Temperature effect on the small-to-large crossover lengthscale of hydrophobic hydration (original) (raw)
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The Journal of Physical Chemistry B, 2012
We present a theoretical model for the effect of water hydrogen bonding on the thermodynamics of hydrophobic hydration. The model is based on a combination of the classical density functional theory with the recently developed probabilistic approach to water hydrogen bonding near a hydrophobic surface. This combination allows one to determine the distribution of water molecules in the vicinity of a hydrophobic particle and calculate the thermodynamic quantities of hydrophobic hydration as well as their temperature dependence. The probabilistic approach allows one to implement the effect of the hydrogen bonding ability of water molecules on their interaction with the hydrophobic surface into the formalism of density functional theory. This effect arises because the number and energy of hydrogen bonds that a water molecule forms near a hydrophobic surface differ from their bulk values. Such alteration gives rise to a hydrogen bond contribution to the external potential field whereto a water molecule is subjected in that vicinity. This contribution is shown to play a dominant role in the interaction of a water molecule with the surface. Our approach predicts that in the temperature range from 293 to 333 K: (a) the free energy of hydration of a planar hydrophobic surface in a model liquid water increases with increasing temperature (although its ratio to the temperature decreases); (b) the hydration process is unfavorable both enthalpically and entropically; (c) the entropic contribution to the hydration free energy is much smaller than the enthalpic one and decreases with increasing temperature, potentially becoming negative. The latter is indirectly supported by the experimental observation that under some conditions the hydration of a molecular hydrophobe is entropically favorable as well as by the molecular dynamics simulations predicting positive hydration entropy for sufficiently large hydrophobes.
The Journal of Physical Chemistry B, 2013
We present a probabilistic approach to water−water hydrogen bonding that allows one to obtain an analytic expression for the number of bonds per water molecule as a function of both its distance to a hydrophobic particle and hydrophobe radius. This approach can be used in density functional theory (DFT) and computer simulations to examine particle size effects on the hydration of particles and on their solvent-mediated interaction. For example, it allows one to explicitly identify a water hydrogen bond contribution to the external potential, whereto a water molecule is subjected near a hydrophobe. The DFT implementation of the model predicts the hydration free energy per unit area of a spherical hydrophobe to be sharply sensitive to the hydrophobe radius for small radii and weakly sensitive thereto for large ones; this corroborates the vision of the hydration of small and large length-scale particles as occurring via different mechanisms. On the other hand, the model predicts that the hydration of even apolar particles of small enough radii may become thermodynamically favorable owing to the interplay of the energies of pairwise (dispersion) water−water and water−hydrophobe interactions. This sheds light on previous counterintuitive observations (both theoretical and simulational) that two inert gas molecules would prefer to form a solventseparated pair rather than a contact one.
Theory and computer simulation studies have predicted that water molecules around hydrophobic molecules should undergo an order−disorder transition with increasing solute size around a 1 nm length scale. Some theories predict the formation of a clathrate-like ordered structure around smaller hydrophobic solutes (<1 nm) and the formation of disordered vapor−liquid interfaces around larger solutes (>1 nm) and surfaces. Experimental validation of these predictions has often been elusive and contradictory. High-resolution Raman spectroscopy has detected that water around small hydrophobic solutes shows a signature similar to that of bulk water at lower temperature (increased ordering and a stronger hydrogen-bonded network). Similarly, water around larger solutes shows an increasing population of dangling OH bonds very similar to higher temperature bulk water. Thus, the solute size dependence of the structure and dynamics of water around hydrophobic molecules seems to have an analogy with the temperature dependence in bulk water. In this work, using atomistic classical molecular dynamics (MD) simulations, we have systematically investigated this aspect and characterized this interesting analogy. Structural order parameters including the tetrahedral order parameter (Q), hydrogen bond distribution, and vibrational power spectrum highlight this similarity. However, in contrast to the experimental observations, we do not observe any length-dependent crossover for linear hydrophobic alcohols (n-alkanols) using classical MD simulations. This is in agreement with earlier findings that linear alkane chains do not demonstrate the length-dependent order−disorder transition due to the presence of a sub-nanometer length scale along the cross section of the chain. Moreover, the collapsed state of linear hydrocarbon chains is not significantly populated for smaller chains (number of carbons below 20). In the context of our computational results, we raise several pertinent questions related to the sensitivity of various structural and dynamical parameters toward capturing these complex phenomena of hydrophobic hydration.
Size and shape dependence of hydrophobic hydration at the level of primitive models
Physical Chemistry Chemical Physics, 2002
The hydrophobic hydration of apolar solutes of different shape and size is studied at an elementary level using two types of extended primitive models of water and representing the solute by hard spheres, cylinders, and spherocylinders, respectively. The structure of the first hydration shell around cylindrical particles is determined and compared to that around spherical ones and at a hard structureless flat wall. It is found that while the two studied models of water give the same hydration structure for small non-polar particles, larger solutes are hydrated in a different way. Whereas one model does not show significant size and shape dependence, the other predicts significant changes in the orientation and hydrogen bonding of water molecules in the vicinity of the hydrophobic surface. These results are in agreement with those found for spherical solutes and confirm sensitivity of hydration phenomena to details of models of water, particularly to the strength and geometry of hydrogen bonding.
Collective properties of hydration: long range and specificity of hydrophobic interactions
Biophysical Journal, 1997
We report results of molecular dynamics (MD) simulations of composite model solutes in explicit molecular water solvent, eliciting novel aspects of the recently demonstrated, strong many-body character of hydration. Our solutes consist of identical apolar (hydrophobic) elements in fixed configurations. Results show that the many-body character of PMF is sufficiently strong to cause 1) a remarkable extension of the range of hydrophobic interactions between pairs of solute elements, up to distances large enough to rule out pairwise interactions of any type, and 2) a SIF that drives one of the hydrophobic solute elements toward the solvent rather than away from it. These findings complement recent data concerning SIFs on a protein at single-residue resolution and on model systems. They illustrate new important consequences of the collective character of hydration and of PMF and reveal new aspects of hydrophobic interactions and, in general, of SIFs. Their relevance to protein recognition, conformation, function, and folding and to the observed slight yet significant nonadditivity of functional effects of distant point mutations in proteins is discussed. These results point out the functional role of the configurational and dynamical states (and related statistical weights) corresponding to the complex configurational energy landscape of the two interacting systems: biomolecule + water.
Long-Time Correlations and Hydrophobe-Modified Hydrogen- Bonding Dynamics in Hydrophobic Hydration
The physical mechanisms behind hydrophobic hydration have been debated for over 65 years. Spectroscopic techniques have the ability to probe the dynamics of water in increasing detail, but many fundamental issues remain controversial. We have performed systematic first-principles ab initio Car−Parrinello molecular dynamics simulations over a broad temperature range and provide a detailed microscopic view on the dynamics of hydration water around a hydrophobic molecule, tetramethylurea. Our simulations provide a unifying view and resolve some of the controversies concerning femtosecond-infrared, THz−GHz dielectric relaxation, and nuclear magnetic resonance experiments and classical molecular dynamics simulations. Our computational results are in good quantitative agreement with experiments, and we provide a physical picture of the long-debated "iceberg" model; we show that the slow, long-time component is present within the hydration shell and that molecular jumps and over-coordination play important roles. We show that the structure and dynamics of hydration water around an organic molecule are non-uniform.
2014
We review some recent progress in understanding the dynamics and thermodynamics of hydration water at low temperature T. Using a theoretical model, we predict two dynamic crossovers for water at the interface with lysozyme at low hydration level and, using dielectric spectroscopy, we observe them. We show that these two crossovers are the consequence of the particular formation and rearrangement of the hydrogen-bond (HB) network of hydration water. At approximately 252 K we predict a large increase in isolated HBs that gives rise to a P-dependent specific heat maximum. This maximum, much broader and smaller than that consistent with calorimetric experiments, is similar to that calculated in many atomistic models for supercooled water and is often associated with the Widom line of the liquid-liquid phase transition. At approximately 181 K we predict that the HB network percolates over the protein surface, inducing a P-independent large structural rearrangement. As a consequence, the specific heat has a large maximum that is consistent with data from calorimetric experiments. We show that this low-T , P-independent maximum is the Widom line, consistent with other recent analyses based on experimental data. Each of the two maxima allows us to predict a dynamic crossover: one between two non-Arrhenius behaviours at 252 K and the other between non-Arrhenius and Arrhenius behaviour at 181 K. Our dielectric experiments for hydrated lysozyme show clear evidence of these two crossovers. Our analysis bridges the gap between experimental data and theoretical predictions.
Hydrophobic hydration at the level of primitive models
Molecular Physics, 1999
Details of structural changes that take place in water near an apolar solute have been studied by Monte Carlo simulations for hard sphere solutes of increasing size, including the limiting case of water at a hard structureless wall. Water has been modelled by two di erent types of extended primitive model, the four-site EPM4 model and ®ve-site EPM5 model. Two di erent patterns of the orientational ordering of the water molecules around the solute as a function of its size have been found. For the EPM5 model, the structure of water and the orientation of its molecules near an apolar solute of ®nite diameter do not seem to be sensitive to the size of the solute, and only become more pronounced when the solute becomes a hard wall. On the other hand, the orientation ordering of the EPM4 molecules gradually changes with increasing size of the solute, and for solutes larger than approximately ®ve times the size of the water molecule it is opposite to that near a small solute. A novel method to evaluate the excess chemical potential of large solutes has been implemented, and some thermodynamic quantities for water (distribution of hydrogen bonds and the excess chemical potential) have been computed as a function of the distance from the solute.