Partial volumes and compressibilities of extended polypeptide chains in aqueous solution: additivity scheme and implication of protein unfolding at normal and high pressure.

An empirical additivity method for calculation of the partial volumes and adiabatic compressibilities of extended oligo- and polypeptides having arbitrary amino acid compositions has been developed and tested by comparison with available experimental data. Its accuracy is the best among the known empirical approaches. Comparison of experimental data on protein denaturation with the results of calculation allows one to discriminate between the unfolded and molten globule states of globular proteins and to estimate the extent of unfolding. For the first time, experimental nonlinear data for the volume-pressure relationship in proteins and model compounds have been used to interpret the high-pressure denaturation of proteins. It has been shown that the two denatured states, molten globule and unfolded ones, can be attained by a pressure rise: the molten globule state by moderate pressure and the unfolded one by high pressure. The relationship between volumetric properties and hydration is briefly discussed.