Effects of water activity and aqueous solvent ordering on thermal stability of lysozyme, alpha-chymotrypsinogen A, and alcohol dehydrogenase

Effects of water activity (a(w)) and solvent ordering were separately analy zed on the thermal unfolding of lysozyme and alpha -chymotrypsinogen A, and also on the thermal deactivation of yeast alcohol dehydrogenase (YADH) in aqueous solutions with various additives. With the coexistence of additives , water activity was the determinant of the extent of the change in the the rmal stability of proteins while solvent ordering was the determinant of th e direction of the change. The parameter alpha, determined from the activit y coefficient of water, representing the deviation of a(w) from that of the ideal solution, was useful as a quantitative index of the solvent ordering showing good correlations with the unfolding temperature and enthalpy of l ysozyme and alpha -chymotrypsinogen A and also with the thermal deactivatio n rate constant of YADH at a constant a(w). Solvent ordering seemed to affe ct the thermal stability of proteins mainly through its effect on the intra molecular hydrophobic interaction among amino acid residues in a protein mo lecule but the contribution of the electrostatic interaction including hydr ogen bonding through the change in permittivity of solution was also sugges ted. (C) 2001 Elsevier Science B.V. All rights reserved.