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Local and Global Effects of Dissolved Sodium Chloride on the Structure of Water Alex P. Gaiduk, and Giulia Galli J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00239 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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The Journal of Physical Chemistry Letters

Local and Global Effects of Dissolved Sodium Chloride on the Structure of Water Alex P. Gaiduk∗,† and Giulia Galli∗,†,‡ †Institute for Molecular Engineering, The University of Chicago, Chicago, Illinois 60637, United States ‡Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States E-mail: [email protected]; [email protected]

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Abstract Determining how the structure of water is modified by the presence of salts is instrumental to understanding the solvation of biomolecules and in general, the role played by salts in biochemical processes. Yet the extent of hydrogen bonding disruption induced by salts remains controversial. We performed extensive first-principles simulations of solutions of a simple salt (NaCl) and found that while the cation does not significantly change the structure of water beyond the first solvation shell, the anion has a further reaching effect, modifying the hydrogen-bond network even outside its second solvation shell. We found that a distinctive fingerprint of hydrogen bonding modification is the change in polarizability of water molecules. Molecular dipole moments are instead insensitive probes of long-range modifications induced by Na+ and Cl− ions. Though noticeable, the long-range effect of Cl− is expected to be too weak to affect solubility of large biomolecules.

Graphical TOC Entry

Keywords Water structure; structure breakers/makers; Hofmeister series; simulations; ab initio molecular dynamics

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The unique properties of water—for example, its high melting and boiling points, and the anomalous temperature dependence of the density—arise due to the hydrogen bonds (HBs) connecting individual molecules arranged into a complex three-dimensional network. 1,2 Various physical and chemical conditions, such as the temperature, pressure, or presence of solutes, change the pattern of hydrogen bonds, and affect the structure of the liquid. However, the microscopic details of how external conditions affect the structure of water, including the effect of salts, are not completely understood. Saline water is one of the basic ingredient of body fluids, serving as a medium for many biochemical reactions. It is known that the solubility of biomolecules increases (salting-in) or decreases (salting-out) depending on the nature and concentration of salts. The ability of several salts/ions to modify solubility of proteins follows a classification, the Hofmeister series, 3 according to which anions appear to have more notable effects than cations. 4 This series has important implications for a variety of processes including enzyme reactions, 5,6 as well as folding 7 and crystallization 8 of proteins. For example, hydrophobic collapse of proteins, a key step in protein folding, critically depends on the stability of protein in fluids, which in turns depends on the presence of salts. 9 Several explanations have been offered for the Hofmeister series: 10 One is based on the direct interaction of the solute ions with biomolecules, 9,11 while according to other theories, salting in/out of proteins is related to the modifications of the water structure in the presence of salts. It is well known that salts modify the structure of water within the first solvation shell of the dissociating ions. 12 However there is not yet a consensus on the extent to which structual changes extend outside the first solvation shell. 13,14 Since water bulk properties such as viscosity, 15 diffusion coefficients, 16 and hydration entropy 17 are modified in the presence of salts, it was often assumed that the structure of the liquid would be affected in a substantial manner as well. 12,16,18–20 This view, however, has recently been challenged by several spectroscopy studies, 21–25 leading to the conclusion that the effect of dissolved salts on the structure of water is weak,

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possibly just confined to the first solvation shells. Overall, both experiments and simulations provide a picture of the effect of ions on water structure that is still controversial: Neutron diffraction, 26–28 dielectric spectroscopy, 29,30 infrared photodissociation spectroscopy of gasphase clusters 31–34 and femtosecond elastic second-harmonic scattering spectroscopy results 35 support the presence of long-range effects of ions, while femtosecond infrared spectroscopy 21 and X-ray absorption experiments 22,25 suggest a more local effect. From a theoretical point of view, most first-principles simulations point at a local effect of ions, 36–41 while classical simulations are largely model-dependent. 24,42–46 Here we employed first-principles molecular dynamics to model solutions of NaCl, with the aim of understanding the effect of Na+ or Cl− ions on the liquid structure. Our calculations of electronic and bonding properties, including maximally localized Wannier functions and polarizabilities of water molecules, indicated modifications of the hydrogen bond network outside the first solvation shell of the Cl− ion, but not of that of the cation Na+ . The modifications induced by Cl− were clearly detectable in our polarizability analysis, although no variations of molecular dipole moments were observed. However, the effects found here on the structure of salty water were overall weak and hence our results indicate that monovalent salt modifications of bulk water hydrogen-bond network are likely not responsible for the altered solubility of proteins in the presence of salts. We simulated a 0.87 M solution of NaCl using first-principles molecular dynamics and the PBE functional. 47 The latter was chosen as it was considered to provide a reasonably accurate description of the structure of the solution, as compared to results obtained with the hybrid functional PBE0 48–51 (see Computational Methods). In addition, we deemed necessary to carry out extensive simulations (which turned out to cover, overall, ∼ 0.5 ns), and such long simulations were not feasible when using hybrid functionals. The concentration chosen here is higher than that of isotonic biological liquids (0.15 M) and probes an upper limit of salt concentrations typically studied in the context of the Hofmeister series. 9,11 We also simulated solutions with individual Na+ and Cl− ions in water at the same concentration

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cases. The difference is small but significant as it clearly falls outside the error bars of our calculations. The results presented in Figure 2 are unaltered when slightly varying the sizes of the solvation shells and when adopting different cutoffs in the definition of hydrogen bonds (Figures S3 and S4). We note that the authors of ref 44 obtained the same conclusions on ion perturbation of the water structure, when using the geometrical definition of hydrogen bonding used here 52 and a topological definition. 54 In our 0.87 M NaCl solution, ions are separated on average by a distance of ∼ 5.8 ˚ A, with a sizable fraction of water shared by the solvation shells of both ions. To understand the effect of individual ions in the absence of counterions, and to exclude effects arising from the overlap of solvation shells, we simulated liquid water containing individual ions, with their net charge neutralized by a uniform background charge of opposite sign. The right panel of Figure 2 shows the effect of individual Na+ and Cl− on water hydrogen bonding. We found that the Cl− ion has the same effect as that observed for the NaCl salt, i.e., it reduces the number of hydrogen bonds throughout the entire solution. The presence of Na+ alone, on the other hand, does not affect the number of hydrogen bonds outside its first solvation shell. The results above indicate that the chlorine and sodium ions have different effect on the structure of water. Cl− weakens hydrogen bonds as far as the third solvation shell, while the effect of Na+ is limited to its first solvation shell. This conclusion is robust, as the statistical error bars associated with the computed numbers of hydrogen bonds in different solvation shells do not overlap, even when using a reasonably tight error analysis criterion (4.73σ, yielding ∼ 95% confidence level). We expect that our results will also hold upon increasing the system size, based on the study of size effects reported in ref 44 using molecular dynamics with empirical potentials. Next, we investigated electronic fingerprints of the structural modifications identified above by computing maximally localized Wannier functions (MLWFs) and molecular polarizabilities. MLWFs are a generalization of Boys orbitals 55 to extended systems, obtained by a unitary transformation of the canonical Kohn–Sham orbitals under the requirement that

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Cl− solution than in the pure liquid, consistent with our previous structural analysis. Since Figure 3 does not include data from molecules in the first two solvation shells of the ions, the results indicate that the effect of Cl− extends beyond its second solvation shell. To compute effective polarizabilities of water molecules in the fluid, we followed ref 60: We evaluated the polarization of the entire supercell containing the solution using densityfunctional perturbation theory, 61 and then projected the result on each individual water molecule using the Wannier functions wn :

∆µ = −4

4 X

n=1

∆E wn r wn = αeff E.

(1)

The summation runs over Wannier function centers of a single molecule, ∆E wn is the response of a Wannier function wn to the perturbing electric field E, and r is the position operator. The effective polarizability αeff is a tensor that measures the response of a molecule to the applied field in different directions defined as shown in the insets of Figures S6 and S7. It is related to the molecular polarizability α via the term accounting for the dipole–induced dipole (DID) interaction with the environment (αDID ; see ref 60):

αeff = α + αDID .

(2)

We found that effective polarizabilities within the molecular plane α↑eff and αkeff are sensitive to the orientation of water molecules in the first solvation shells of ions (Figure S6), eff while the polarizability in the direction perpendicular to the molecular plane α⊥ is sensitive eff to modification of hydrogen bonding (Figure S7). A decrease in α⊥ is a fingerprint of broken

hydrogen bonds, while an increase is indicative of stronger hydrogen bonds between water molecules. We computed polarizabilities for molecules outside two solvation shells of Na+ and Cl− ions, and compared them with molecular polarizabilities in pristine water. The results, shown in Figure 4, indicate that H2 O molecules in Cl− and Na+ solutions have overall lower and higher polarizabilities than in pure water, respectively. This is the fingerprint of what 9




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to the first solvation shell; that of Cl− extends instead beyond the second solvation shell, however it is weaker, locally, than that of the cation. Overall, the effect of ions on the structure of bulk salty water found here is weak, and the choice of the appropriate probes to identify it, is crucial. We found that the number of hydrogen bonds per water molecule in different solvation shells of the ion is indicative of small differences in the structure of the solvent. Other characteristics of water molecules noticeably perturbed by the presence of ions are the effective polarizabilities of water molecules perpendicular to the molecular plane. An example of a property which was instead not sensitive to long-range modifications is the distribution of dipole moments of water molecules. Use of this property may have led to the underestimation of the effect of the ion on the structure of water. 37–40 Our findings are consistent with the observation that anions in general have stronger effect on the structure of water than cations. At the same time, the extent of the hydrogen bond network modification makes it unlikely that the structure of salty water with monovalent ions plays any significant role in the solubility of proteins, which may be modified in the presence of salts due to direct interaction with ions. Our results are consistent with those of Chen et al. 35 indicating that the structural modifications of bulk water by the action of ions, although undoubtedly real, appear to be too weak to explain many of the dynamical and thermodynamic properties of solutions. Work is in progress to analyze structural modifications by ions on salty water interfaced with solids and to analyze how surface effects might modify the conclusions obtained here for bulk solutions.

Computational Methods We modeled NaCl in water with a periodic 64-water-molecule unit cell in which two molecules were replaced by Na+ and Cl− ions. The periodic cell was cubic with the side length of 12.414 ˚ A. We chose to perform simulations using PBE functional as it provides a reasonably

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accurate structure of the solution, comparable to that at the PBE0 level of theory. 48,51 In order to provide a realistic description of the structure of solution at the ambient conditions, 51,62,63 we performed simulations at the elevated temperature of 400 K. First-principles simulations were carried out using the massively parallel Qbox code 64 Typical simulation used between 128 and 512 compute nodes (2048–8192 cores) on mira BlueGene/Q system at the Argonne Leadership Computing Facility. Norm-conserving Hamann– Schl¨ uter–Chiang–Vanderbilt (HSCV) pseudopotentials 65,66 were used to represent the core region of oxygen, hydrogen, and chlorine atoms, and Troullier–Martins pseudopotentials were used for sodium atom. The 1s electron of H, 2s2 2p4 electrons of O, 2s2 2p5 electrons of Cl, and 2p6 3s1 electrons of Na were treated explicitly. The simulations were conducted at an energy cutoff of 85 Ry, using a time step of 0.242 fs. Deuterium was used instead of hydrogen to allow for a longer simulation time step. To separate the effects of Na+ and Cl− ions on the structure of water, we also studied solutions of these ions individually. In each case, we simulated a solution of one of these ions mixed with 63 water molecules in a cell of the same size as the full system. The resulting systems were neutral as the charge of a single Na+ or Cl− ion was compensated by a uniform background charge of an opposite sign. Our previous study has shown that the use of the background charge has minimal effect on the solution structure. 48 Since the effect of ions was expected to be weak, we chose to carry out extensive simulations so as to accumulate sufficient statistical data. Instead of running a single long molecular-dynamics trajectory, we performed several simultaneous simulation runs, so as to better sample the configuration space of the system as well as to improve the statistical quality of the results. 62,67 Each of our solutions was represented by 8 independent runs of 19.8 ps (81,920 MD steps). Our results for salts were compared with those of pure water from the PBE400 database (http://www.quantum-simulation.org/reference/h2o/pbe400/s32/index.htm). All electronic properties for pure water were computed using the samples s0004, s0006, s0012, s0015,

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s0017, s0019, s0025, and s0031 from that database, in the range of md100 and md120 simulations (truncated up to a total of 81,920 MD steps). The structural properties such as radial distribution functions of water and the number of hydrogen bonds were computed using the last 163,840 MD steps of each of the 32 independent trajectories. Each computed quantity was then averaged between either 8 or 32 independent measurements. With this many independent simulations, statistical analysis was an essential tool to decide if the differences observed between solutions were significant. The error was estimated as a standard deviation of the mean σmean across 8 or 32 samples as explained in the Supporting Information. To account for the finite sample size, we multiplied errors by a factor obtained from the Student’s t-test, yielding a two-sided interval at the 95% level of confidence for each measured quantity. For 8 independent measurements, the number of degrees of freedom is 7, and σmean was multiplied by 2.365 to yield a 4.73σmean interval for the true mean. For 32 independent measurements, the factor from the Student’s test was 2.04 at the same level of confidence. We adopted these values in all of the analyses presented in this work.

Acknowledgement The authors gratefully acknowledge helpful discussions with Dr. Tuan Anh Pham, Nicholas Brawand, Dr. Quan Wan, Dr. Peter Scherpelz, Dr. Federico Giberti, and Prof. Fran¸cois Gygi. The work of G.G. was supported by MICCoM as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division (5J-30161-0010A). Work by A.P.G. was supported by the MICCoM and by the postdoctoral fellowship from the Natural Sciences and Engineering Research Council of Canada. An award of computer time was provided by the INCITE program. This research used resources of the Argonne Leadership Computing Facility, which is a DOE Office of Science User Facility supported under contract DEAC02-06CH11357.

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Supporting Information Available Details of error analysis; definition of hydrogen bond and sensitivity of our results to the definition cutoffs; polarizabilities, dipole moments, and tilt angles of water molecules in different solvation shells of Na+ and Cl− ions. This material is available free of charge via the Internet at http://pubs.acs.org/.

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