Hydrogenic Stretch Spectroscopy of Glycine-Water Complexes

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Hydrogenic Stretch Spectroscopy of GlycineWater Complexes: Anharmonic AICSP Calculations Lior Sagiv, Barak Hirshberg, and Robert Benny Gerber J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b05378 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

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The Journal of Physical Chemistry

Hydrogenic Stretch Spectroscopy of Glycine-Water Complexes: Anharmonic AICSP Calculations Lior Sagiva, Barak Hirshberga,1 and R. Benny Gerbera,b* a) Institute of Chemistry and the Fritz Haber Center for Molecular Dynamics, the Hebrew University, Jerusalem 9190401, Israel b) Department of Chemistry, University of California, Irvine, California 92697, USA ABSTRACT The anharmonic frequencies of the O-H, C-H and the N-H stretching modes of hydrogen-bonded glycine-H2O complexes are calculated using the ab initio Classical Separable Potentials approximation. In this approach, ab initio molecular dynamics simulations are used to determine an effective classical potential for each of the normal modes of the system. The frequencies are calculated by solving the time-independent Schrödinger equation for each mode using time-averaged potentials. Three complex structures are studied, that differ in the location of the water molecule on the amino acid. Significant differences are found between the spectra of the three structures and signatures of individual complexes are established. It is demonstrated that anharmonic effects are essential in the discrimination between the different structures, while frequency differences at the harmonic level are much smaller. Intensities are also computed and found to carry information on differences between structures, but the role of anharmonicity in this is small.

Current address: Department of Chemistry and Applied Biosciences, ETH Zurich, 8092 Zurich, Switzerland and Institute of Computational Sciences, Università della Svizzera italiana, via G. Buffi 13, 6900 Lugano, Switzerland 1

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1. Introduction Water can have a crucial influence on the structure and function of biologic molecules in solutions1,2. As many biological processes occur in aqueous environment3, it is essential to understand the behavior of biological molecules, or of building blocks of biological molecules in water environment. In particular, the effect of the hydrogen bonding between the biomolecule and water on the vibrational spectroscopy of the system is of substantial interest4,5. Interactions of glycine with water have been studied previously using various approaches, both theoretically and experimentally. Theoretical studies of glycine-water complexes investigated the interaction strength and the preferred binding site of the water6–8, and the electronic and vibrational properties of the glycine-water hydrogen bonds9. The intermolecular hydrogen bonding interactions are found to be very strong and to affect vibrational frequencies and infrared intensities of both the glycine and the water molecule to a very large extent. The microsolvation of neutral and zwitterionic glycine was also studied computationally10,11. Experimental studies used Fouriertransform infrared (FTIR) spectra of matrix-isolated complexes12,13, supersonic jet methods combined with Fourier-transform (FT) microwave spectroscopy14, Raman spectroscopy15, photoelectron spectroscopy16,17 and helium droplet IR spectroscopy18 to investigate structures and other properties of hydrated glycine. Vibrational spectroscopy is an essential tool in the study of hydration of biomolecules19–22. Much information can be obtained in cryogenic conditions, in which hydration and hydrated species are often studied experimentally, e.g., in cryogenic matrices23–27 or in molecular beams28–32. Such experiments typically result in sharply defined, well-resolved features, but, at the same time, have implications relevant also for ambient conditions. On one hand, it is obviously important to


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understand properties at biologically relevant temperatures. On the other hand, fundamental spectroscopic information is inevitably lost in the presence of more than a few molecules, or at room temperature, where broadening effects can be strong. Thus, it is useful to treat spectroscopy both at low temperatures and at room temperature in order to have the advantages of both regimes. The vibrational methodology employed should desirably be suitable for both of these regimes. From the computational point of view, classical ab initio molecular dynamics (AIMD) methods are widely employed in studying molecular systems and, particularly, biomolecular processes33– 35.

These methods include, among others, the Born-Oppenheimer molecular dynamics (BOMD)

approach36 and the Car-Parrinello molecular dynamics (CPMD) method37. In contrast to pure classical molecular dynamics simulations, which employ parametrized empirical potentials to model the interatom interactions, these methods derive the force fields on-the-fly from accurate electronic structure calculations38. Yet, the nuclear dynamics is treated classically, by integrating the classical equations of motion to compute the evolution of the system. While the neglect of nuclear quantum mechanical effects, such as zero-point energy, tunneling or quantum interference can be justified for heavy atoms or at relatively high temperatures, for light atoms at low temperatures these effects play an essential role in the dynamics. A full solution of the timedependent Schrödinger equation (TDSE) is currently limited to no more than a few atoms due to the exponential dependence of the solution’s complexity on the number of degrees of freedom39. Many alternative approaches have been devised for the approximate quantum-mechanical treatment of larger molecular systems. Some of these are time independent, such as the vibrational self-consistent (VSCF) method40–45 and its perturbation-theory-based extension, the VSCF-PT2, and the second-order vibrational perturbation theory (VPT2)46,47. Time dependent methods include semiclassical approaches such as the initial value representation method48,49 or Gaussian wave 3 ACS Paragon Plus Environment


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packet based approaches50–54. The ab initio classical separable potentials (AICSP) method, used in the current study, belongs to the wide class of approximate, time-dependent, quantum mechanical methods, comprising, among many others, the path-integral-based methods, like the centroid molecular dynamics (CMD)55,56 and the ring-polymer molecular dynamics (RPMD)57,58. The bases of the AICSP method are the time-dependent self-consistent field (TDSCF) approach59– 62

and the time-dependent multi-configurational Hartree method (MCTDH)63–65, which are based

on the description of the wavefunction as an expansion in Hartree products, and employ the DiracFrenkel variational principle to solve the equations of motion. As will be shown below, the AICSP method approximates the multidimensional integrals of the TDSCF method by an average value obtained from classical trajectories simulations66,67. AICSP is a specific case of the more general CSP method, which was applied for a range of problems, such as ultrafast dynamics in large noblegas clusters68–70, electron photodetachment from (C60-)71 and others72,73. Recently, a timeindependent variant of the AICSP method was employed for several polyatomic systems, including several amino acids, by solving the Schrödinger equation for the anharmonic, time-averaged CS potentials74,75. Another application is Fourier-transforming the time dependent CS potential75. The resulting spectrum serves as a measure for the coupling strength between the modes of the system. An important aim of this paper is to establish whether vibrational spectroscopy can distinguish between different structures of the glycine-water complex, and thus identify each of them. Specifically, it is of much interest to establish whether vibrational spectroscopy can lead to determination of the sites where the water molecule is found in the different structures of the complex. Finally, a question arises as to the role of anharmonic effects in the spectroscopic analysis, and in addressing the above issues. Many studies indicate that anharmonic effects can be important in relating spectroscopy to possible structures of biomolecular complexes74,76–78. This 4 ACS Paragon Plus Environment

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topic requires further quantification and benchmarking. We will explore here the role of anharmonic effects for the fundamental glycine-water complex, using AICSP as our computational anharmonic algorithm. We note that the experimental challenge here is essentially similar to that encountered in previous applications of the AICSP method. We had already applications for biomolecular systems such as the guanine−cytosine pair of nucleobases74, for which agreement with experiment was obtained. Furthermore, AICSP was found in several tests to be in good accord with VSCF, and the latter was already applied to similar issues, e.g. for sugar-water complexes78, in cooperation with experiment. The article is structured as follows. Section 2 presents the three glycine-water stationary structures examined in the current study. In Section 3 we briefly describe the methodology used, including an outline of the CSP method. In Section 4, results of the calculations are presented and discussed. Concluding remarks are given in Section 5.



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2. Systems The complex of the lowest-energy glycine conformer with a single water molecule has several minima on its potential energy surface. Three of these structures were investigated in the current study. As shown in figure 1, these structures differ in the position at which the water molecule is bonded. Changes in the structures of the amino acid itself are very small. Geometry optimizations were obtained using the ωB97X-D density functional theory with the aug-cc-pVDZ basis set. The structure denoted (a) was found to be the global minimum, while structures (b) and (c) are 3.8 and 7.0 kcal/mol higher in energy.

Figure 1. Glycine-water structures investigated in the current study. The configurations were optimized at the ωB97X-D/aug-cc-pVDZ level of theory. Blue, red, black and gray sphere represent nitrogen, oxygen, carbon and hydrogen atoms, respectively.

3. Methodologies A. The CSP Method The CSP approximation is primarily based on two presumptions: (i) the quantum-mechanical dynamics characterizing the system under study deviates only moderately from the classical dynamics, and (ii) the coupling between the vibrational normal modes of the system is not very 6 ACS Paragon Plus Environment

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strong, weak enough to justify the representation of the wavefunction for the system as a product of single mode wavefunctions. The implementation of the CSP method then consists of four steps: first, a set of classical initial coordinates and momenta of the system is generated. These initial conditions are sampled either classically, e.g., from Maxwell-Boltzmann distribution, assuming a certain temperature, or simply from the classical harmonic approximation to the vibrational mode, or, alternatively, quantum mechanically, assuming an initial quantum state, where the initial conditions are weighted in proportion to the square of the wavefunction amplitude or sampled from the corresponding Wigner distribution. In the second step, each of these initial conditions is used for computing a classical molecular dynamics trajectory for a relatively short time scale, usually around 1ps, at which the validity of the underlying assumptions is usually guaranteed. At the third step, a time dependent potential is calculated for each of the trajectories, where the displacement Q for the mode under study is taken as an independent variable. This results in a set of a 2-dimensional potentials in the (Q, t) space. These potentials are then averaged to give a separable, time dependent mean-field potential, N

(1) 𝑉𝑗(𝑄𝑗,𝑡) = ∑𝑖 = 1𝑊𝑖𝑉(𝑄𝑖1(𝑡),𝑄𝑖2(𝑡)…,𝑄𝑖j ― 1(𝑡),𝑄𝑗,𝑄𝑖j + 1(𝑡),…,𝑄𝑖𝑛(𝑡)). Here, V̅j is the average, single-mode potential for the jth mode, Wi is the ith trajectory weight, N is the number of MD trajectories, n is the number of modes, and Qij(t) are the jth mode displacements, taken from the ith trajectory at time t. The time-dependent Schrödinger equation is then solved numerically for each of the individual modes, on the same time scale as the classical MD trajectories. The total wavefunction for the system is then constructed as the product of the single mode wave functions, 𝑛

(2) Ψ(𝑄1,…,𝑄𝑛,𝑡) = ∏𝑗 = 1𝜓𝑗(𝑄𝑗,𝑡), 7 ACS Paragon Plus Environment


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constituting the fourth step of the CSP procedure. This approximate wavefunction may serve for the calculation of various dynamical quantities and properties, such as linewidths and lineshapes of vibrational bands, dephasing timescales of vibrational excitations, expectation values of displacements or kinetic energies and many more. It is of interest to note that although the effective potential and related wavefunction are separable, the intermode coupling and energy transfer are implicit in the classic MD propagation. Alternatively, time independent information is derived from the time dependent classical separable potential by time averaging over the propagation period 1 𝜏

(3) 𝑉𝑗(𝑄𝑗) = 𝜏 ∫0𝑑𝑡𝑉𝑗(𝑄𝑗,𝑡), where τ is the averaging time, which must be larger than the vibration period. This results in a time independent effective potential for each of the modes, for which the time-independent Schrödinger equation can be solved. The resulting anharmonic wave functions and energy eigenvalues yield IR and Raman spectra, including frequencies of fundamentals and overtones, as well as intensities. The reliability of the AICSP method was tested extensively in several previous studies, both against other state of the art methods such as VSCF, and against experiments. This includes tests for several amino acids, the G-C complex of nucleic acid bases74, and several carbonic acid structures and complexes75. Both the tests against experiment, and the tests against VSCF demonstrate that the method gives frequencies of good accuracy. Comparisons against VSCF, a purely quantum mechanical method considered widely to be of state-of-the-art accuracy for anharmonic calculations, shows that the two methods generally give very similar frequencies. In particular, the mean field treatment takes into account the influence of the other modes in calculating the wavefunction of a given mode, despite the substantial coupling between different 8 ACS Paragon Plus Environment

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modes, as was verified by the quality of results in previous applications and test calculations, some of which involving strong H-bonds74,75. B. Computational Details The current study explores the anharmonic effects for the hydrogenic stretches of the IR spectrum of the glycine-water complex structures, using the time-averaged potentials, as described in the last paragraph of section A. The calculation is aimed to simulate the dynamics at a temperature as close as possible to 0 K, where the system is at its ground state. Following previous studies74,75, the initial conditions were sampled classically, modelling each of the normal modes as a classic harmonic oscillator of the corresponding frequency. The equipartition of energy was introduced by providing each mode with an equal energy, chosen to be high enough for the anharmonic effects to influence the dynamics, but, at the same time, not too high to allow any conformational changes or large energy flow between different modes, equivalent to a temperature of the order of magnitude of 0.01 K. For the optimization of the glycine-water complex configurations, as well as for the calculation of the effective classical potentials themselves, we used the ωB97X-D hybrid density functional79 with the aug-cc-pVDZ basis set80. This is a dispersion-corrected functional which was found in previous studies to describe reliably the structure and energetics of several molecular systems, such as amino acids, guanine-cytosine pair of nucleobases74 and carbonic acid and its complexes, including isotopologues75. The optimization stage results in the harmonic frequencies, and in the eigenvectors of the projected mass-weighted Hessian matrix, serving as the Cartesian-to-normal coordinates transformation matrix, in addition to the optimized configuration. 10 classical MD trajectories were performed for each of the configurations, using a time step of 20 atomic units (0.484 fs), for a total time of 1.2ps. The time step was chosen to be short enough



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relative to the period of the highest frequency mode, about 8.4 fs for the O-H stretch. At the same time, the total simulation time was chosen to be long enough relative to the longest period, about 11.2 fs for the lowest-frequency hydrogenic stretching mode considered, in order to verify the validity of the time-averaging of the effective potentials. The CS potentials were constructed on a spatial-temporal grid using equation (1) with equal weighting for all of the trajectories, as the proper weighting is implicit in the prior sampling. The spatial axis spans 10 standard deviations of the ground state harmonic probability distribution for the mode under study, distributed over 17 equally spaced spatial points. In the temporal axis we used a time step of 48 fs, distributed over 1.2ps, resulting in 25 time points. The adequacy of these parameters was verified in previous studies74,75 and the numerical error due to variations in the number of trajectories or the grid parameters lies within ±15 cm-1. A more comprehensive description of the numerical implementation of the time-averaged CSP procedure can be found in previous publications74,75,81.


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4. Results and Discussion There are seven hydrogenic stretches in the glycine-water complex spectrum, all of which were investigated in the current study. The lowest energy one is the symmetric CH2 stretching vibration, while its counterpart, the asymmetric mode is the next one above it. As seen in figure 2, these two modes manifest different behavior regarding the anharmonic effects. Concerning the symmetric mode, the harmonic frequencies are exactly identical between the different structures, while the anharmonic vibrations are red-shifted by about 90 cm-1 and, similarly, are virtually identical to each other, with maximum difference of 7 cm-1. However, for the asymmetric mode, the anharmonic frequencies are significantly different among the three structures, while the harmonic frequencies are very close to each other. While for structure (b) no anharmonic effect is observed, structures (a) and (c) show a red shift of 24 and 52 cm-1, respectively. This is an interesting

Figure 2. AICSP anharmonic vs. harmonic vibrational energies for the CH2 symmetric and asymmetric stretches (inset).



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spectroscopic near-degeneracy at the harmonic level, which the anharmonic interactions seem to split. The next mode, in order of the harmonic frequency, is the NH2 symmetric vibration, except for structure (a), for which the glycine’s O-H stretch is less energetic. This mode shows the largest red-shift of the harmonic frequencies, relative to the anharmonic frequencies. As shown in figure 3, the harmonic frequencies are fairly close to each other, and vary between 3532 and 3544 cm-1. In contrast, the anharmonic frequencies vary considerably, between 3374 cm-1 for structure (a) and 3434 cm-1 for structure (b). The large anharmonicity characterizing this mode, expressed in a red-shift of the anharmonic frequencies of 163 and 159 cm-1 for structures (a) and (c), respectively, is cancelled partially in structure (b) by the hydrogen bond between the water’s oxygen and one of the glycine’s hydrogens, resulting in a smaller shift of 98 cm-1.

Figure 3. AICSP anharmonic vs. harmonic vibrational energies for the NH2 symmetric and asymmetric stretches (inset).


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The harmonic frequencies for the asymmetric NH2 mode are virtually identical, and vary between 3621 and 3624 cm-1. This mode is considerably less anharmonic than its counterpart. As for the symmetric mode, structure (a) shows the largest anharmonic red shift, while structure (b) shows the smallest one, of 64 and 19 cm-1, respectively. On the other hand, in contrast to the symmetric NH2 mode, structure (c) shows a small anharmonic shift of 25 cm-1, almost identical to the one of structure (b).

Figure 4. AICSP anharmonic vs. harmonic vibrational energies for the glycine’s O-H stretch (inset). Figure 4 shows a very large difference of 385 cm-1 between the harmonic frequencies of the glycine’s O-H stretch of structure (a) and those of structures (b) and (c), due to the hydrogen bond between the oscillating hydrogen and the water’s oxygen in structure (a). While the harmonic frequencies for this mode are virtually identical between structures (b) and (c), the hydrogen bond between the glycine’s oxygen and the water’s hydrogen affects considerably the anharmonic frequency in structure (c), resulting in a large red shift of 263 cm-1. In addition, this bonding 13 ACS Paragon Plus Environment


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couples the O-H stretch of glycine and the symmetric H-O-H stretch of water, and, as expected, breaks the symmetry between the hydrogens of H2O, as indicated by the arrows denoting the displacements of the atoms in figure 4. Nevertheless, the anharmonic nature of this mode is revealed in structure (b) in a large anharmonic red shift of 165 cm-1. Concerning structure (a), the anharmonic red shift for this mode shows an intermediate value between those of structures (b) and (c), and the AICSP-based frequency agrees well with the experimental data obtained by refs. 12 and 13, as indicated in table 1. The two remaining modes of the glycine-water complex are associated with the water molecule. Obviously, the H2O symmetry is broken due to the glycine-water interaction, and the extent of the symmetry breaking varies between structures (a), (b) and (c). Figure 5 presents the anharmonic against the harmonic frequencies for the water modes. In structure (a), the symmetry breaking between the water modes is expressed in a considerable difference between the O-H oscillation amplitudes, yet, the phase-antiphase characteristics are maintained. A similar behavior is observed in structure (c), though a weak coupling exists between the glycine’s O-H stretch and the symmetric mode of the water, as mentioned above and can be seen in figure 4. In structure (b), the hydrogen bonding between the glycine’s oxygen and one of the water’s hydrogens results in a complete decoupling between the water’s O-H stretches. Obviously, the lower frequency mode is the one associated with the hydrogen bonded H-atom of the water. This mode is lower in frequency also than the free O-H stretch of the glycine. The highest frequency mode of the whole complex is the free O-H stretch of the water. Regarding the anharmonic effects, while the harmonic difference between the lower frequency modes of structures (a) and (b) is 37 cm-1, the anharmonic frequencies are almost identical. The anharmonic red shifts, 160 and 194 cm-1, respectively, reflect this difference. Structure (c) shows the largest anharmonic shift for this mode, 205 cm-1. 14 ACS Paragon Plus Environment

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Conversely, while the harmonic frequencies for the most energetic mode are close to each other and vary between 3953 and 3976 cm-1, the anharmonic shifts change considerably between the different structures. While for structure (a) the anharmonic shifts are similar for the water’s stretching modes, in structure (c) the asymmetric mode is considerably less anharmonic (shift of 98 cm-1 compared to 205 cm-1 for the symmetric mode). The situation is reversed in structure (b), for which, while the hydrogen-bonded stretch shows a moderate anharmonic shift of 165 cm-1, the “free” stretching shows the largest red shift, reaching 305 cm-1. We note that the AICSP red shifts underestimate the experimental shifts for the water modes of structure (a), reported in ref. 12, as shown in table 1.

Figure 5. AICSP anharmonic vs. harmonic vibrational energies for the water molecule stretches (inset). The lower frequencies (3700 – 3850 cm-1 in the harmonic range) are associated primarily with the bonded O-H stretch, while the higher ones (3950 – 4000 cm-1) are associated with the free stretch, though only structure (b) shows a complete decoupling between the O-H stretches of the water. 15 ACS Paragon Plus Environment


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An interesting phenomenon, observed in all of the studied structures, is related to the order of the frequencies of the modes. While in the harmonic case, the asymmetric NH2 stretch has a lower frequency than all of the O-H stretches, except for the lower frequency O-H stretch of the glycine, in structure (a), the order is reversed in the anharmonic calculation, and the lower frequency mode of the water molecule in structures (a) and (b), and the O-H stretch of the glycine in (c), are less energetic than the NH2 mode. Several of the above mentioned anharmonic effects may provide useful information required for the distinction between the different glycine-water structures, in contrast to the harmonic spectrum, for which the frequency differences are small. The first mode to show a clear configuration-dependent anharmonic frequency is the asymmetric CH2 stretch, as shown in figure 2. Unlike the symmetric CH2 mode, for which the anharmonic frequencies vary between 2978 and 2985 cm-1, the range of frequency variation for the asymmetric mode is much wider, between 3074 and 3125 cm-1. Similarly, both of the NH2 stretching modes of the glycine show a wide range of anharmonic frequency variability, 3374 to 3434 cm-1 for the symmetric and 3558 to 3602 cm-1 for the asymmetric mode (figure 3). We note that concerning the latter, several bands exist in the anharmonic spectrum in the vicinity of the symmetric NH2 frequencies, which may obscure the underlying distribution of structures. These include the lower frequency H-O-H stretch of the water, and the O-H vibration of the glycine. As mentioned before, the most prominent harmonic/anharmonic difference is manifested by the highest frequency, asymmetric mode of the water (right-hand side of figure 5). While the harmonic spectrum shows a narrow distribution of frequencies, 23 cm-1 in width, the anharmonic frequencies 16 ACS Paragon Plus Environment

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are distributed nearly evenly between 3665 and 3878 cm-1. It seems that this mode provides the clearest signature by which to distinguish between the structures of the complex. All frequencies are summarized in table 1.

(a) mode type

harm.

CSP

(cm-1)

(cm-1)

CH2, sym. CH2, asym.

3071 3118

2978 3094

O-H, glycine

3442

3238

NH2, sym. NH2, asym. O-H, water, bonded O-H, water, free

3543 3622

3374 3558

3698

3538

3953

3765

(b)

(c)

harm.

CSP

harm.

CSP

(cm-1)

(cm-1)

(cm-1)

(cm-1)

3071 3120

2981 3125

3071 3126

2985 3074

3825

3660

3826

3563

3532 3621

3434 3602

3544 3624

3391 3599

3410b

3735

3541

3838

3633

3690b

3970

3665

3976

3878

exp. (cm-1)

3205a 3220-3240b

Table 1. Vibrational frequencies (cm-1) of the hydrogenic stretching modes of glycine-water structures at the harmonic and anharmonic AICSP levels, along with experimental results for the O-H stretches of structure (a). a ref. 13. b ref. 12. Figure 6 compares the CSP based anharmonic peak intensities to those extracted from the harmonic calculation. All of the intensities were normalized relative to the one associated with the asymmetric stretch of the water molecule of structure (a). Basically, a significant variation is found between the peak intensities distribution of the 3 structures studied. Like the frequency difference, the intensities variation too yields a unique pattern associated with each of the glycine-water structures. However, as can be seen in the figure, most of the anharmonic intensities are virtually identical to their harmonic counterparts, and anharmonicity affects only slightly the peak intensities. Some exceptions are the symmetric mode of the water and the O-H stretch of the glycine of structure (c), whose anharmonic intensities deviate in 7.5% and -9.2% relative to the



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corresponding harmonic intensities, and the O-H stretch of the glycine in structure (a), the intensity of which is larger than its harmonic counterpart by 9.9%.

Figure 6. AICSP anharmonic vs. harmonic vibrational peak intensities for the glycine-water structures. The intensities are normalized relative to the asymmetric stretching mode of the water molecule of structure (a). As a concluding remark, we note that several glycine conformers were recognized, each of which can interact with water molecule to form different glycine-H2O structures. While the current study focuses on the lowest energy, most populated glycine conformer, revealing the complete picture requires considering the higher-energy structures as well. We hope to pursue this in a future study. 18 ACS Paragon Plus Environment

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5. Conclusions The results presented in this paper demonstrate the importance of pursuing anharmonic calculations when attempting to determine structures of biomolecule-water complexes from IR spectroscopic data. It is evident that anharmonic effects determine a large part of the signatures that can distinguish between different structures of such species. Calculations at the harmonic level only may fail to determine the correct geometry. It further seems that while both frequencies and intensities carry information on differences between structures, accurate frequencies are the most useful data in this respect. Approximate anharmonic calculations, not necessarily of the highest accuracy, as used here, may suffice for the purpose of structure determination of the biomoleculewater complex. Since very many of the biomolecule-water complexes of interest are large and therefore computationally very demanding, the possibility of using a relatively simple anharmonic approximation is therefore of practical importance.

ACKNOWLEDGEMENTS B.H. was supported through an Adams fellowship of the Israel Academy of Sciences and Humanities.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] REFERENCES (1)

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TOC Graphic


Hydrogenic Stretch Spectroscopy of Glycine-Water Complexes

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