2O from the optimized geometry of [Gly
H]
3 (H2O)n. EH2O is the total energy of a water molecule in the gas phase. A prime corresponding to equation marks the optimized energies, which are achieved at the MP2/6-311þþG(d,p) level of theory. The thermodynamic functions have been approached at a temperature of 298.15 K and a pressure of 1 atm.

III. RESULTS AND DISCUSSION According to analyze the PESs of the hydrated complexes with one to eight water molecules, the starting structures were generated to be optimized at MP2/6-311þþG(d,p) theory level. A. First Hydration Shell. Here, we defined the first hydration shell as a set of solvent molecules directly interacting with solvated molecule. For example, the first hydration shell of the deprotonated glycine is defined as a network of hydrogenbonded water molecules interacting with carboxylate (
CO2
), amino (
NH2), and methylene (
CH2). 6214

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B. Structures of Deprotonated Glycine and Its Hydrated Complexes. We performed a conformational search for the

deprotonated glycine, and then the resulting low-energy structures were classified into three groups. During the optimization, the representative structures of each group will converge to the same energy-minimized structure. The optimized structure is shown in Figure 1. As you can see in Figure 1, a weak intramolecular H-bond takes place between the amino and carboxylate groups. 1. [Gly
H]
3 (H2O). On the basis of the optimized conformer of [Gly
H]
, the PESs of the deprotonated glycine monohydrate were analyzed. Successively, 31 representive structures were found on the PESs of [Gly
H]
3 (H2O). Four conformers are favorable according to the optimized energies. The structures and the relative energies are shown in Figure 2. Here, a systematic nomenclature for the hydration complexes is proposed: the capital letter G designates the deprotonated glycine and the number indicates the number of water molecules that are associated with the deprotonated glycine. If necessary, the conformational energies are distinguished from each other by appending the letters a, b, c, etc. For example, conformer G3a is the lowest-energy conformation of [Gly
H]
3 (H2O)3. Let us turn our attention to Figure 2. The most stable structure forms two hydrogen bonds (H-bonds) between H2O and the carboxylate group (see G1a in Figure 2). The other stable structure forms one H-bond between H2O and the carboxylate group (see G1b in Figure 2), and thus it is 11.4 kJ/mol higher in Table 1. Enthalpies (kcal/mol), Entropies (cal/(mol 3 k)), and the Gibbs Free Energies (kcal/mol) for the Hydration of the Deprotonated Glycine in the Gas Phase, Which Were Corrected for the Basis Set Superposition Error (BSSE)a

a

thermodynamic parameters


ΔH

ΔS


ΔG

b

expt

16.0 (0.3)

26.6 (0.4)

8.2 (0.4)

CP-MP2c

15.4

28.0

7.0

this work

17.4

28.2

9.0

The values in parentheses are experimental uncertainties. b Reference 25. c Reference 22.

energy than G1a. In the G1c conformer, the water molecule provided a bridge between carboxylate and amino groups, which may be defined as a bridging structure. It is obvious that there is a complexation with a nitrogen lone pair in the G1d conformer (see Figure 2). The relative energies of G1c and G1d are 12.3 and 18.8 kJ/mol, respectively. The thermodynamic parameters corresponding to the structures in Figure 2 are listed in Table 1, along with the values obtained from CP-MP2 calculation22 and experiment25 for comparison. In the present work, all values for [Gly
H]
3 (H2O) are reasonably consistent with those obtained from the experiment and the previous calculation. 2. [Gly
H]
3 (H2O)2. Considering addition of the second water molecule to the possible [Gly
H]
3 (H2O) conformers, according to the discusstion in the section II, 29 representive structures were generated based on the random conformational searches for [Gly
H]
3 (H2O)2. After the MP2 geometry optimization, 12 of [Gly
H]
3 (H2O)2 remained distinguishable. In Figure 3, the lowest five stable structures and the relative energies are shown. The most stable dihydrate structure forms two H-bonds at the carboxylate site and one H-bond between water molecules (see G2a in Figure 3). Also the added water molecule provided a bridge between the carboxylate and amino groups in the G2b conformer. However, the G2c conformer lacks a bridging structure making a 0.3 kJ/mol energy higher than G2b. It is interesting to note that the G2d and G2e conformers show sharp increase in energy, especially for G2e, which has a water molecule in the second hydration shell. The bridge structure is supported by the electron density distribution of the highest occupied molecular orbital (HOMO) of [Gly
H]
and [Gly
H]
3 (H2O) shown in Figure 4. The electron density at the carboxylate is clearly larger than that at the amino in [Gly
H]
, which leads to the favorable hydrate site. It can be proved by the lowest-energy structure G1a. When another water molecule is attached to G1a, the electron density at the carboxylate tends to be very weak, resulting in a strong competition of the bridge position in [Gly
H]
3 (H2O)2. 3. [Gly
H]
3 (H2O)3 and [Gly
H]
3 (H2O)4. Starting structures of [Gly
H]
3 (H2O)n are generated by sequentially adding water molecules to the possible [Gly
H]
3 (H2O)n
1

Figure 3. Five low-energy conformers of [Gly
H]
3 (H2O)2. In each frame, the dashed lines indicate the H-bonds between water molecule and the deprotonated glycine. The values in parentheses are the relative energies (kJ/mol). 6215

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Figure 4. HOMO electron density distributions for the [Gly
H]
(a) and [Gly
H]
3 (H2O) (b).

Figure 5. Energy-mimimized conformers and the relative energies (kJ/mol) of [Gly
H]
3 (H2O)3.

conformers. Of the 65 000 conformational searches for the [Gly
H]
3 (H2O)3, 46 representative conformers are found out of 12 208 conformers. After the optimization, 13 unique structures remain. Eight of the 13 structures are favorable in energy, as shown in Figure 5. The lowest-energy structure of [Gly
H]
3 (H2O)3 forms three H-bonds (see G3a in Figure 5). The three water molecules are directly hydrated to the deprotonated glycine to form three H-bonds, and a ringlike H-bond is generated among three water molecules. As we can see in Figure 5, it seems that the G3b, G3c, and G3g conformers can be achieved in geometry by adding another water molecule to the G2B conformer, while the other low-energy structures (see G3d, G3f, G3e, and G3h in Figure 5) are generated in geometry by adding two water molecules to the G1a conformer. Similar to building the [Gly
H]
3 (H2O)3 structure, starting geometries are generated on the basis of the resulting energyminimized conformations of [Gly
H]
3 (H2O)3. By running the 165 000 conformational searches for the [Gly
H]
3 (H2O)4, 104 representative conformers are generated out of 34 107 conformers. After the optimization, 19 unique structures remain. Six of the 19 conformers are shown in Figure 6. The most favorable conformer of [Gly
H]
3 (H2O)4 (see G4a in

Figure 6) also has a ringlike H-bond among four water molecules. Likewise, the G4e structure can be generated in geometry by adding a water molecule to the G3a conformer. Too many structures will be generated by the conformational search with increasing number of water molecules, and the conformers with H2O in the second hydration shell can make the structures more complicated. One could also question if it is necessary that the second hydration shell be considered in the further calculations. A deep insight into analyzing the infrared spectra and structural parameters in the gas phase will help us to answer the question. For the G2e conformer, a water molecule is located in the second hydration shell. In this case, the water molecule indirectly reacts with the deprotonated glycine and forms a H-bond network among water molecules. How large will be the effect of the water molecule in the second hydration shell on the deprotonated glycine? Figure 7 shows the infrared spectra of the G2e and G1a conformers in the gas phase. It is clear that the main peaks, which are labeled in Figure 7, of the G2e conformer are similar to those of the G1a conformer. That is, the first hydration shell is dominant to the water molecules interacting with the deprotonated glycine. Moreover, the main structural parameters of G1a and G2e are calculated in the 6216

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Figure 6. Low-energy conformers and the relative energies (kJ/mol) of [Gly
H]
3 (H2O)4.

Figure 7. Infrared spectra of the G2e and G1a conformers. A few main peaks are labeled to identify the vibrations.

MP2/6-311þþG (d,p) theory, such as the bond length, bond angle, and dihedral angle. The results show that the water molecules in the second hydration shell have few effects on the hydrated deprotonated glycine. Therefore, the effect of the second hydration shell on the hydrated complexes can be negligible and the following studies will be carried out on the complexes with the first hydration shell. After analyzing the possible structure of hydrated deprotonated glycine, the hydration sites fall into two typical categories: site I and site II. As shown in Figure 1, site I is divided into five positions (see I1, I2, I3, I4, and I5 in Figure 1) according to the different spatial orientations. Site II locates nearby the amino group. From the low-energy structures of [Gly
H]
3 (H2O)n=2
4 above, the water molecules tend to occupy the hydrate site I compared to the hydrate site II. The charge of the deprotonated glycine appears partial selfsolvation.12 As a result, the carboxylate group has a strong negative charge, while the amino group is a weak hydrogen donor bearing

less negative charge. It is indicated that the hydrate processes are significantly driven by the self-solvation charged group. In this case, it is preferred that the carboxylate group interacts with the netlike arrangement of water molecules. Figure 8 illustrates the most stable structures of [Gly
H]
3 (H2O)n=1
4 with the hydrate site I and the conformers of [Gly
H]
3 (H2O)n=1
4 with the hydrate site II. The relative energies suggest that water molecules could not saturate the hydrate site I until four molecules and show clear preferrence for the hydrate site I. Site I will be saturated with adding more water molecules to the hydrate site I. Then, it is possible that the hydrate site II will be considerable in the hydrate processes instead of the site I. The hydration energies and interaction energies would give us an insight into the competition between the hydrate site I and the hydrate site II. It is clear that the hydration complexes with water molecules at the hydrate site I1, site I2, site I3, and site I5 form a strong hydrogen-bonded network. However, the hydrate site II is not dominant because of a weak interaction between water molecules and the amino group. Therefore, the competition between the hydrate site I and the hydrate site II only referred to the hydration energies of the conformers with water molecules at the hydrate site I4 and site II. The energies of H-bond of hydration complexes with water molecules at the hydrate site I and site II can be obtained by the interaction energies among water molecules. Figure 9 shows the plots of the hydration energies and interaction energies of [Gly
H]
3 (H2O)n=1
4 as a function of the number of water molecules. Both energies have nearly linear dependence on the number of adatoms. As shown in Figure 9a, the hydration energies decrease with increasing number of water molecules. It is obvious that the hydration energies of the conformers with water molecules at the hydrate site I4 decrease rapidly. Obviously, the lines show a noticeable trend in making a crossover in the process of sequentially adding water molecules to [Gly
H]
. The results also suggest that the interaction energies follow a similar trend with increasing the number of water molecules. The decreasing rate of interaction energy is different between the hydrate site I and the hydrate site 6217

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Figure 8. Comparison between (a) the most stable structures of [Gly
H]
3 (H2O)n=1
4 with the hydrate site I, clockwise from top left G1a, G2a, G3a, and G4a, and (b) the most stable structures of [Gly
H]
3 (H2O)n=1
4 with the hydrated site II, clockwise from top left G1d, G2d, G3g, and G4e.

II. In the monohydrate G1a, single water molecule forms two H-bonds with the carboxylate group for the hydrated site I, and in the G1d conformer, water molecule forms one H-bond with the amino group. With sequentially adding water molecules to the deprotonated glycine, the H-bonds at the carboxylate site become weak and can be nearly equivalent to that at the amino site. Therefore, we expected that the hydrate site II will be considered to an appropriate hydrate site when the hydrate site I is saturated. 4. [Gly
H]
3 (H2O)n=5
8. Clearly, the analysis of the hydrate site revealed that the conformational searches depend to a high degree on finding all possible hydrate sites. Also from the structures of [Gly
H]
3 (H2O)n=1
4, we can see that all the lowest-energy conformations of [Gly
H]
3 (H2O)n are based on adding one water molcule to the lowest-energy conformation of [Gly
H]
3 (H2O)n
1. Our concern here is with the structural information of the lowest-energy conformation. The statement above can be extended generally to generate low-energy conformations of [Gly
H]
3 (H2O)n=5
8, and the lowest-energy conformations of [Gly
H]
3 (H2O)n are effective and suitable to generate starting geometries of [Gly
H]
3 (H2O)nþ1. Therefore, conformational searches for the [Gly
H]
3 (H2O)n=5
8 have been performed on the basis of the lowestenergy conformation of [Gly
H]
3 (H2O)n
1 and the corresponding structures are shown in Figure 10. In the [Gly
H]
3 (H2O)5 conformers, the structure with water molecule at the hydrate site II (see G5c in Figure 10) is 2.2 kJ/mol higher than the G5a conformer in energy. It is predicted that the energy gap between the conformer with the hydrate site I and the conformer with the hydate site II is relatively small, which means the competition of the hydrate site II becomes stronger. The competition between the hydrate site II and the hydrate site I is distinguished in [Gly
H]
3 (H2O)6. The investigation of [Gly
H]
3 (H2O)6 indicated the energy gap between the G6a conformer and the G6b conformer is only 0.2 kJ/mol. The

small energy difference suggests the hydrate site II can contend with the hydrate site I when the number of water molecules reached six. These results have been predicted by the plots of hydration energies and interaction energies of hydration complexes containing water molecules up to four. A further proof will be given by [Gly
H]
3 (H2O)7. The hydrate site II is considered to be favorable in the lowest-energy structure (see G7a in Figure 10). Moreover, the G7a conformer is 3.5 kJ/mol lower than the G7c conformer in energy. On the basis of the [Gly
H]
3 (H2O)7 structure, a conformational research has been employed to explore the possible conformers of [Gly
H]
3 (H2O)8. It is surprising that only one conformer is found on the PESs of octahydrate complex. Then the conformer had the energy optimization and the frequency analysis at the MP2/6-311þþG(d,p) theory level. The optimized structure is shown in Figure 11. Compared to [Gly
H]
3 (H2O)7, the added water molecule at the hydrate site I5 provided a bridge between the carboxylate and amino group in [Gly
H]
3 (H2O)8. On successively adding a water molecule to [Gly
H]
3 (H2O)8, we found that the extra water molecule will indirectly interact to the deprotonated glycine. It is deduced that always the first hydration shell is formed by eight water molecules. Here, the first hydration shell of the deprotonated glycine has until now been built up by the discrete hydration model. C. Vibrational Properties of Deprotonated Glycine in Solution. To our knowledge, the vibrational properties of small amino acid are highly sensitive to the rearrangement of the neighboring water molecules in the first hydration shell.26 Indeed, the CdO and N
H stretching modes of the amide linkage have been widely used to characterize the structure of peptide in solution.27,28 These studies suggest that the vibrational properties should be considered in the theoretical simulation. Thus, the vibrational properties of CdO and N
H are presented in detail to characterize the structure of deprotonated glycine in solution. 6218

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Figure 9. Plots of hydration energy and interaction energy of hydration complex have been addressed as a function of the number of water molecules added to the different hydration sites. For the hydrate site I (see panel a), the interaction energy refers to an interaction between the deprotonated glycine and added water molecules. For the hydrate site II (see panel b), the interaction energy is caused by an interaction between the H2O at the hydrate site II and the remaining part.

Figure 10. Lowest-energy conformers of [Gly
H]
3 (H2O)5, [Gly
H]
3 (H2O)6, and [Gly
H]
3 (H2O)7.

Based on the MP2/6-311þþG(d,p) geometry optimization and frequency analysis, the asymmetric stretching vibrations of CdO and N
H in [Gly
H]
3 (H2O)8 have been determined by the discrete hydration model and the CPCM model in the gas phase. The calculated frequencies are listed in Table 2. In the case of the N
H group, the band of asymmetric stretching vibrations in [Gly
H]
3 (H2O)8 is located at 3423 cm
1 by the discrete hydration model; they are blue-shifted by 28 cm
1 with respect to the deprotonated glycine (3395 cm
1), but are red-shifted much more strongly to lower wavenumbers by 50 cm
1 using the

CPCM model. This blue shift of N
H asymmetric stretching vibrations is due to weakening the interaction of the H-bond between the carboxylate and amino group. For the case of [Gly
H]
3 (H2O)8, the H-bond length increased from 2.264 Å in the unsolvated deprotonated glycine to 2.848 Å in [Gly
H]
3 (H2O)8. As a result, The asymmetric stretching vibrations of N
H in [Gly
H]
3 (H2O)8 should be followed by a blue shift. The asymmetric stretching vibrations of CdO produced a red shift of 52 cm
1 referred to the unsolvated deprotonated glycine (1540 cm
1) by the discrete hydration 6219

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MCMM and ab initio methods provide a computationally efficient and accurate model of interaction between water molecules and glycine amino acid, which provide mechanisms of the structural organization within the hydration shell for understanding the biological activity of proteins.

’ ASSOCIATED CONTENT

bS

Supporting Information. Computed conformer of deprotonated glycine; the lengths (Å) of inter-hydrogen bonds for all the most stable the hydration of the deprotonated glycine; and some basic geometric parameters for all the most stable structures with each particular hydration number. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (B.L.); [email protected] (M.D.). Figure 11. Structure of the octahydrate complex. The first hydration shell is formed by eight water molecules.

Table 2. Asymmetric Stretching Vibration of N
H and CdO Obtained by the Discrete Hydration Model and the CPCM Model in the Gas Phasea

a

’ ACKNOWLEDGMENT This work is supported by the Ministry of Education of China foundation of Supported Program for New Century Excellent Talents in University (NCET-07-0256), National Natural Science Foundation of China (NSFC 30900280).

model

Gly
unsolvated

Gly
CPCM

[Gly
H]
3 (H2O)8

νas(N
H)

3395

3345

3423

νas(CdO)

’ REFERENCES

1592

1503

1540

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All calculated frequencies were scaled by a factor of 0.95.

model, and are red-shifted by 89 cm
1 using the CPCM model. The red shift associated with the carboxylate group can be explained by the heavily reinforced interaction between water molecules and the carboxylate group. Binding water molecules to the deprotonated glycine differently affected the vibrational properties of the CdO and N
H groups. The discrete hydration model is employed to calculate the unharmonic frequencies of N
H in [Gly
H]
3 (H2O)8, which provide good agreement with the previous work.28

IV. CONCLUSIONS MCMM conformational search analysis and the discrete hydration model have been employed to study the low-energy strucutres of deprotonated glycine and its hydration complexes. The resulting representative structures are energetically optimized at MP2/6-311þþG(d,p). The calculated structural parameters and infrared spectra of the G2f and G1a conformers suggested that the second-shell water molecules around the anion of glycine could be neglected to determine the structure of deprotonated glycine in solution. Furthermore, the hydration energies and interaction energies of [Gly
H]
3 (H2O)n=1
4 have nearly linear dependence on the number of water molecules. Eventually, the lines will make a crossover in the hydration process. The competition between the hydrate site I and the hydrate site II has been confirmed in [Gly
H]
3 (H2O)6. On the basis of the octahydrate complex, the first hydration shell of the deprotonated glycine has been built up and the asymmetric stretching vibrations of N
H and CdO in [Gly
H]
3 (H2O)8 should be followed by a blue shift and a red shift, respectively.

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