Model for the hydration of nonpolar compounds and polymers

Understanding the role of hydrogen bonds in water dynamics and protein stability

The mechanisms of cold and pressure denaturation of proteins are a matter of debate, but it is commonly accepted that water plays a fundamental role in the process. It has been proposed that the denaturation process is related to an increase of hydrogen bonds among hydration water molecules. Other theories suggest that the causes of denaturation are the density fluctuations of surface water, or the destabilization of hydrophobic contacts as a consequence of water molecule inclusions inside the protein, especially at high pressures. We review some theories that have been proposed to give insight into this problem, and we describe a coarse-grained model of water that compares well with experiments for proteins' hydration water. We introduce its extension for a homopolymer in contact with the water monolayer and study it by Monte Carlo simulations in an attempt to understand how the interplay of water cooperativity and interfacial hydrogen bonds affects protein stability.

Low-temperature-induced swelling of a hydrophobic polymer: A lattice approach

The authors investigate equilibrium properties of a simple model of hydrophobic polymer in aqueous solution by means of dynamic Monte Carlo simulations. The solvent is described by a simplified two-dimensional model, defined on a triangular lattice, which has been previously shown to account for most thermodynamic anomalies of pure water and of hydrophobic solvation for monomeric solutes. The polymer is modeled as a self-avoiding walk on the same lattice. In this framework, the degrees of freedom of water are taken into account explicitly, and in principle there is no need to introduce effective self-contact interactions for the polymer in order to mimic the hydrophobic effect. In certain conditions, the authors observe low-temperature-induced swelling, i.e., expansion of the polymer globule upon decreasing temperature. The authors discuss the relationship between this phenomenon and the anomalous properties of the solvent.

Thermodynamic anomalies in a lattice model of water: Solvation properties

We investigate a lattice-fluid model of water, defined on a three-dimensional body-centered-cubic lattice. Model molecules possess a tetrahedral symmetry, with four equivalent bonding arms. The model is similar to the one proposed by Roberts and Debenedetti [J. Chem. Phys. 105, 658 (1996)], simplified by removing distinction between "donors" and "acceptors." We focus on the solvation properties, mainly as far as an ideally inert (hydrophobic) solute is concerned. As in our previous analysis, devoted to neat water [J. Chem. Phys. 121, 11856 (2004)], we make use of a generalized first-order approximation on a tetrahedral cluster. We show that the model exhibits quite a coherent picture of water thermodynamics, reproducing qualitatively several anomalous properties observed both in pure water and in solutions of hydrophobic solutes. As far as supercooled liquid water is concerned, the model is consistent with the second critical-point scenario.

Possible mechanism for cold denaturation of proteins at high pressure

We study cold denaturation of proteins at high pressures. Using multicanonical Monte Carlo simulations of a model protein in a water bath, we investigate the effect of water density fluctuations on protein stability. We find that above the pressure where water freezes to the dense ice phase (approximately 2 kbars) the mechanism for cold denaturation with decreasing temperature is the loss of local low-density water structure. We find our results in agreement with data of bovine pancreatic ribonuclease A.