Protein stability and solubility depend strongly on the presence of osmolytes, because of the protein preference to be solvated by either water or osmolyte. It has traditionally been assumed that only this relative preference can be measured, and that the individual solvation contributions of water and osmolyte are inaccessible. However, it is possible to determine hydration and osmolyte solvation (osmolation) separately using Kirkwood-Buff theory, and this fact has recently been utilized by several researchers. Here, we provide a thermodynamic assessment of how each surface group on proteins contributes to the overall hydration and osmolation. Our analysis is based on transfer free energy measurements with model-compounds that were previously demonstrated to allow for a very successful prediction of osmolyte-dependent protein stability. When combined with Kirkwood-Buff theory, the Transfer Model provides a space-resolved solvation pattern of the peptide unit, amino acids, and the folding/unfolding equilibrium of proteins in the presence of osmolytes. We find that the major solvation effects on protein side-chains originate from the osmolytes, and that the hydration mostly depends on the size of the side-chain. The peptide backbone unit displays a much more variable hydration in the different osmolyte solutions. Interestingly, the presence of sucrose leads to simultaneous accumulation of both the sugar and water in the vicinity of peptide groups, resulting from a saccharide accumulation that is less than the accumulation of water, a net preferential exclusion. Only the denaturing osmolyte, urea, obeys the classical solvent exchange mechanism in which the preferential interaction with the peptide unit excludes water.
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