A polypeptide chain can adopt very different conformations, a fundamental distinguishing feature of which is the water accessible surface area, WASA, that is a measure of the layer around the polypeptide chain where the center of water molecules cannot physically enter, generating a solvent-excluded volume effect. The large WASA decrease associated with the folding of a globular protein leads to a large decrease in the solvent-excluded volume, and so to a large increase in the configurational/translational freedom of water molecules. The latter is a quantity that depends upon temperature. Simple calculations over the -30 to 150 °C temperature range, where liquid water can exist at 1 atm, show that such a gain decreases significantly on lowering the temperature below 0 °C, paralleling the decrease in liquid water density. There will be a temperature where the destabilizing contribution of the polypeptide chain conformational entropy exactly matches the stabilizing contribution of the water configurational/translational entropy, leading to cold denaturation.

Many of the mixture models of water seek to explain the large free energy change associated with hydrophobic hydration by means of changes in the number and character of the hydrogen bonds in water. All of these models, regardless of detail, are in clash with the idea that hydrogen bond rearrangements will produce changes in both enthalpy and entropy, which largely compensate to produce little net free energy change. One of the simplest and most recent of these mixture models is Muller's two-state model, which produces small enthalpy and large negative entropy changes. In this paper, Muller's model is examined in detail. It is found that only slight changes are required in order for the model to produce nearly compensating enthalpy and entropy changes.

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