Dynamics of Hydration Water in Sugars and Peptides Solutions - The

Jun 4, 2013 - The remaining 20% of the hydration layer is more strongly retarded (ξ > 6) and corresponds to the water molecules in proximity of HB ac...
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Dynamics of Hydration Water in Sugars and Peptides Solutions Stefania Perticaroli,*,†,‡ Masahiro Nakanishi,†,‡ Eugene Pashkovski,§,¶ and Alexei P. Sokolov†,‡,# †

Chemical and Materials Sciences Division at Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States # Joint Institute for Neutron Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Unilever R&D Trumbull, 40 Merritt Boulevard, Trumbull, Connecticut 06611, United States ‡

S Supporting Information *

ABSTRACT: We analyzed solute and solvent dynamics of sugars and peptides aqueous solutions using extended depolarized light scattering (EDLS) and broadband dielectric spectroscopies (BDS). Spectra measured with both techniques reveal the same mechanism of rotational diffusion of peptides molecules. In the case of sugars, this solute reorientational relaxation can be isolated by EDLS measurements, whereas its contribution to the dielectric spectra is almost negligible. In the presented analysis, we characterize the hydration water in terms of hydration number and retardation ratio ξ between relaxation times of hydration and bulk water. Both techniques provide similar estimates of ξ. The retardation imposed on the hydration water by sugars is ∼3.3 ± 1.3 and involves only water molecules hydrogen-bonded (HB) to solutes (∼3 water molecules per sugar OH-group). In contrast, polar peptides cause longer range perturbations beyond the first hydration shell, and ξ between 2.8 and 8, increasing with the number of chemical groups engaged in HB formation. We demonstrate that chemical heterogeneity and specific HB interactions play a crucial role in hydration dynamics around polar solutes. The obtained results help to disentangle the role of excluded volume and enthalpic contributions in dynamics of hydration water at the interface with biological molecules.

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responsible not only for the magnitude of the retardation imposed on the surrounding solvent, but also for the number of dynamically perturbed water molecules (nH).1−5 For instance, according to an NMR database of nH obtained for small solutes, the COO− group can affect up to 10−20 water molecules.4 At the same time, MD simulations and light scattering measurements suggested that each OH group of glucose and trehalose molecules perturbs ∼3.3 water molecules.6 Sugars and peptides have been widely used as model systems to mimic and study the hydrophilic and hydrophobic interactions taking place in protein solutions. These studies demonstrated that chemical specificity, and not just topology of molecular surfaces, controls the dynamical features of water near biological interfaces.4,7−9 However, substantial elements of controversy still remain, especially regarding the characteristic mobility of hydration water and the extent of the induced perturbation. MD simulation studies and several experimental techniques have provided contradictory results, with retardation factors going from 1.5−2 up to orders of magnitude10 and hydration numbers corresponding to only the first hydration layer11 or to 3−4 shells around the solute.12 It is important to recognize that

ost of the crucial molecular events in biology occur at the protein−water interface. Therefore, drawing a quantitative picture of water dynamics in the hydration shell surrounding biomolecules is of great importance. This stimulates the intense interest in the dynamical features of water interacting with the surface of biomolecules, and multiple efforts are made to describe how properties of hydration water differ from those of the bulk. It is now well established that the dynamics of hydration water are slowed down relative to bulk water. Recently, Sterpone et al.1 conducted a detailed microscopic mapping of the water reorientation and hydrogen-bond (HB) dynamics around lysozyme. The 80% of water molecules in the hydration shell experience a moderate slowdown relative to bulk water, with a retardation factor ξ of ∼2−3 controlled mostly by an excluded volume (entropic) effect. Those water molecules are mainly located near hydrophobic and HB donor (HBd) groups of the protein. The remaining 20% of the hydration layer is more strongly retarded (ξ > 6) and corresponds to the water molecules in proximity of HB acceptor (HBa) groups confined within pockets and clefts. In this case the slowdown is essentially controlled by the water−protein HB strength (enthalpic effect). However, this study does not establish a method to clearly distinguish the effects of topology versus the chemical identity, e.g., HBd/HBa. Different chemical functionalities seem to be © XXXX American Chemical Society

Received: April 14, 2013 Revised: May 23, 2013

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Figure 1. Imaginary part of the light scattering dynamic susceptibility χ″(ν) (A) and dielectric loss permittivity ε″(ν) (B) of pure water (symbols) and total fit (lines). Blue areas represent the bulk water relaxation.

Each peptide contains chemical functionalities that can potentially act as HBa or HBd. In fact, lone pairs of O and N atoms can potentially serve as HBa. This corresponds to 9 HBa for NAHYP, 8 for GLYGLY, 11 for ALAGLN and 17 for GSH. On the other hand, H atoms directly connected to an oxygen or a nitrogen can show acidic nature. Thus, NAHYP has 2 HBd groups, GLYGLY 4, ALAGLN 5 and GSH 6. Therefore, the propensity of the peptides to HB formation in water can be written in the order NAHYP < GLYGLY < ALAGLN < GSH, with GSH being the molecule most involved in the HB network. NAHYP, with its proline ring, is the most hydrophobic solute. The hydration dynamics of those peptide prototypes has not been explored much in literature, despite a large biologic interest in food and medical applications.17 The molecular weights and chemical structure of all the systems investigated are reported in the Supporting Information together with the details of the EDLS and BDS experiments. All the investigated samples were at 10% solute concentration by weight (10 mg solute/90 mg water) and measured at T = 292 K. The spectral windows covered with the light scattering and dielectric experiments were 0.5 GHz to 4THz and 1 MHz to 50 GHz, respectively. We analyzed dielectric loss ε″(ν) and the imaginary part of the light scattering dynamic susceptibility χ″(ν), calculated according to the relation χ″(ν) ∝ I(ν)/[nB(ν) + 1], where I(ν) is the measured intensity and nB(ν) = 1/ [exp(hν/kT) − 1] is the Bose-Einstein occupation number. Bulk Water. BDS and EDLS spectroscopies probe dipole and polarizability anisotropy relaxation dynamics, respectively. The relaxation dynamics of pure water probed by light scattering at room temperature (Figure1A) can be described by a Cole Davidson distribution function χ″CD = −Im{Δ[1 + iωτ]−β} with stretching parameter β = 0.6, amplitude Δ and a characteristic relaxation time of τ ∼ 0.76 ps (blue area in Figure 1A). The two additional high frequency components, at ∼1.5 and ∼5.1 THz, correspond to bending and stretching water resonant modes, respectively,7 and can be fitted by two damped harmonic oscillator (DHO) peaks, χ″DHO = Im{Δω20[ω2 − ω20 − iωΓ]−1} where position ω0, width Γ, and amplitude Δ of the peaks are free fitting parameters. On the other hand, the main dielectric relaxation process in water at 292 K (Figure 1B) has essentially a Debye shape (no stretching) with τ ∼ 9.7 ps. This contribution has been assigned to a cooperative relaxation specific to hydrogen-bond liquids and arises from long-range Hbond mediated dipole−dipole interactions.14

the retardation value strongly depends on the physical property probed and on the collectivity of the response, i.e., on the number of water molecules dynamically involved.13 In particular, it would be important to clarify the confusion existing between Extended Depolarized Light Scattering (EDLS) and Broadband Dielectric Spectroscopies (BDS) results. In fact, both techniques, covering several orders in frequency and being sensitive to both solute and solvent motions, are particularly suitable for the analysis of biological aqueous solutions. Nevertheless, only very few attempts have been made to directly compare EDLS and BDS data of aqueous systems.14 Recently, Fukasawa et al.14 rationalized the comparison between the dielectric (rank l = 1) and depolarized light scattering (l = 2) spectra of pure water in the microwave to terahertz frequency range. However, the dynamics of hydration water for different classes of small solutes has not been studied using both techniques. The main goals of this work are (i) to determine the origin of the slowdown of hydration water and to quantify retardation factors and spatial extent of the perturbation for the different solutes; (ii) to unravel the role of specific chemical structures of solutes in dynamics of hydration water; and (iii) to analyze and directly compare the dynamical information contained in the dielectric and depolarized light scattering spectra of sugars and polar peptides aqueous solutions. Our results suggest that sugars affect ∼3 water molecules per each OH group with retardation factors ∼3. On the other hand, polar peptides, containing HBd and mainly HBa groups, affect water dynamics more strongly, with ξ up to ∼8 and nH beyond one hydration shell. To characterize the role of chemical heterogeneity and to disentangle the HBd/HBa character from the topological effects of protein surface, we analyzed binary aqueous solutions of sugars and polar peptides of increasing complexity. We selected two monosaccharides (D-(+)-glucose and D-(+)-fructose), a disaccharide (sucrose), and an oligosaccharide (maltodextrin, Dextrose Equivalent 16.5−19.5 corresponding to molecular weight ∼1025 amu and ∼6.2 glucose units per molecule). Carbohydrates are considered amphiphilic15 or mildly hydrophilic solutes13 having OHs of different polarity acting as HBd and HBa.6,16 The number of OHs is 5 for the two smallest sugars, 8 for sucrose and ∼22 for maltodextrin. For the second category of compounds, a peptide (N-acetyl-L-hydroxyproline, NAHYP), two dipeptides (glycyl-glycine, GLYGLY; alanylglutamine, ALAGLN), and a tripeptide (L-glutathione reduced, GSH) composed of differing chemical groups were chosen. B

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Figure 2. EDLS (left panels) and BDS (right panels) of sugars (A and B) and peptides (C and D) samples compared to pure water (gray symbols).

This significant difference between light and dielectric relaxation spectroscopy of water is well-known. In a recent MD simulations study,18 Laage and Hynes proposed a molecular jump mechanism for water picosecond reorientation in which rotational and translational motions are strongly coupled. In this model, the reorienting water OH group forms a new H-Bond with another water molecule. When the new water molecule arrives, the existing H-bond elongates and a transition state characterized by a symmetrically bifurcated Hbond with two water molecules is established. Switching to the new H-bonded partner leads to reorientation of the water molecule. The original H-bond breaks, the new H-bond with the second water molecule is stabilized, and the initial partner leaves the coordination shell. The translational dynamics and the structural reorganization of HB network involved in this large amplitude reorientational jump modulate the picosecond polarizability and are therefore visible in the EDLS spectra.19,13,20 On the other hand, the polarizability of water molecule is approximately isotropic;21 therefore, molecular orientational contributions of the solvent to the observed EDLS signal are expected to be negligible. On the contrary, BDS experiments are not sensitive to the translational contributions involved in the concerted mechanism but are affected by the reorientational component (long-range H-bond mediated water dipole reorientation). Hydration Water. Figure 2 shows EDLS (left panels) and BDS (right panels) spectra of sugars (A and B) and peptides (C and D) aqueous solutions. All the EDLS spectra have been

scaled at the intensity of the librational band of water (ν > 10 THz) which depends weakly on temperature and concentration variations. Addition of solutes leads to appearance of two more relaxation processes in the light scattering spectra: one at around 30−50 GHz related to hydration water relaxation, and the peak at lower frequency region (ν < 20 GHz). Position of the latter depends on molecular weight of the solute and the amplitude depends on both solute optical anisotropy and concentration5 (Figure 2A,C). Previous studies demonstrated that the latter can be related to rotational diffusion of single solute molecules.7,22 The solute rotational relaxation times estimated from the light scattering are in good agreement with NMR results.23,24 At frequencies between 0.4 and 5 THz, EDLS spectra of sugars aqueous solutions resemble the spectrum of pure water, whereas those of peptides show an additional contribution due to low-frequency vibrations of the solutes.7,20 The EDLS spectral region between 10 GHz and 1 THz7 contains information on the relaxational dynamics of water molecules in solution. Thanks to the slow exchange between hydration shell and the bulk, two distinct relaxation processes can be isolated. The fast process (around 200 GHz) is due to bulk water molecules dynamics essentially unaffected by the presence of the solute, whereas the slow one, located at around 30−50 GHz, is related to relaxation of hydration water. Similar changes can be found also in the dielectric spectra (Figures 2B,D). Several experimental and theoretical studies25 have pointed out that depending on the solute and its concentration the low frequency contributions (ν < 10 GHz) to C

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the ε″(ν) spectrum of the solution can lead to a bimodal shape or retain a single peak characteristic of pure water.8,26,25 Our data show that at 10% solute concentration only GLYGLY and ALAGLN solutions display well separated double-peak features in this frequency range (Figure 2D). Maltodextrin, NAHYP and GSH solutions spectra also have an intense conductivity tail below 1 GHz that can be properly described by the typical ν−1 power law. This tail is distinct from the relaxation processes and does not affect our data analysis. There is still a significant disagreement in literature concerning the interpretation of the low frequency part (ν < 4 GHz) of the dielectric spectra of aqueous solutions. Solute− solvent cross-correlations terms, subjective definitions of hydration shells, non- trivial amplitudes of dipolar couplings between first and outer hydration layers and the constant exchange of water molecules between hydration and bulk make the decomposition of the dielectric signal quite ambiguous.8 However, both experiments and MD simulations agree in considering the intermediate relaxation region corresponding to 20−60 ps as essentially due to hydration water.8 Residence times for the bulk-exchanging surface water have been estimated by dielectric, NMR and MD simulations studies. Literature presents significant discrepancies for both small solutes and protein solutions, with estimates going from a few picoseconds27 up to tens4,28 and hundreds29 of picoseconds. The absence of a clear exchange process in the dielectric spectra is consistent with MD simulations26 on oligosaccharides solutions, that showed a broad distribution of exchange times between hydration shell and bulk but with negligible dielectric amplitudes. Some MD simulations studies presented twocomponents decompositions of the dielectric spectra of sugars and peptides solutions, as well as for proteins samples, evaluating contributions from solvent, solute, and solute− solvent cross terms. For such model systems, the low frequency signal is due to the superposition of several effects, consisting of the low-frequency tail of the water self-correlation, strong crossterms interactions13 and self-contributions of the solutes. The latter components are almost negligible for sugars solutes13,26 but significant for peptides.8 In fact, no evidence of the sugar rotational diffusion has been explicitly found in dielectric spectra.22,24 On the contrary, reorientation and tumbling of peptide molecules have been singled out between 0.2 and 20 GHz in the permittivity measurements of several peptide and amino-acids aqueous solutions at temperature and concentration comparable to those employed in our experiments.30,31 The frequency of reorientational correlation function of rank l is proportional to l(l + 1). Since BDS and EDLS correlation functions have rank l = 1 (dipole) and l = 2 (polarizability), respectively,32 a factor 3 shift in the frequency has to be considered. Therefore, in order to verify if both techniques probe the same peptide rotational dynamics at low frequency, we plotted the ε″(ν) spectrum shifted by a factor of 3 toward higher frequencies, together with the EDLS susceptibility χ″(ν) spectrum for the GLYGLY-water sample (Figure 3). The agreement between low-frequency sides of the EDLS and the BDS spectra is quite good, demonstrating that also dielectric spectrum presents rotational diffusion of the solute. Analogous considerations regarding this low-frequency EDLS/BDS superposition can be performed also for all the other peptides studied here. To provide a quantitative characterization of the hydration water dynamics, we fit the EDLS and BDS spectra (Figure 4) following the procedure presented in refs 24 and 7. The χ″(ν)

Figure 3. Comparison of GLYGLY-water EDLS (green circles) and BDS (orange squares) spectra. The BDS spectrum has been shifted by a factor of 3 toward higher frequency.

difference spectra of the peptides solutions, obtained after removal of the vibrational peptides contribution (0.4−5 THz)7 and the total χ″(ν) spectra of the sugars samples, have been described as a superposition of five components: two DHO functions for the intermolecular stretching (purple line) and bending (magenta line) modes of water, two Cole-Davidson functional forms, to account for bulk (blue area) and hydration (cyan area) water dynamics and a Debye peak for the solute relaxation process (orange area). Each Cole-Davidson is characterized by the relaxation time τ, amplitude Δ and stretching parameter β. The β values were fixed to 0.6 for the water relaxations, as previously determined for pure solvent and biomolecules aqueous solutions.7 The ε″(ν) spectra were reproduced by Debye-like peaks, as commonly reported in literature (Figure 4B,D).26 We want to emphasize that in the case of sugar solutions the contribution assigned to the low frequency solute relaxation is negligible (orange curve in Figure 2B), especially for the monosaccharides, and that two Debye functions are sufficient to reproduce the bulk water dynamics (blue area) and dynamics of hydration water (cyan area). The fit of EDLS and BDS spectra has been performed with either fixing the relaxation time of bulk water component at the value of pure water or letting it to be free. For each technique, the relaxation times of hydration ⟨τH⟩ and bulk ⟨τB⟩ water dynamics, ξ = τH/τB and nH values were obtained by averaging the parameters provided by both fits (Table 1). Retardation Factors ξ and Hydration Numbers nH. The two experimental techniques provide surprisingly consistent results for the water retardation factors ξ, for both sugars and peptides solutions (Figure 5). This finding is remarkable in that it demonstrates the water translational dynamics in proximity of small solutes have retardation factors similar to those of reorientational relaxation. It supports previous hints from NMR4 and MD simulations on lysozyme.28 This might suggest that variation of the HB lifetime and changes in the rearrangement of the HB network at the interface of polar solutes affect both translations and rotations in the same way. We found that water molecules located in the hydration shell of carbohydrates are slowed down by ∼3.3 ± 1.3 times relative to the bulk solvent, whereas those surrounding peptides are more strongly retarded with ξ rising up to ∼8 ± 1 in the case of GSH. D

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Figure 4. Best fits obtained for EDLS (left panels) and BDS (right panels) spectra of sucrose (A and B) and GLYGLY (C and D) aqueous solutions. Rotational relaxations of the solutes are marked in orange. Cyan and blue areas represent hydration and bulk water dynamics, respectively.

Table 1. Average Relaxation Times of Hydration ⟨τH⟩ and bulk ⟨τB⟩ Water Dynamics, Together with the Retardation Factor ξa EDLS

WATER GLUC. FRUCT. SUCR. MALT. NAHYP GLYGLY ALAGLN GSH

⟨τH⟩

⟨τB⟩

(ps)

(ps)

− 2.4 2.2 2.7 2.6 2.1 4.0 5.5 6.5

0.76 0.76 0.76 0.77 0.77 0.76 0.77 0.77 0.77

ξ − 3.2 2.8 3.6 3.4 2.7 5.3 7.1 8.4

BDS nH

nH/nOH

− 14 13 29 50 46 42 75 77

2.8 2.6 3.6 2.3

n1st shell − 14 14 25 52 15 13 20 26

⟨τH⟩

⟨τB⟩

(ps)

(ps)

− 25.8 22.0 32.2 29.9 42.8 40.1 46.3 70.6

9.7 9.9 9.8 9.8 9.7 9.7 9.5 9.9 9.7

ξ − 2.6 2.3 3.3 3.1 3.1 4.2 6.0 7.3

Hydration numbers nH extracted from EDLS spectra are compared to the number of water molecules contained in the first hydration shell (n1st shell) calculated as in ref 33. In the case of sugars, the number of hydration water molecules per number of OHs is also listed (nH/nOH).

a

estimates of nH. Such parameter can be provided from EDLS data (Table 1), applying the formula nH = fsolΔH/(ΔH + ΔB), where fsol is the solute/water molar ratio and ΔH and ΔB are the amplitudes of hydration (ΔH) and bulk (ΔB) water relaxation processes.7 In the case of sugars solutions, the estimated nH agrees well with the number of molecules contained in the first hydration shell (n1st shell, Table 1), that have been calculated from the solvent accessible surface area.33 On the other hand, the perturbation propagates deeper into the bulk in the case of peptides, extending beyond the first hydration layer.

Increasing the structural complexity of sugars seems to affect only marginally the dynamical slowdown of hydration water, whereas the different chemical nature of the peptides affects ξ drastically. In particular, ξ increases with the number of HBd/ HBa groups in the peptide molecule and with the ability of the solute to participate in formation of the HB network. Due to rather strong dipole−dipole interactions, dielectric relaxation spectra of systems with high dipole moments can be distorted by cross-terms, dipole−dipole coupling and superposition effects. Thus, we decide not to use the dielectric data for E

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Figure 5. Retardation factors, ξ, obtained from EDLS (green circles) and BDS (orange squares) for both sugar (left) and peptide (right) solutions. Within each class of solutes, the samples are listed from left to right starting from the lowest number of chemical groups involved in HB network to the highest. Error bars denote uncertainties obtained by different curve fitting procedures.

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DISCUSSION AND CONCLUSIONS Results obtained for the sugars solutions are consistent with EDLS data previously published.5,34 The ξ values reported here are only slightly smaller than those presented in refs 5 and 34, most probably due to averaging results of the two fitting procedures. Good agreement is also found with dielectric data published by Weingärtner et al.25 for glucose solutions at similar experimental conditions (ξ ∼2.9). Recently, Halle et al. have reported slightly smaller retardation factor around 1.67 for trehalose aqueous solutions at 293 K analyzed by NMR, with a perturbation limited to the first hydration shell.35 This difference can be explained considering that the NMR technique probes single molecule rotational motion, while EDLS and BDS are sensitive to collective HB network rearrangements and are affected by the interaction effects induced among different molecules. It is worth reiterating that the NMR retardation factor of 1.67 has been observed in solutions diluted 100−200 times more than the solutions analyzed here. These dilute solutions exist in a regime where hydration shell superposition effects can be justifiably neglected. Our results also agree with MD simulation studies.15,36 Lee et al. have shown that at 300 K the rotation of water around glucose and sucrose is 2−4 times slower relative to pure water.36 Similar results have been recently provided also for other disaccharides solutions, with the rotational anisotropy decay of hydration water showing a maximum retardation value ξ = 4.4.15 Our estimate of the nH suggests that on average each hydroxyl group of the sugar molecules slows down the dynamics of ∼3 water molecules (Table 1). These findings confirm that in sugars solutions only the water molecules Hbonded to the solutes are dynamically perturbed. This is consistent with previous evidence provided by both EDLS5 and MD simulation studies5,6 for glucose and trehalose diluted solutions, and extends the result also to sugars of higher structural complexity. The small perturbation observed for sugars is compatible with the view that carbohydrate molecules are able to partially replace water in the HB network16 due to the similar OH functionality and comparable strength of the water−sugar and water−water HBs.37 In the case of polar peptides, we argue that the different chemical groups form HBs of different strength (compared to

those of pure water) and give rise to more disruptive interactions in comparison to the sugar case. In particular, GSH known to create several HB bridges with water,38 strongly perturbs the HB network. Since the picosecond dynamics of the peptides analyzed here have not been investigated in literature, we make reference to data available for similar samples. MD simulations conducted on hydrophobic sites of amphiphilic molecules suggested modest retardations (ξ ≤ 2) in reorientation of water molecules in the solvation shell and ascribed this evidence to geometric excluded volume effects.39 Such result has been supported by NMR experiments on partially hydrophobic solutes, which found water rotation in the hydration shell of apolar moieties retarded by a factor ∼2 relative to bulk water.40 Dielectric spectroscopy studies of reorientation dynamics in the hydration shell of two hydrophobic solutes tetramethylurea (TMU) and trimethylamine N-oxide (TMAO) in diluted solutions reported ξ ∼3.41,42 The same authors also found that for TMU the reorientational dynamics of water molecules in the hydration shell slows down with concentration reaching ξ∼ 10.41 Recently, ultrafast Optically Heterodyne Detected−Optical Kerr Effect (OHD-OKE), which measures the relaxation of induced polarizability anisotropy in real time, has been applied to study hydrophilic, hydrophobic and amphiphilic peptides,9,43 as well as proteins. The magnitude of the retardation factors is consistent with our findings; hydration water is 2−3 times slower than that in bulk water for the hydrophobic HBa solutes TMAO and TMU, and 6−7 times slower for urea and formamide, that contain both HBa and HBd groups and are considerably more hydrophilic.9 The same authors found slightly larger retardation factors for the hydration dynamics surrounding amphiphilic dipeptides, namely, N-acetyl-glycinemethylamide (NAGMA), N-acetyl-alanine-methylamide (NAAMA), and N-acetyl-leucine-methylamide (NALMA). Slower hydration dynamics were observed for the more hydrophilic peptide (NAGMA), indicating that hydrophilic sites have a larger effect on the retardation of water dynamics than hydrophobic ones.43 The same observation was also made by Murarka et al. in an MD simulations study investigating the dielectric properties of NAGMA and NALMA solutions.8 An EDLS experiment carried out by one of us on analogous NALMA solutions recently proposed consistent retardation factors ranging from 9 to 7, and nH varying from 62 to 50 F

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corresponding to more than two hydration layers.7 The large spatial perturbation revealed for peptides in our studies (Table 1) agrees with EDLS results on NALMA solutions7 and with MD simulation data for THz absorption of sugars, amino acids and proteins solutions.13 The latter study suggested that in the presence of these biomolecules the polarization is long-ranged and that the perturbation, expressed in terms of spatial crosscorrelation between solute and water dipoles, propagates 2−4 nm from the solute surface into bulk water. Unfortunately, analysis of the number of water molecules affected by peptides per H-bonding site in the peptides is not as straightforward as in sugars. First of all, peptides have HBa and HBd groups that will interact differently with water. Second, not all of them are accessible for hydrogen bonding with water molecules due to steric hindrance. For example, MD simulations by Zhang et al.38 show that not all the HBd and HBa sites of GSH peptide are bonded to water molecules. Detailed MD simulations can help in this quantitative analysis and provide deeper insight into longer scale effects of peptides on dynamics of hydration water. The analysis of hydration water properties here presented disentangles effects of topology and chemical functionalities composing the biomolecules. Entropic effects related to excluded volume, which can explain the moderate slowdown (ξ around 2) obtained for reorientation of hydration water near mainly hydrophobic solutes, cannot explain the strong retardation of hydration dynamics obtained here for sugars and polar peptides solutes. We suggest that chemical heterogeneity and enthalpic effects related to the ability of such polar solutes to participate in the HB network are crucial factors to rationalize the hydration properties in solution. The origin of the perturbation can be found in a collective effect, possibly related to frustration of the rearrangement dynamics of the HB network and probably induced by the presence of molecular groups of different polarity. In summary, we were able to provide a general picture of the picosecond dynamics associated with the structural reorganization of HB network in aqueous solutions of sugars and polar peptides. For the first time with aqueous solutions of these two biologically relevant species, we combine two techniques, EDLS and BDS, to isolate and disentangle the solute and solvent contributions. Influence of sugars on hydration water can be described by a retardation factor ξ ∼ 3 affecting approximately 3 water molecules per each OH group (Table 1). From this, it appears that the complexity of chemical structure of the sugars is not of primary importance. The situation is more complex in polar peptides, with retardation factor increasing up to ξ ∼ 8 and perturbation of water extending beyond the first hydration shell. The hydration properties of the different systems were quantitatively characterized in terms of hydration numbers and retardation factors relative to bulk water. We present an explanation of the origin of the solute induced perturbations, emphasizing the crucial role of the chemical structures in dynamics of hydration water, specifically for these industrially and biologically important samples.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Address: University of Tennessee, Department of Chemistry, 552 Buehler Hall, 1420 Circle Dr., Knoxville, TN 37996-1600; Chemical and Materials Sciences Division at Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA. Present Address ¶

Eugene Pashkovski: The Lubrizol Corporation, 29400 Lakeland Blvd., Wickliffe, OH 44092, USA.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank Professor R. Germani for helpful discussions about HB properties of peptides. This work was supported by DOE through the EPSCoR program (grant DEFG02-08ER46528) and by Spallation Neutron Source (SNS) through UT-Battelle (LLC for the U.S. Department of Energy under contract No. DEAC05-00OR22725). We also acknowledge the financial support from the Unilever corporate research program.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Samples description and experimental EDLS and BDS details. This material is available free of charge via the Internet at http://pubs.acs.org. G

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dx.doi.org/10.1021/jp403665w | J. Phys. Chem. B XXXX, XXX, XXX−XXX