THE ROLE OF HYDROGEN BONDING IN THE FORMATION OF MOLECULAR CLUSTERS IN GLYCINE: RAMAN, IR SPECTRA AND DFT CALCULATIONS

ABSTRACT

Raman scattering and IR absorption spectra of glycine (Gly) in its crystalline form were analyzed in aqueous solution. Density functional theory (DFT) calculations were performed on Gly+n·H₂O (where n=1-10) complexes in the B3LYP/6-311++G(d,p) basis set, and a polarizable continuum model (PCM) was used to account for solvent effects using the integral equation formalism variant (IEFPCM). It was found that the Raman scattering and IR absorption spectra of glycine were consistent with the models of complexes formed with water molecules. A model of the most stable complex of the zwitterionic state of glycine in the presence of water molecules hydrogen-bonded to −COO⁻ and −NH₃⁺ groups was proposed and is in good agreement with experiment.

AННОТАЦИЯ

Спектры комбинационного рассеяния света (КРС) и ИК-поглощения глицина (Gly) в его кристаллической форме были проанализированы в водном растворе. Расчеты теории функционала плотности (DFT) были выполнены для комплексов Gly+n·H₂O (где n=1-10) в базисном наборе B3LYP/6-311++G(d,p), а для учета эффектов растворителя использовалась модель поляризуемого континуума (PCM) с использованием варианта формализма интегральных уравнений (IEFPCM). Было обнаружено, что спектры комбинационного рассеяния света (КРС) и ИК-поглощения глицина согласуются с моделями комплексов, образованных молекулами воды. Была предложена модель наиболее устойчивого комплекса цвиттер-ионного состояния глицина в присутствии молекул воды, связанных водородными связями с группами −COO⁻ и −NH₃⁺, которая хорошо согласуется с экспериментом.

Keywords: Glycine, Infrared and Raman spectra, DFT, glycine-water complexes, hydrogen bonding.

Ключевые слова: глицин, инфракрасные и рамановские спектры, DFT, комплексы глицин-вода, водородные связи.

Introduction

Vibrational spectroscopy is one of the most effective methods, and this method is widely used in the study of complex biological molecules. Amino acids have been studied in the works of many scientists. In particular, Ref [1] presents the results of studying the transfer of protons in water and in bioenergetic proteins. The authors concluded that the nature of proton transfer is a function of distance. The hypothesis that water is the main component in the chains of amino acids connected by hydrogen bonds is discussed, and the importance of intermolecular forces as the reason for the existence of biological compounds such as DNA and RNA is shown [2].

Currently, increasing attention is paid to aqueous solutions of amino acids, which are components of proteins and play an important role in various biological and biochemical processes.

Glycine, a representative of amino acids, is an important structural element of proteins and has two centers capable of forming hydrogen bonds, namely, a proton donor through a hydroxyl group and a proton acceptor through a NH2 group. Glycine is of interest not only for fundamental research, but also has a high practical value. In addition, they are convenient model systems for studying the processes of intermolecular interactions. In such cases, Raman and IR spectra provide more complete information about the molecular structure of more complex amino acids [3]. Amino acids, in particular glycine, are one of the most important biological molecules, as they constitute the "building blocks" of peptides and proteins. Many of their remarkable properties are due to their amphoteric nature, which is due to the presence of two functional groups of different polarities (amino acid, NH2, and carboxyl, COOH, groups) in their structure [4-6]. These functional groups play a key role in the formation of hydrogen bonds both with polar solvents and with other molecules in crystals. The presence of such interactions can be detected by infrared (IR) or Raman spectroscopy experiments [7]. Most amino acids exist in zwitterionic structures in solution and crystal phases [8] and interact with surrounding molecules through hydrogen bonding. The vibrational spectra of amino acids in the crystal and solution states have been extensively studied [9-10]. However, the vibrational data of most amino acids are insufficient.

Glycine (Gly) is a simple amino acid, and although its vibrational spectra have been studied by a number of researchers, studies on the effect of solvents on vibrational spectra are still ongoing [11-13]. Gly is one of the three amino acids necessary for the biosynthesis of creatine, a non-protein nitrogenous material in human and animal tissues [14]. Gly exists in the gas phase as a neutral NHCHCOOH structure, and in the liquid or solid phase as a zwitterion NHCHCOO+.

As a result of experiments [15-16], the authors comprehensively revealed the specific properties of different types of hydrogen bonds in two glycine-water complexes [17].

The study of the vibrational spectra of amino acids using Raman spectroscopy methods shows that the formation of molecular complexes through intermolecular hydrogen bonding leads to a change in the spectral parameters of the interacting molecules [18-19].

Spectral studies have also focused on the field of molecular interactions, in which the vibrational degrees of freedom of amino acid residues, as well as vibrations of C-H and N-H groups, are manifested [20].

The Raman spectra of all standard amino acids under different pressure conditions with temperature changes have been studied in comparison [21] and important conclusions have been drawn. In many studies [22], the corresponding depolarization ratios of the infrared and Raman spectra have been calculated using several calculation methods for vibrational spectra.

Modeling is one of the convenient solutions for studying the processes of intermolecular interactions [23]. DFT and MP2 calculations have shown that the most stable states in glycine–water 1:1 complexes are stabilized by strong double hydrogen bonds, with the zwitterionic form dominating in solution [24]. DFT studies have shown that glycine is stabilized in the zwitterionic form when fully microsolvated with a water molecule [25]. DFT calculations have shown that hydrolysis of glycine amide with water in the gas phase is energetically impossible and that such a reaction does not occur under space conditions [26]. Theoretical calculations have shown that the zwitterionic form of glycine in aqueous media is energetically and electronically more stable than the neutral form, which is preferable as a model close to biological systems [27].

The changes in intensity in the studied bands of amino acids are explained by the additional contribution of the charge distribution of the electronically excited state due to the formation of hydrogen bonds. When the polarization and temperature change, the spectral width of the band changes, and the relaxation time changes in accordance with the change in the motion of molecules with hydrogen bonds. To verify the correctness of these conclusions, we analyzed the literature.

The analysis of the literature showed that the existing theory and knowledge do not allow us to fully explain the mechanism of formation of molecular complexes of glycine with water molecules and its spectral manifestation. The use of theoretical calculations for a detailed analysis of the Raman and IR spectra obtained in the experiment gives good results. Therefore, in this work, we studied the mechanism of formation of molecular complexes in glycine and its aqueous solution using vibrational (Raman and IR) spectroscopy and DFT calculations.

The geometry of the molecular complexes formed by glycine with water molecules was optimized using the DFT method at the B3LYP/6-311++G(d,p) level. The nature of intermolecular interactions in the molecular complexes was evaluated using Non-Covalent Interaction (NCI), Reduced Density Gradient (RDG), ELF, and LOL topological analyses.

2. Experimental and computational details

The IR absorption spectra of pure glycine and its aqueous solution at the best solubility concentration (0.249 g/liter) [28] in the range of 4000-400 cm^-1 were recorded using a Shimadzu IRTracer-100 FTIR spectrophotometer. The spectrometer was stabilized for high sensitivity. LabSolutions IR software was used for spectral analysis. The Raman spectrum of glycine in the range of 4000-100 cm^-1 was recorded using a Renishaw Invia Raman spectrometer. A laser with a wavelength of 532 nm and a power of 50 mW was used as the excitation light source. A standard Renishaw CCD Camera detector was used to record the scattered light. The exposure time was 10 s and the resolution was 0.5 cm-1. A diffraction grating with a period of 1200 lines/mm was used as the dispersive element. All spectra were recorded at normal atmospheric pressure and room temperature.

All calculations in this study were performed in the Gaussian 09 W package [29]. The optimal geometry and vibrational spectra of glycine water clusters were calculated using the DFT method with the B3LYP hybrid functional and the 6-311++G(d,p) basis set. At the same time, the AIM and RDG methods are of great importance in determining non-covalent interactions, as well as in analyzing weak interactions between atoms in the molecule, as well as bonds. By using the AIM and RDG methods together, it is possible to accurately analyze non-covalent interactions [30]. These methods have been used to explain the dynamics of interactions and molecular structure.

3. Results and discussion

3.1. Experimental results

IR and Raman spectroscopy complement each other and provide information about vibrations in molecular systems. Sometimes IR and Raman spectral lines (bands) do not match each other due to selection rules, molecular symmetry, fundamental transitions, overtones, and their combinations.

Analysis of a number of results shows that in the Raman spectrum of amino acids, a line with a frequency of 1650 cm^-1 and intense lines with a frequency of 3380 cm^-1 characteristic of the carboxyl group are observed, as well as vibrations with a frequency of 3340 cm^-1 for the NH₂ amino group. In order to better understand the molecular vibrations of glycine in its crystalline form and in aqueous solution, FTIR and Raman spectra were recorded. In the vibrational spectra of pure glycine in the crystalline state obtained by the two methods, the frequencies of the bands do not coincide. Such spectral lines are complex, and the problem of explaining the inconsistency of their spectral parameters remains a mystery. By analyzing the complexity of the bands in the 2800-3200 cm^-1 region, which correspond to the symmetric and antisymmetric O-H and N-H vibrations of the glycine molecule, it is possible to understand the mechanism of formation of molecular complexes. There are various assumptions to explain such complexity of the bands [31].

If the analysis of the structure of the bands in the 2800-3200 cm^-1 region shows that this complex band, which corresponds to the O-H and N-H vibrations of glycine, consists of several lines with different bulk depolarization coefficients, then this complexity can be easily explained, which is also confirmed by the results presented in the literature [32].

In this study, we focused on the bands corresponding to the COO group of the molecule and the stretching vibration of the NH₃ group in the crystalline state of glycine and its aqueous solutions. Because these groups actively participate in intermolecular interactions. Figure 1 depicts the Raman spectra of glycine and its aqueous solution in the range of 500-1700 cm^-1. The C=O stretching vibration band in the Raman spectra of glycine in the pure state corresponds to 1670 cm⁻¹ and is shifted to a lower frequency by 36 cm^-1 in the aqueous solution (1624 cm^-1). The reason for this shift of the peak to a lower frequency may be due to the formation of hydrogen bonds between the solute and solvent molecules. It can also be seen that the intensity of some vibration bands has decreased and the half-width has changed. This may be due to van der Waals interactions. At the same time, there is a band corresponding to the O-H band formed as a result of the interactions of water molecules in the range of 3000-3500 cm^-1. The stretching vibration of the NH₃ group in pure glycine corresponds to 3141 cm^-1 and cannot be observed due to the fact that it remains in the background of the O-H vibration band in an aqueous environment.

Figure 1. Experimental Raman spectra of glycine and its aqueous solution

Figure 2. Experimental IR absorption spectra of glycine and its aqueous solution

Very weak and broad peaks are observed in the IR absorption spectrum (Fig. 2), and it can be seen that these peaks (maxima) decrease in intensity in aqueous solution and the bands are somewhat simplified. It is known that the vibrations of the functional groups of the glycine molecule and the translational motion of the molecule lead to the appearance of dipole moments.

A broad band peak associated with the stretching vibrations of the NH₃ group, which have different dipolarization ratios, was observed in the IR spectrum at 3170 cm^-1. The band maximum associated with the bending vibration of the NH₃ group of glycine was recorded at 1612 cm^-1 and shifted to a higher frequency by 14 cm^-1 (1626 cm^-1) in aqueous solution. This upward shift of this band occurs due to hydrogen bonding between the NH₃ group of the glycine molecule and the oxygen atom of the water molecule. This suggests that glycine contains complex molecular clusters. Spectroscopic data confirmed that glycine and its aqueous solution consist of complex molecular clusters. In order to confirm the conclusions drawn from the experimental results, we performed quantum-chemical calculations.

3.2. Geometrical analysis

Raman scattering and IR absorption spectra of glycine in crystalline form and aqueous solution were recorded. In order to further understand the spectra, quantum chemical calculations were performed using the density functional theory (DFT) method in the B3LYP/6-311++G(d,p) basis set Gly+n·(H₂O) (where n=1-10) and IEFPCM was used to take into account the solvent effect, and the calculated optimal geometries were depicted.

Figure 3. Optimal geometric structures of Gly+n·(H₂O) (n=1-10) clusters

The optimal geometry of molecular clusters that can be formed with glycine and up to 10 water molecules is presented in Figure 3. The calculation results showed that all clusters formed by glycine with one to ten water molecules are formed through H-bonding. In the calculated monomer, glycine formed an internal H-bond between O2···H8-N with a bond length of 1.81Å. In the complex formed by glycine with 1 water molecule, the internal H-bond between O2···H8-N was slightly extended, i.e., 1.85Å, and between the water molecule and C=O1···H was 1.75Å. In the complex formed by glycine with 2 water molecules, the internal H-bond is lost, forming a C=O1···H with the water molecule, which is 1.75Å, and with the second water molecule, C=O2···H, which is 1.8Å, and H8-N···O, which is 1.78Å. In the complex formed by glycine with 3 water molecules, C=O1···H, which is 1.75Å, C=O2···H, which is 1.85Å, C=O2···H, which is 1.86Å, H8-N···O, which is 1.78Å, and there is also an O-H···O H-bond between the water molecules, which is 1.95Å. In the complex formed by glycine with 4 water molecules, the C=O1···H bond length is 1.81Å, the C=O1···H bond length with the next water molecule is 1.71Å, the C=O2···H bond length is 1.87Å, the C=O2···H bond length is 1.9Å, the N-H8···O bond length is 1.76Å, and the H-bond length between the water molecules is 1.92Å. In the complex formed with glycine and 5 water molecules, it was found that the C=O1···H, 1.8Å, with the next water molecule, C=O1···H, 1.77Å, C=O2···H, 1.87Å, C=O2···H, 1.86Å, N-H8···O, 1.83Å, N-H9···O, 1.78Å, and the H-bond length between water molecules changed to 1.93Å. In the complex formed by glycine with 6 water molecules, it was found that C=O1···H is 1.8Å, C=O1···H is 1.76Å, C=O2···H is 1.86Å, C=O2···H is 1.83Å, N-H8···O is 1.87Å, N-H9···O is 1.82Å, N-H10···O is 1.78Å, and the H-bond length between water molecules is unchanged at 1.93Å. The complex formed by glycine with 7 water molecules differs from the other complexes in that it began to interact with water, C=O1···H, 1.8Å, C=O1···H, 1.76Å, C=O2···H, 1.86Å, C=O2···H, 1.84Å, N-H8···O, 1.87Å, N-H9···O, 1.82Å, N-H10···O, 1.89Å, and the H-bond lengths between water molecules were found to be 1.93Å and 1.95Å. In the complex formed by glycine with 8 water molecules, the H-bond lengths between C=O1···H are 1.94Å, C=O1···H is 1.77Å, C=O2···H is 1.86Å, C=O2···H is 1.84Å, N-H8···O is 1.86Å, N-H9···O is 1.93Å, N-H10···O is 1.79Å, and the H-bond lengths between water molecules are 1.93Å, 1.8Å, 1.79Å, and 1.9Å. In the complex formed by glycine with 9 water molecules, the H-bond lengths between C=O1···H are 1.74Å, C=O1···H is 1.77Å, C=O2···H is 1.86Å, C=O2···H is 1.85Å, N-H8···O is 1.88Å, N-H9···O is 1.93Å, N-H10···O is 1.79Å, C-H7···H is 2.3Å, and the H-bond lengths between water molecules are 1.93Å, 1.79Å, 1.89Å, 1.8Å, and 1.74Å. In the complex formed by glycine with 10 water molecules, it was found that the H-bond lengths of C=O1···H are 1.75Å, C=O1···H is 1.77Å, C=O2···H is 1.87Å, C=O2···H is 1.77Å, N-H8···O is 1.96Å, N-H9···O is 1.92Å, N-H10···O is 1.78Å, C-H7···H is 2.3Å, and the H-bond lengths between water molecules are 1.93Å, 1.85Å, 1.79Å, 1.9Å, 1.8Å, and 1.75Å. The geometric parameters of molecular clusters that can be formed with glycine and up to 10 water molecules were determined.

Table 1.

The total energies (E) and dipole moments of Gly+n·(H₂O) (n=1-10) clusters

The results showed that the cluster formed by Gly with 7 water molecules was found to be relatively energetically stable compared to the remaining clusters, and the hydrogen bond energy between Gly and water was higher than that between water and water.

3.3. Vibrational analysis

If we look at the band corresponding to the C=O group, it corresponds to 1668 cm^-1 in the monomeric state, while if we look at the clusters formed with Gly and up to 10 water molecules, it can be seen that with an increase in the number of water molecules, it shifts to a lower frequency of 87 cm^-1 (Figure 4). In the experiments, the C=O band in the Raman spectra shifted to 1670 cm⁻¹, and to a lower frequency of 36 cm^-1 in the aqueous solution. This situation was also confirmed by the analysis of the calculated spectra. It is reasonable to say that the mechanism of cluster formation is due to hydrogen bonding between Gly and water molecules.

Figure 4. Dependence of the C=O stretching vibration frequency on the number of water molecules in the clusters

The experimentally obtained IR absorption spectra of Gly had a band maximum corresponding to 1612 cm^-1 shifted to a higher frequency by 14 cm^-1 (1626 cm^-1) in aqueous solution. If we look at the analysis of the calculated spectra, it can be seen that while the asymmetric bending vibration belonging to the NH₃ group in the monomer corresponds to 1634 cm^-1, in the clusters formed with water molecules, it shifts to a higher frequency by up to 92 cm^-1 with an increase in the number of water molecules (Fig. 5). Considering that the depolarized asymmetric bending vibration state is active precisely in the IR absorption spectra, the high frequency shift can be explained by the nonclassical hydrogen bond of the N-H···O type between glycine and the water molecule.

Figure 5. Graph of the dependence of the asymmetric bending vibration frequency of the NH₃ on the increase in the number of water molecules in the clusters

3.4. Mulliken atomic charge analysis

Atomic charges in molecules are used to investigate the electronegativity of atoms and to describe charge transfer processes in chemical reactions, as well as to model the electrostatic potential outside molecular surfaces [15,16]. Table 2 presents the Mulliken atomic charges for the Gly monomer and Gly+(H2O)10 clusters. According to the table, the atoms O1, O2, N3 and C4 of the Gly molecule are negatively charged, while the remaining carbons C5, H6, H7, H8, H9, H10 are positively charged, which shows the proton-donor and proton-acceptor properties of the molecule. Therefore, the possibility of these atoms participating in the interaction is high, and the charge modulus of the atoms of the Gly molecule is stabilized in the zwitterion state with an increase in the number of water molecules in the cluster.

Table 2.

Mulliken atomic charges of Gly in Gly+n·(H2O) (n=1-10) clusters

3.5. Molecular electrostatic potential surface (MEPS) analysis

Molecular electrostatic potential surface (MEPS) analysis is useful in determining the electron distribution in molecular systems, the electrophilic and nucleophilic domains for chemical reactions, and the points of intermolecular interaction [17-21]. The electrostatic potential (U(r)) of a molecular system depends on the electron density () on the atoms and is determined by the following relationship:

                                                      (1)

where r' is the charge distribution vector that generates the electrostatic potential at any point r. Figure 7 shows the MEP diagram for Gly and its aqueous clusters. In the surface diagram, nucleophilic regions are shown in blue and electrophilic regions are shown in red. The order of decreasing potential value is presented in the sequence blue>green>yellow>orange>red. The nucleophilic region on the hydrogen atoms (H8, H9 and H10) bonded to the nitrogen atom in the Gly monomer molecules is shown in blue. The electrophilic region is represented by the red area around the oxygen atoms (O1, O2). The green regions indicate the zero potential and neutral regions. According to the results of the molecular electrostatic potential surface (MEPS) analysis, the nucleophilic and electrophilic zones continue to be preserved even when the number of water molecules in the aqueous clusters of Gly increases.

Figure 7. MEP analysis for Gly-(H₂O)_n (n=1-10) clusters

3.6 Non-covalent interaction (NCI) and reduced density gradient (RDG) analyses

Noncovalent interactions (NCI) and reduced density gradient (RDG) analysis are widely used methods for characterizing weak intermolecular interactions [22-23]. The NCI index is used to characterize intermolecular interactions and assess the nature of weak interactions. The NCI index is based on the reduced density gradient (RDG) and provides more information in the study of noncovalent interactions. The reduced density gradient (RDG) is a fundamental dimensionless quantity consisting of the density and its first derivative, and is expressed by formula (2) [23]:

                                                     (2)

The determination of the electron density of the   with respect to the RDG provides information about the nature of intermolecular interactions and the magnitude of these interactions. In molecular systems, blue indicates mutual attraction, while red indicates repulsion.  is important in predicting the nature of the interaction: for example,  <0 indicates repulsion between bonded atoms, while  >0 indicates repulsion between unbonded atoms.

The right side of Figure 8 shows the RDG scattering plot of the Gly monomer and its complexes (Gly-(H₂O)_1-10) formed with up to 10 water molecules. According to the expression given in the upper part of the figure, red color represents strong repulsive forces (steric or cyclic effect), blue color represents H-bonding, and green color represents the presence of Van der Waals interactions. According to the results, there is an effect between oxygen and hydrogen in the Gly monomer, it can also be seen from the RDG scattering plot that the value of the scattering sign(λ₂)ρ is equal to -0.04, which is related to H-bonding in this range, and is equal to 0.03, which is also related to simultaneous repulsive forces. It can be seen that the repulsive forces in the Gly-(H₂O) complex decrease sharply while the H-bonding with the water molecule is maintained. In the Gly-(H₂O)₂ cluster, the value of sign(λ₂)ρ is -0.04, indicating that the internal H-bonding of the Gly molecule with H-bonding between the water molecules and the Gly molecule decreases, and Van der Waals interactions and repulsive forces appear in the RDG scattering plot. In the Gly-(H₂O)₃ complex, the value of sign(λ₂)ρ is between -0.04 and -0.03, indicating that the internal H-bonding of the Gly molecule with H-bonding between the water molecules and the Gly molecule decreases, and Van der Waals interactions and repulsive forces appear in the RDG scattering plot. In the RDG scattering plots, the values ​​of sign(λ_2 )ρ in the Gly+(H₂O)₄ to (H2O)6 complexes range from -0.04 to -0.03, indicating that H-bonding exists between water molecules and Gly molecules and repulsive forces arise. In the Gly-(H₂O)₇ to Gly-(H2O)10 complexes, the value of sign(λ₂)ρ is maintained, and with the increase in the number of water molecules, the H-bonding between molecules increases, while the Van der Waals interactions increase, and no significant change in repulsive forces is observed, as can be seen in the RDG scattering plot. In general, the main reason for the formation of Gly-(H₂O)_1-10 clusteres is H-bonding. At the same time, it can be seen that the probability of H-bonding increases with the increase in the number of water molecules. It can be concluded that the main reason for the compression of the polysaccharide in the experiment is the H-bonding between Gly and water molecules.
 

Figure 8. NCI and RDG analyses for Gly-(H₂O)_n (n=1-10) clusters

3.6. Atoms in molecules (AIM) analysis for Gly-(H₂O)_n (n=1-10) clusters

Atoms in molecules (AIM) analysis is widely used to determine noncovalent interactions in molecular systems, especially intramolecular and intermolecular hydrogen bonds. Due to the electron density, a bonding path appears between two interacting atoms, which creates critical points (CPs) in the electron density. At these critical points, the electron density gradient disappears (∇ρ(r) = 0). In the monomeric state of Gly, internal H-bonding through N-H...O can be observed, while the clusters formed by Gly and its water molecules form mainly through H-bonding interactions. The AIM analysis was performed at the DFT: B3LYP/6-311++G(d,p) level and IEFPCM was used to take into account solvent effects. Figure 1 shows the molecular diagram for Gly and its water molecules. The orange dots in the diagram represent bond critical points (BCPs), and the yellow dots represent ring critical points (RCPs). Table 3 lists some topological parameters in the BCP.

Figure 9. The AIM molecular diagrams of glycine-(H₂O)_n (n=1-10) clusters

Table 3.

Topological parameters of MEK-(H₂O)₂ clusters

The presence of hydrogen bonds or other interactions in molecular complexes is confirmed by topological analysis of electron density. Table 3 lists the topological parameters for Gly-(H₂O)₂ cluster: H-bond length, electron density ρ(r), electron density Laplacian ∇²ρ(r), energy density H(r), Lagrangian kinetic energy density G(r) and potential energy density V(r) in BCPs, which provide a lot of information about the nature of hydrogen bonding (Table 3). [33] According to the literature, the hydrogen bonding interaction can be calculated as follows:

  and kcal/mol – weak hydrogen bond;

  and kcal/mol – moderate hydrogen bonding;

  and kcal/mol – strong hydrogen bond.

Also,

                                                            (3)

The hydrogen bond energy can be determined using the formula [34]. The electron density in BCPs has values ​​in the range of 0.0040–0.0534 au, which corresponds to the H-bond region (0.0033–0.168 au). The Laplacian electron density lies in the range of 0.0144–0.1447 au. According to formula (1), the H-bond energy is in the range of 0.72–16.5 kcal/mol, and strong, medium, and weak H-bonds are present, as well as Van der Waals interactions. In all other complexes, the total energy density in H-bonds is positive (H>0). This means that the electrostatic effect plays a dominant role in such hydrogen bonds.

AIM analysis revealed that the H-bond energy between the atoms of the molecules involved in cluster formation, the bond lengths, and the direct effect of the Gly with 1-7 water molecules. Topological analysis showed that the H-bond energy between Gly and the water molecule is larger than that between the water molecules in the cluster and the water molecules.

Conclusions

Infrared (IR) and Raman spectral analyses showed that the Gly molecule undergoes significant spectral changes in aqueous solution. In the IR spectrum, a broad band corresponding to the stretching vibrations of the NH₃ group was observed at 3170 cm⁻¹, and a shift of the NH₃ bending vibration band from 1612 cm⁻¹ to 1626 cm⁻¹ to a higher frequency was noted. This shift is explained by the formation of strong hydrogen bonds of the N–H···O type between the NH₃ group of Gly and the oxygen atoms of the water molecules.

In the Raman spectrum, a shift of the C=O stretching vibration band in pure Gly from 1670 cm⁻¹ to a lower frequency in aqueous solution was observed from 1624 cm⁻¹ to a lower frequency in aqueous solution. This is due to the formation of hydrogen bonds between the solute and solvent molecules, and the decrease in the intensity and change in the half-width of some bands indicates the presence of intermolecular interactions of the van der Waals type.

In the O-H vibration region (3000–3500 cm⁻¹), broad bands due to the interaction of water molecules were detected, and the NH₃ stretching vibration of Gly in this region was not distinguished by the background effect. This indicates that water molecules formed a strong hydrogen bond network around Gly.

DFT calculations and cluster analysis showed that the most stable state in the Gly–(H₂O)₁–₁₀ clusters was a cluster formed by seven water molecules. Also, the fact that the Gly–water hydrogen bond energy was higher than the water–water hydrogen bond energy confirmed that the Gly molecule has a strong binding property with the solvent.

Mulliken charge analysis revealed that the O₁, O₂, N₃ and C₄ atoms were negatively charged, and the C₅ and H atoms were positively charged. An increase in the number of water molecules leads to an increase in the charge modulus of the atoms, and as a result, the zwitterionic state of Gly is stabilized.

MEPS analysis showed that nucleophilic and electrophilic zones in Gly clusters are preserved even with an increase in the number of water molecules, which indicates the strength of the hydrogen bonding network. RDG analysis showed that the hydrogen bonding density and van der Waals interactions increase with an increase in the number of water molecules, but there are no significant changes in the intermolecular repulsive forces.

The results of AIM topological analysis clearly showed the N–H···O type hydrogen bond energies and bond lengths between Gly molecules and water molecules in the clusters. According to the analysis results, the Gly–water hydrogen bond energy is higher than the water–water bond energy, which confirms the active participation of Gly as a proton donor and proton acceptor in the formation of the cluster.

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