Thermodynamical implications of a protein model with water interactions

We refine a protein model that reproduces fundamental aspects of protein th ermodynamics. The model exhibits two transitions, hot and cold unfolding. T he number of relevant parameters is reduced to three: (1) binding energy of folding relative to the orientational energy of bound water, (2) ratio of degrees of freedom between the folded and unfolded protein chain, and (3) t he number of water molecules that can access the hydrophobic parts of the p rotein interior upon unfolding. By increasing the number of water molecules in the model., the separation between the two peaks in the heat capacity c urve comes closer, which is more consistent with experimental data. In the end we show that if we, as a speculative assumption, assign only two distin ct energy levels for the bound water molecules, better correspondence with experiments can be obtained. (C) 2001 Academic Press.