N-Methylacetamide Aqueous Solutions: A Neutron Diffraction Study

Publication Date (Web): February 9, 2019. Copyright © 2019 American Chemical Society. Cite this:J. Phys. Chem. B XXXX, XXX, XXX-XXX ...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCB

Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

N‑Methylacetamide Aqueous Solutions: A Neutron Diffraction Study Michael Di Gioacchino, Fabio Bruni, and Maria Antonietta Ricci*

J. Phys. Chem. B Downloaded from pubs.acs.org by UNIV OF SUSSEX on 02/24/19. For personal use only.

Dipartimento di Scienze, Universitá degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy ABSTRACT: The hydration of N-methylacetamide (NMA) in solution has been determined by neutron diffraction with isotopic Hydrogen/ Deuterium substitution (NDIS), augmented by Monte Carlo simulation. This study is representative of the hydration of the peptide bonds characteristic of proteins and might shed light on aggregation phenomena in intrinsically disordered proteins. It is found that NMA forms hydrogen bonds with water at both O and H peptide sites, although of different lengths and strengths. The comparison with the case of tripeptide glutathione evidences differences in both hydration and propensity for aggregation.

■

INTRODUCTION Proteins control almost all processes occurring in cells through well-defined structural parts, responsible for their biological activity.1 In particular, most interactions involving these protein structural parts are guided by the fundamental link holding together amino acid units, that is, the peptide bond (HN−CO). Water is another important player of many biological processes, triggering enzymatic activity and maintaining the native conformation of proteins.2,3 Thus understanding the hydration properties of the peptide bond may be very helpful in order to characterize both structure and function of proteins. Given the complexity of proteins, this can be attempted by steps, the first one being the study of the hydration pattern of a single peptide bond. This can be accomplished considering the molecule N-methylacetamide (NMA), which represents the simplest model system made up by a single peptide bond. In addition, in aqueous solution the NMA molecules interact with each other in a way similar to intrinsically disordered proteins (IDPs).1 Indeed they have one hydrogen-bond-accepting oxygen atom and one donating hydrogen atom, which allows fast forming and breaking of hydrogen bonds (HBs) and rapid fluctuations between different conformations. Consequently, on the one hand knowledge of NMA interactions with water can provide the basis for understanding the geometric constraints, imposed by the peptide bond and determining protein structure and on the other hand can give insight on aggregation phenomena characteristic of IDP.1,4 The preferred peptide bond conformation of the NMA molecule (CH3CONHCH3) in aqueous solution is the trans one, where the CO group (C1−Oc sites in Figure 1) and NH group (N−Hn sites in Figure 1) are at the opposite sides with respect to the C−N−C−C “backbone” chain, as shown in Figure 1.5 Therefore, both groups are exposed to water and in principle available for H-bonding. The NMA can form three HBs with water, two at the CO acceptor group, and one at the NH donor group. The influence of such bonds on the structure and dynamics of NMA has been investigated since © XXXX American Chemical Society

Figure 1. Structure of the NMA molecule and atom labeling for the NMA molecule used in EPSR simulation. The carbon atoms on the NMA molecule are labeled Cn, C1, and Cc according to their different environments; all hydrogens bonded to carbon sites are labeled Mc; N, Hn, and Oc are the labels of the remaining atoms, respectively. The red dashed box highlights the peptide bond HN− CO.

the fifties of the last century by experimental and theoretical techniques, as for instance resonant Raman, infrared, and ultraviolet spectroscopy, classical molecular dynamics, quantum mechanical studies, and ab initio density functional simulations.6−18 In particular, refs 7 and 10 show that the intense amide I vibrational band in the resonant Raman spectrum of NMA vapor disappears in aqueous solution because of the frequency shift brought by H-bonding with water at the Oc site. It is also suggested that the CO intramolecular bond length increases, whereas that of the N− H decreases as a consequence of hydration. Moreover, firstprinciple simulations suggest that the HB between water and the Oc site of the NMA molecule has a longer lifetime than water−water HBs.17 In highly concentrated aqueous solutions or in the pure liquid, NMA can form chain aggregates, with a dynamics which Received: January 9, 2019 Revised: February 7, 2019 Published: February 9, 2019 A

DOI: 10.1021/acs.jpcb.9b00246 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B may suggest similarities with protein aggregation, folding, and misfolding,14,19−23 as suggested, for instance, by the anomalous temperature dependence of the diffusive timescale measured above 380 K by ultrafast spectroscopy.24 It has been proposed by Allison and co-workers,25 by classical molecular dynamics simulations, that also at low NMA concentration in aqueous solution there is a considerable NMA self-association and significative disruption of the bulk water HB network structure. In this context, we have performed a neutron diffraction study with isotopic Hydrogen/Deuterium (H/D) substitution (NDIS)26,27 of aqueous solutions of natural and deuterated NMA at ambient temperature. This study, interpreted by using a Monte Carlo simulation,28,29 gives access to the structural properties of the NMA hydration shell and, in particular, to the H-bonding with water at the Oc and Hn sites. The occurrence of NMA−NMA bonding and dimer formation, chain precursor, is investigated as well. Finally, the peptide hydration has been compared with that found in the case of the tripeptide glutathione (GSH).27

■

MATERIALS AND METHODS Sample Preparation. All solutes have been purchased from Sigma-Aldrich and used without further purification. NMA CH 3 CONHCH 3 (NMAH7) and its isotope CD3CONDCD3 (NMAD7) were dissolved either in H2O or D2O at the concentration of 1 mol of NMA per 15 mol of water, in order to obtain three distinct isotopic mixtures (see Table 1) and exploit the isotopic H/D substitution26,27 on both solvent (water) and solute (NMA). Table 1. Labels and Composition of the Investigated NMA− Water Samples sample label

NMAD7 (molar ratio)

NMAH7 (molar ratio)

D2O (molar ratio)

H2O (molar ratio)

NMAD7 D2O NMAH7 H2O NMAH7/D7 HDO

1 0 0.5

0 1 0.5

15 0 7.5

0 15 7.5

Experimental and Data Analysis. Neutron Diffraction Experiment. NDIS is an efficient method to investigate the structure of aqueous mixtures and in particular the hydration of molecules of biological interest, as neutrons are strongly scattered by hydrogens and can distinguish between its isotopes.30 The latter property is the basis of the NDIS technique exploited in this work, which gives access to richer information, compared to standard neutron diffraction and Xray diffraction experiments. The experiment has been performed at the ISIS spallation source (STFC, UK), on the SANDALS diffractometer,31 at ambient conditions (298 K), using standard Ti−Zr sample containers (1 mm thickness). Each sample, listed in Table 1, has been measured for ∼8 h corresponding to a total proton current at the ISIS target of 1000 μA. Measurements on empty containers, empty instrument and vanadium−niobium standard have also been performed. These data allow to obtain the total interference differential scattering cross section, F(Q), shown in Figure 2, after correction for systematic errors and normalization via the GUDRUN routine.32 The F(Q) is the linear combination of all partial structure factors, Sαβ(Q), where α and β label all atomic pairs present in

Figure 2. F(Q) of the NMA−water solution: data (empty circles), fits (solid thick lines), and residuals (solid thin lines, arbitrarily downshifted by 0.2 b/atom sr). (a) Totally hydrogenated sample; (b) totally deuterated sample; (c) equimolar mixture.

the sample, each weighted by their concentration, c, and scattering length, b,33 so that F (Q ) =

∑ cαcβbαbβ(Sαβ(Q ) − 1) α ,β

(1)

where Q is the exchanged momentum in the scattering event. The structural information, which are the radial distribution functions gαβ(r), is obtained from their Fourier transforms, Sαβ(Q). Thus, the more isotopic substituted samples are available, the better structural information is obtained, provided that the isotopic substitution does not significantly alter the structure of the solution.26 B

DOI: 10.1021/acs.jpcb.9b00246 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B Empirical Potential Structure Refinement Simulation. The experimental F(Q)’s have been modeled using the empirical potential structure refinement (EPSR) code,28,29 that is, a Monte Carlo simulation constrained by the experimental data. The cubic simulation box, used in the present case, has a side equal to 31.52 Å and contains 50 NMA and 750 water molecules, giving the same density and composition as the real samples. We have used in this simulation only the trans NMA conformation, as the cis one represents a negligible minority (≤1.5%).5 The atomic structure of the NMA molecule is shown in Figure 1, along with the labels hereafter assigned to the individual atomic sites. No dihedral angle rotations have been allowed during the simulation, to keep stable the trans configuration of NMA in the simulation box. The parameters of the reference potential used as seed to start the simulation are shown in Table 2.

These SDFs give a three-dimensional (3D) representation of the NMA hydration and neighboring shells. This is accomplished by looking at the regions where the probability to find a water or first neighbor NMA molecule within a given distance from a central NMA molecule is higher than a chosen threshold value.37

■

RESULTS AND DISCUSSION Focusing our attention on Figure 3, we notice that the pair distribution functions of the NMA sites (Hn, Oc) candidate

Table 2. Reference Potential Parameters Used in the EPSR Simulation Box for NMA34 and Water35a molecule

atom type

ϵ (kJ/mol)

σ (Å)

q (e)

NMA

N C1 Cc Oc Cn Hn Mc OW HW

0.71128 0.43932 0.66944 0.87864 0.71128 0 0 0.65 0

3.25 3.75 3.91 2.96 3.80 0 0 3.166 0

−0.55 0.58 0.00 −0.53 0.20 0.30 0 −0.8476 0.4238

water a

Atoms are labeled according to Figure 1. Figure 3. Pair distribution functions of the hydrophilic sites of NMA, namely, Oc and Hn and water atoms (HW and OW). The aggregation of NMA molecules is investigated considering the g(r)’s of the binding sites of NMA molecules. The functions have been shifted for clarity. The gHn−OW(r) (solid) and gNOW(r) (dashed) functions are reported in black (shifted by 1.2); the gOcHW(r) (solid) and gC1HW(r) (dashed) in magenta (shifted by 2.2); the gMcOW(r) (shifted by 3.5) and gNHW(r) (shifted by 5.0) in blue and green respectively; the NMA−NMA radial distribution function, gHnOc(r), is reported in red.

The EPSR code refines the interaction potential model against the experimental data, starting from a molecular configuration equilibrated by using the reference potential. The potential model is updated until the best fit of the data is achieved and a production run can start. The output of this procedure is a set of pair distribution functions, gαβ(r), and a numerical correction to the site−site reference potential. In the present instance, the potential refinement has been switched on after about 1500 iterations, required to bring the simulation box at equilibrium. The quality of the EPSR fits is shown in Figure 2: we notice that the fit is excellent in the case of the fully deuterated sample and that for the two hydrogencontaining samples the main structures of the F(Q) functions are well reproduced, although residuals between data and fit are visible at Q values smaller than 2 Å−1. These are ascribed to the inelastic contributions to the scattering, particularly severe in the hydrogenated samples, which have been not properly corrected. The fit residuals appear as smooth increasing or decreasing functions at low Q, which do not affect the results in r-space in the distance range of interest. At this stage, the simulation box represents a realistic model of the experimental samples. After this, we have recorded 4500 configurations in order to get smooth gαβ(r) functions, which give the density of probability that given an atom α at the origin of the reference frame, a β atom lies at distance r. In this work, we report only those g(r)’s, useful to evidence properties of the NMA hydration shell, the NMA−NMA aggregation, and the influence of NMA on the water structure. In addition, information on the spatial distribution of molecules can be visualized by the spatial density functions (SDFs) obtained by spherical harmonic approximation of EPSR configurations.36

for hydrogen bonding with water at sites OW and HW or with another NMA molecule at sites Hn, Oc show a peak at distances between 1.8 and 2.1 Å. In particular, the first peak of the gOcHW(r) is centered at ∼1.81 Å, and that of the gHnOW(r) at ∼2.04 Å. These distances, in overall agreement with literature values,11,15 are compatible with hydrogen bonding, although of different bond strengths, the Hn−OW bond being longer and thus weaker than the Oc− HW one. The presence of HBs is further supported by the first peaks of the gC1HW(r) and gNOW(r) at ∼2.79 and ∼2.97 Å, respectively. Furthermore, we notice that there is evidence for possible aggregation of NMA molecules (Figure 3 red curve) not mediated by water. Indeed, the intermolecular first peak of the gHnOc(r) is centered at ∼2.04 Å, suggesting a bond length comparable with that of the Hn−OW pair. It is useful to calculate the coordination number, namely, the number of first neighbors of the water oxygens of a Hn site; the number of first neighbors of the water hydrogens of an Oc site, or the number of Oc−Hn first neighbor pairs, according to the definition C

DOI: 10.1021/acs.jpcb.9b00246 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 4. (a) Radial distribution functions of pure water38 (dashed line) and water in the presence of NMA (solid line): gHWHW(r), gOWHW(r), and gOWOW(r) are reported in red, green, and blue, respectively, and the green and blue curves are shifted for clarity by 1.5 and 3. (b) Comparison between the gOWOW(r) functions on a reduced scale, that is, as a function of r/r1, where r1 is the position of the first peak.

Nαβ(R1 , R 2) = 4πρcβ

∫R

R2

1

r 2gαβ (r ) dr

confirmed by the gOWOW(r) function, showing a shift to shorter distances of both first and second peaks. In particular, the ratio between the positions of these latter two peaks is 1.55 in the solution, to be compared with 1.63 (corresponding to the tetrahedral coordination) in pure water,39 as shown in Figure 4b. This is a clear evidence of loss of tetrahedrality of water in the presence of NMA. We have calculated the SDFs around an NMA molecule (see Figure 5), showing the 3D arrangement of hydration water.36

(2)

where R1 is the minimum distance between the atom pair and R2 is the first minimum of the gαβ(r); α is the atomic site at the origin of the reference frame and β is the second atom of the pair. We find that the number of water oxygens bonded to the Hn site is of the order of 0.7, whereas the number of water hydrogens bonded to the Oc site is ∼1.5. This is in agreement with basic chemistry considerations, which imply that there are two binding sites on the Oc and only one on the Hn site.8,15 However, the number of NMA−NMA bonds, given by NOcHn, is of the order of 0.2, suggesting that NMA tends to form dimers also at the low concentration investigated in the present study, as proposed by the theoretical study of Allison’s group.25 Not surprisingly, both the methyl hydrogens (Mc) and the nitrogen (N) do not form bonds with water and their pair distribution functions increase smoothly with distance r, in agreement with the simulations reported in ref 11. In particular the absence of direct N−HW contacts may be due to the steric hyndrances around the N site, whereas the case of the methyls is a signature of the hydrophobicity of the CH3 group. At this point, we focus our interest on the water structure in the investigated NMA solution (Figure 4). As predicted,25 the structure of water in the presence of the NMA solute is clearly changed in comparison with the structure of pure water.38 In general, it is evident that the interaction between first neighbor water molecules is stronger in the presence of NMA, as in all g(r)’s shown in Figure 4a the first peaks move to shorter distances. Moreover, the second peaks are always much broader, suggesting increased disorder in the solution. In particular, the hydrogen bonding between solvent water molecules is stronger compared to pure water, as the first peak of the gOWHW(r) is centered at 1.77 Å, corresponding to a HB length shorter than in pure water (1.86 Å). At the same time, the second peak of the same gOWHW(r) broadens, without moving, suggesting increased disorder in the solute. This is

Figure 5. (a) Yellow regions represent the area occupied by water molecules in the first three hydration shells of NMA in solution. (b) Focus on the first hydration shell of NMA. For the water distribution, the shown regions enclose 15% of the water molecules in the distance range 0−15 Å, for (a), and 0−8.8 Å, for (b), from the central NMA molecule.

The yellow clouds represent the region of space around a central NMA solute, where the probability to find a water molecule exceeds a threshold value. Figure 5a shows all the hydration shells within a distance of 15 Å from the central molecule. This latter distance allows to look at the first three hydration shells on top of the oxygen site and on the bottom of the hydrogen one. The hydrophobicity of the two methyl groups is evidenced by the low probability to find a water D

DOI: 10.1021/acs.jpcb.9b00246 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B molecule on the equatorial plane. Figure 5b shows the first hydration shell on a wider scale. We notice immediately the different sizes and shapes of the two yellow clouds. Indeed, the one around the Oc site is concave and larger than that on the Hn site (convex). This is in agreement with the number of HBs reported above and confirms the symmetry of the molecular orbitals. Therefore, despite its double bond with carbon (C1), the Oc oxygen has a higher tendency to bond with water than the Hn hydrogen. This situation is somewhat reversed for the second and third shells (see Figure 5a). Figure 6 shows the SDF of the N atoms of the NMA solutes, around a central NMA molecule. The N atom on this latter

Figure 7. Pair distribution functions of the hydrophilic sites of NMA, namely, Oc and Hn, and water atoms OW and HW, along with the radial distribution functions describing NMA−NMA bonding, in comparison with the corresponding sites, Op and Hp, of a GSH peptide. Data for the solutions are reported in magenta and black (shifted by 2.0) for the HB site interaction with water and in red (shifted by 3.5) for the NMA−NMA and GSH−GSH interactions. The NMA data are reported as solid lines, the GSH ones as dashed lines. Data for GSH are taken from ref 27 and the atomic labels are assigned according to Figure 1 of that reference.

Figure 6. Purple clouds represent the area occupied by the first neighbors of the nitrogen atoms of the central NMA molecule, in the distance range 0−8.8 Å. The cloud on top of the molecule corresponds to regions where the probability of finding an N atom exceeds 15%, whereas the ring around the Hn atom corresponds to a threshold value of ∼2% to accommodate the intramolecular geometry.

molecule has been chosen as the origin of the reference frame. On the top of the Oc site, there is a hat which suggests the possible formation of dimers resulting from intermolecular Hn−Oc HBs. Because of the choice of the N site as the center of the NMA molecule, the same Hn−Oc bonds give rise to the purple ring region at the bottom of the figure. This configuration is compatible with a first neighbor NMA molecule, exposing its Oc site to the Hn one of the central NMA. This aggregation scheme suggests the formation of NMA H-bonded chains at a much higher solute concentration. Finally, we compare the NMA hydration with that of another model peptide, namely, the tripeptide GSH. In particular, the peptide groups of the two molecules will be compared. The radial distribution functions of GSH in water have been determined after a neutron diffraction experiment by some of us.27 Also, that experiment was performed on SANDALS and analyzed following the same procedure as the present data on the NMA solution. The comparison is reported in Figure 7. The atoms belonging to the peptide groups of GSH are labeled Op and Hp, following ref 27. Notice that the atoms belonging to the two peptide bonds in GSH have the same labels. Interestingly, the HBs on the oxygen sites are stronger and with a sharper length distribution in the case of GSH, given the shorter bond distance and sharpness of the first peak of the gOpHW(r) (magenta dashed line), compared to the gOcHW(r) (magenta solid line). In detail, the first peak position is 1.66 Å for GSH and 1.81 Å in the NMA solution. Conversely, the hydrogen site, Hp, forms fewer and weaker HBs with water, as the gHpOW(r) (black dashed line) shows only a shoulder, instead of the peak at 2.04 Å, visible in the gHnOW(r) (black solid line). Furthermore, the formation of GSH−GSH chains is denied, as suggested by the absence of first Op−Hp contact at distances shorter than ∼2.9

Å. This may be due to the stronger interaction between the oxygen site and water in the case of GSH, with respect to NMA, and to the steric hindrance of its large residuals. This work confirms that NMA is an excellent model to investigate the interactions of the peptide bond with water, whereas the tripeptide GSH, being more articulated and complicated, can be used to approach studies of hydration and folding of peptides and proteins.

■

CONCLUSIONS NMA is a simple model molecule to study relevant aspects of the interactions of the peptide bond with water, avoiding the complexity of folding, shown by larger biomolecules. Indeed, NMA allows to investigate geometrical and structural peptide bond properties in solution. Formation of NMA chains at a high solute concentration21,24 might be of help to investigate aggregation phenomena, mimicking IDPs.1 In this paper, we have determined the structure of low concentration NMA aqueous solutions at the atomic scale and, in particular, the hydration structure of the atomic sites available for HBs with water. We have shown that the Oc site binds with water via a shorter HB in comparison with the Hn site. Also, the average number of HBs formed at the Oc site is almost twice the number of HBs at the Hn site. Then, we have confirmed the hydrophobicity of the methyl groups and of the N site, the latter likely being of steric origin. Finally, we have seen that NMA can form H-bonded dimers and possibly chains also at a low concentration. These dimers are formed by HBs of the same strength as those between NMA and water molecules, suggesting a relative stability of the aggregates, once they are formed. E

DOI: 10.1021/acs.jpcb.9b00246 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

(8) Guo, H.; Karplus, M. Ab Initio Studies of Hydrogen Bonding of N-Methylacetamide: Structure, Cooperativity, and Internal Rotational Barriers. J. Phys. Chem. 1992, 96, 7273−7287. (9) Williams, R. W. Effects of Hydration on Scale Factors for ab Initio Force Constants. IV. Trans and Cis N-Methylacetamide. Biopolymers 1992, 32, 829−847. (10) Markham, L. M.; Hudson, B. S. Ab Initio Analysis of the Effects of Aqueous Solvation on the Resonance Raman Intensities of NMethylacetamide. J. Phys. Chem. 1996, 100, 2731−2737. (11) Gao, J.; Freindorf, M. Hybrid ab Initio QM/MM Simulation of N-Methylacetamide in Aqueous Solution. J. Phys. Chem. A 1997, 101, 3182−3188. (12) Iuchi, S.; Morita, A.; Kato, S. Molecular Dynamics Simulation with the Charge Response Kernel: Vibrational Spectra of Liquid Water and N-Methylacetamide in Aqueous Solution. J. Phys. Chem. B 2002, 106, 3466−3476. (13) Kwac, K.; Cho, M. Molecular Dynamics Simulation Study of NMethylacetamide in Water. I. Amide I Mode Frequency Fluctuation. J. Chem. Phys. 2003, 119, 2247−2255. (14) Torii, H. Vibrational Interactions in the Amide I Subspace of the Oligomers and Hydration Clusters of N-Methylacetamide. J. Phys. Chem. A 2004, 108, 7272−7280. (15) Dannenberg, J. J. Enthalpies of Hydration of N-Methylacetamide by One, Two, and Three Waters and the Effect upon the CO Stretching Frequency. An Ab Initio DFT Study. J. Phys. Chem. A 2006, 110, 5798−5802. (16) Heyda, J.; Vincent, J. C.; Tobias, D. J.; Dzubiella, J.; Jungwirth, P. Ion Specificity at the Peptide Bond: Molecular Dynamics Simulations of N-Methylacetamide in Aqueous Salt Solutions. J. Phys. Chem. B 2010, 114, 1213−1220. (17) Yadav, V. K.; Chandra, A. First-Principles Simulation Study of Vibrational Spectral Diffusion and Hydrogen Bond Fluctuations in Aqueous Solution of N-Methylacetamide. J. Phys. Chem. B 2015, 119, 9858−9867. (18) Biernacki, K. A.; Kaczkowska, E.; Bruździak, P. Aqueous Solutions of NMA, Na2HPO4, and NaH2PO4 as Models for Interaction Studies in Phosphate-Protein Systems. J. Mol. Liq. 2018, 265, 361−371. (19) Han, W.-G.; Suhai, S. Density Functional Studies on NMethylacetamide-Water Complexes. J. Phys. Chem. 1996, 100, 3942− 3949. (20) Albrecht, M.; Rice, C. A.; Suhm, M. A. Elementary Peptide Motifs in the Gas Phase: FTIR Aggregation Study of Formamide, Acetamide, N-Methylformamide, and N-Methylacetamide. J. Phys. Chem. A 2008, 112, 7530−7542. (21) Forsting, T.; Gottschalk, H. C.; Hartwig, B.; Mons, M.; Suhm, M. A. Correcting the Record: the Dimers and Trimers of Trans-NMethylacetamide. Phys. Chem. Chem. Phys. 2017, 19, 10727−10737. (22) Torii, H.; Tatsumi, T.; Kanazawa, T.; Tasumi, M. Effects of Intermolecular Hydrogen-Bonding Interactions on the Amide I Mode of N-Methylacetamide: Matrix-Isolation Infrared Studies and ab Initio Molecular Orbital Calculations. J. Phys. Chem. B 1998, 102, 309−314. (23) Salamatova, E.; Cunha, A. V.; Bloem, R.; Roeters, S. J.; Woutersen, S.; Jansen, T. L. C.; Pshenichnikov, M. S. Hydrophobic Collapse in N-Methylacetamide-Water Mixtures. J. Phys. Chem. A 2018, 122, 2468−2478. (24) Hunt, N. T.; Wynne, K. The Effect of Temperature and Solvation on the Ultrafast Dynamics of N-Methylacetamide. Chem. Phys. Lett. 2006, 431, 155−159. (25) Allison, S. K.; Bates, S. P.; Crain, J.; Martyna, G. J. Solution Structure of the Aqueous Model Peptide N-Methylacetamide. J. Phys. Chem. B 2006, 110, 21319−21326. (26) McLain, S. E.; Imberti, S.; Soper, A. K.; Botti, A.; Bruni, F.; Ricci, M. A. Structure of 2 molar NaOH in Aqueous Solution from Neutron Diffraction and Empirical Potential Structure Refinement. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 094201. (27) Scoppola, E.; Sodo, A.; McLain, S. E.; Ricci, M. A.; Bruni, F. Water-Peptide Site-Specific Interactions: A Structural Study on the Hydration of Glutathione. Biophys. J. 2014, 106, 1701−1709.

Moreover, we have found that the water structure is perturbed by the presence of NMA molecules in solution with respect to pure water. In particular, HBs are shorter and the HB network is distorted, with respect to the tetrahedral symmetry of pure water. In conclusion, the results of our experiment substantially confirm the predictions of computer simulations and theoretical models, along with those provided by spectroscopic experiments. The structure and hydration of NMA chains, formed at a high concentration, as a function of temperature deserve future attention. Such studies could help in understanding the differences found in the present work between the hydration shells of NMA and GSH.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 0655737226. ORCID

Michael Di Gioacchino: 0000-0001-7465-2456 Maria Antonietta Ricci: 0000-0002-6904-6686 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been performed within the agreement no. 0018318 (02/06/2014) between STFC and CNR, concerning collaboration in scientific research at the spallation neutron source ISIS and with partial financial support of CNR. Beamtime awarded by ISIS under RB numbers 1510043 and 1510044 is gratefully acknowledged. The Grant of Excellence Departments, MIUR (Articolo 1, commi 314 337 Legge 232/ 2016) is greatly acknowledged. The authors thank S. Imberti for the assistance during the experiment at ISIS.

■

REFERENCES

(1) Cunha, A. V.; Salamatova, E.; Bloem, R.; Roeters, S. J.; Woutersen, S.; Pshenichnikov, M. S.; Jansen, T. L. C. Interplay between Hydrogen Bonding and Vibrational Coupling in Liquid NMethylacetamide. J. Phys. Chem. Lett. 2017, 8, 2438−2444. (2) Rupley, J. A.; Careri, G. Protein Hydration and Function. In Advances in Protein Chemistry; Anfinsen, C., Richards, F. M., Edsall, J. T., Eisenberg, D. S., Eds.; Academic Press, 1991; Vol. 41, pp 37−172. (3) Ball, P. Water as an Active Constituent in Cell Biology. Chem. Rev. 2008, 108, 74−108. (4) Mennucci, B.; Martínez, J. M. How to Model Solvation of Peptides? Insights from a Quantum-mechanical and Molecular Dynamics Study of N-Methylacetamide. 1. Geometries, Infrared, and Ultraviolet Spectra in Water. J. Phys. Chem. B 2005, 109, 9818− 9829. (5) Beicastro, M.; Marino, T.; Mineva, T.; Russo, N.; Sicilia, E.; Toscano, M. Recent Developments and Applications of Modern Density Functional Theory. In Theoretical and Computational Chemistry; Seminario, J., Ed.; Elsevier, 1996; Vol. 4, pp 743−772. (6) Mizushima, S.-i.; Simanouti, T.; Nagakura, S.; Kuratani, K.; Tsuboi, M.; Baba, H.; Fujioka, O. The Molecular Structure of NMethylacetamide. J. Am. Chem. Soc. 1950, 72, 3490−3494. (7) Mayne, L. C.; Hudson, B. Resonance Raman Spectroscopy of NMethylacetamide: Overtones and Combinations of the CarbonNitrogen Stretch (Amide II’) and Effect of Solvation on the CarbonOxygen Double-Bond Stretch (Amide I) Intensity. J. Phys. Chem. 1991, 95, 2962−2967. F

DOI: 10.1021/acs.jpcb.9b00246 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B (28) Soper, A. K. Empirical Potential Monte Carlo Simulation of Fluid Structure. Chem. Phys. 1996, 202, 295−306. (29) Soper, A. K. Partial Structure Factors from Disordered Materials Diffraction Data: An Approach Using Empirical Potential Structure Refinement. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 104−204. (30) Bruni, F.; Di Mino, C.; Imberti, S.; McLain, S. E.; Rhys, N. H.; Ricci, M. A. Hydrogen Bond Length as a Key To Understanding Sweetness. J. Phys. Chem. Lett. 2018, 9, 3667−3672. (31) For information: https://www.isis.stfc.ac.uk/Pages/sandals. aspx. Last access on February 14, 2019. (32) Soper, A. GudrunN and GudrunX: Programs for Correcting Raw Neutron and X-ray Diffraction Data to Differential Scattering Cross Section; Science & Technology Facilities Council, 2011. (33) Sears, V. F. Neutron Scattering Lengths and Cross Sections. Neutron News 1992, 3, 26−37. (34) Jorgensen, W. L.; Gao, J. Cis-Trans Energy Difference for the Peptide Bond in the Gas Phase and in Aqueous Solution. J. Am. Chem. Soc. 1988, 110, 4212−4216. (35) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269− 6271. (36) Svishchev, I. M.; Kusalik, P. G. Structure in Liquid Water: A Study of Spatial Distribution Functions. J. Chem. Phys. 1993, 99, 3049−3058. (37) Rhys, N. H.; Bruni, F.; Imberti, S.; McLain, S. E.; Ricci, M. A. Glucose and Mannose: A Link between Hydration and Sweetness. J. Phys. Chem. B 2017, 121, 7771−7776. (38) Soper, A. K. The Radial Distribution Functions of Water as Derived from Radiation Total Scattering Experiments: Is There Anything We Can Say for Sure? ISRN Phys. Chem. 2013, 2013, 1−67. (39) Soper, A. K.; Ricci, M. A. Structures of High-Density and LowDensity Water. Phys. Rev. Lett. 2000, 84, 2881−2884.

G

DOI: 10.1021/acs.jpcb.9b00246 J. Phys. Chem. B XXXX, XXX, XXX−XXX