Osmolytes are small organic solutes accumulated at high concentrations by cells/tissues in response to osmotic stress. Osmolytes increase thermodynamic stability of folded proteins and provide protection against denaturing stresses. The mechanism of osmolyte compatibility and osmolyte-induced stability has, therefore, attracted considerable attention in recent years. However, to our knowledge, no quantitative study of osmolyte effects on the strength of hydrophobic interactions has been reported. Here, we present a detailed molecular dynamics simulation study of the effect of the osmolyte trimethylamine-N-oxide (TMAO) on hydrophobic phenomena at molecular and nanoscopic length scales. Specifically, we investigate the effects of TMAO on the thermodynamics of hydrophobic hydration and interactions of small solutes as well as on the folding-unfolding conformational equilibrium of a hydrophobic polymer in water. The major conclusion of our study is that TMAO has almost no effect either on the thermodynamics of hydration of small nonpolar solutes or on the hydrophobic interactions at the pair and many-body level. We propose that this neutrality of TMAO toward hydrophobic interactions-one of the primary driving forces in protein folding-is at least partially responsible for making TMAO a "compatible" osmolyte. That is, TMAO can be tolerated at high concentrations in organisms without affecting nonspecific hydrophobic effects. Our study implies that protein stabilization by TMAO occurs through other mechanisms, such as unfavorable water-mediated interaction of TMAO with the protein backbone, as suggested by recent experimental studies. We complement the above calculations with analysis of TMAO hydration and changes in water structure in the presence of TMAO molecules. TMAO is an amphiphilic molecule containing both hydrophobic and hydrophilic parts. The precise balance of the effects of hydrophobic and hydrophilic segments of the molecule appears to explain the virtual noneffect of TMAO on the strength of hydrophobic interactions.

We present results from extensive molecular dynamics simulations of collapse transitions of hydrophobic polymers in explicit water focused on understanding effects of lengthscale of the hydrophobic surface and of attractive interactions on folding. Hydrophobic polymers display parabolic, protein-like, temperature-dependent free energy of unfolding. Folded states of small attractive polymers are marginally stable at 300 K and can be unfolded by heating or cooling. Increasing the lengthscale or decreasing the polymerwater attractions stabilizes folded states significantly, the former dominated by the hydration contribution. That hydration contribution can be described by the surface tension model, ⌬G ‫؍‬ ␥(T)⌬A, where the surface tension, ␥, is lengthscale-dependent and decreases monotonically with temperature. The resulting variation of the hydration entropy with polymer lengthscale is consistent with theoretical predictions of Huang and Chandler [Huang DM, Chandler D (2000) Proc Natl Acad Sci USA 97:8324 -8327] that explain the blurring of entropy convergence observed in protein folding thermodynamics. Analysis of water structure shows that the polymer-water hydrophobic interface is soft and weakly dewetted, and is characterized by enhanced interfacial density fluctuations. Formation of this interface, which induces polymer folding, is strongly opposed by enthalpy and favored by entropy, similar to the vapor-liquid interface.dewetting ͉ folding ͉ hydration entropy ͉ hydrophobic hydration ͉ hydrophobic interaction H ydrophobic interactions are one of the major contributors to biological self-assembly in solution, including protein folding and aggregation, micelle and membrane formation, and biomolecular recognition (1-5). Recent work in this area has focused on the lengthscale dependencies of hydrophobic hydration and interactions (4,(6)(7)(8)(9). In particular, a recent theory by Lum, Chandler, and Weeks (6) highlighted the different physical mechanisms of solvation of small and large hydrophobic solutes in water. Small solutes are accommodated in water through molecular-scale density fluctuations (10, 11), whereas solvation of larger solutes requires formation of an interface similar to that between a liquid and a vapor (4,6,12). This change in physics is also reflected in thermodynamic (entropy vs. enthalpy dominated hydration) (9) and structural (wetting vs. dewetting of the solute surface) (4, 12, 13) aspects of hydration. Similarly, interactions between larger hydrophobic solutes in water (14-18) are characteristically distinct from those between their molecular counterparts (19,20).The differences between the hydration and interactions of small and large solutes characterize many-body effects in hydrophobic phenomena. Effects of similar origin are also at work in association of small hydrophobic solutes into a larger aggregate (21,22) and are quantified by the n-particle potential of mean force (PMF) (23-26). For n Ͼ 3, however, the dimensionality of the system makes calculations of n-particle PMFs computa...

Salts and additives can significantly affect the strength of water-mediated interactions in solution. We present results from molecular dynamics simulations focused on the thermodynamics of hydrophobic hydration, association, and the folding-unfolding of a hydrophobic polymer in water and in aqueous solutions of NaCl and of an osmolyte trimethylamine oxide (TMAO). It is known that addition of NaCl makes the hydration of hydrophobic solutes unfavorable and, correspondingly, strengthens their association at the pair as well as the many-body level (Ghosh, T.; Kalra, A.; Garde, S. J. Phys. Chem. B 2005, 109, 642), whereas the osmolyte TMAO has an almost negligible effect on the hydrophobic hydration and association (Athawale, M. V.; Dordick, J. S.; Garde, S. Biophys. J. 2005, 89, 858). Whether these effects are enthalpic or entropic in origin is not fully known. Here we perform temperature-dependent simulations to resolve the free energy into entropy and enthalpy contributions. We find that in TMAO solutions, there is an almost precise entropy-enthalpy compensation leading to the negligible effect of TMAO on hydrophobic phenomena. In contrast, in NaCl solutions, changes in enthalpy dominate, making the salt-induced strengthening of hydrophobic interactions enthalpic in origin. The resolution of total enthalpy into solute-solvent and solvent-solvent terms further shows that enthalpy changes originate primarily from solvent-solvent energy terms. Our results are consistent with experimental data on the hydration of small hydrophobic solutes by Ben-Naim and Yaacobi (Ben-Naim, A.; Yaacobi, M. J. Phys. Chem. 1974, 78, 170). In combination with recent work by Zangi, Hagen, and Berne (Zangi, R.; Hagen, M.; Berne, B. J. J. Am. Chem. Soc. 2007, 129, 4678) and the experimental data on surface tensions of salt solutions by Matubayasi et al. (Matubayasi, N.; Matsuo, H.; Yamamoto, K.; Yamaguchi, S.; Matuzawa, A. J. Colloid Interface Sci. 1999, 209, 398), our results highlight interesting length scale dependences of salt effects on hydrophobic phenomena. Although NaCl strengthens hydrophobic interactions at both small and large length scales, that effect is enthalpy-dominated at small length scales and entropy-dominated for large solutes and interfaces. Our results have implications for understanding of additive effects on water-mediated interactions, as well as on biocompatibility of osmolyte molecules in aqueous solutions.

We focus on the hydration of a methane and spherical single and multisite C60 and C180 solutes over a range of solute-water attractions to quantify the vicinal water structure and their hydration thermodynamics using extensive molecular dynamics simulations and theory. We show that water structure near larger solutes is more sensitive to solute-water attractions compared to that near smaller ones. To understand the sensitivity, we separate the solute-water potential of mean force into a direct solute-water interaction and an indirect or solvent contribution [omega(r)]. In the absence of omega(r), water density in the solute vicinity would increase exponentially with solute-water interactions. Instead, omega(r) becomes increasingly repulsive with strengthening of solute-water attractions thereby opposing those direct interactions. We term this phenomenon "competitive expulsion," which characterizes the repulsion of a test water molecule by the hydration shell solvent waters. We develop a physically motivated theoretical approach to predict changes in omega(r) with attractions. We call this approach the modified-EXP (M-EXP) approximation owing to the similarity of ideas and especially our final expression with that of the EXP approximation of Chandler and Andersen [J. Chem. Phys. 57, 1930 (1972)]. Solute-water radial distribution functions and chemical potentials calculated using the M-EXP approach are in good agreement with simulation data. These calculations highlight the sensitivity of hydration structure and thermodynamics of bucky ball like solutes to solute-water interactions. We find that excess chemical potentials of bucky balls with standard alkane-like carbon-water interactions parameters are negative, suggesting the need for a careful calibration of those parameters for predictions of solubility, wetting, and water-mediated interactions using molecular simulations.

We explore the prospects of a perturbation approach for predicting how weak attractive interactions affect collapse thermodynamics of hydrophobic polymers in water. Specifically, using molecular dynamics simulations of model polymers in explicit water, we show that the hydration structure is sensitive to the strength of the van der Waals attractions but that the hydration contribution to the potential of mean force for collapse is not. We discuss how perturbation theory ideas developed for small spherical apolar solutes need to be modified in order to account for the effect of attractions on the conformational equilibria of polymers.