Effect of Surface Tension on the Stability of Heat-Stressed Proteins:  A Molecular Thermodynamic Interpretation

The stability behavior of four globular proteins (glucose oxidase, ribonuclease, lysozyme, and carbonic anhydrase) in pure buffer and in the presence of water-miscible hydroxylic additives (alcohols, polyols, and sugars) was analyzed. Attention was focused on the influence of these compounds on the melting temperature of the proteins. For all of the proteins examined, this latter quantity was found to be linearly related to the bulk surface tension of the mixed solvent. To provide a quantitative interpretation to the above observation, a molecular thermodynamic model, based on the additive-induced perturbation of the equilibrium between the folded and the unfolded protein forms, was developed. It is shown that, under some limiting conditions, the Gibbs equilibrium criterion applied to the two-state unfolding process yields a linear dependence of the melting temperature on the bulk surface tension, as observed for the proteins considered. The results obtained appear to indicate that the conformational stability of heat-stressed proteins in water−hydroxylic cosolvent mixtures does not rely on any special property of these substances but rather on their ability to affect the interfacial free energy between the protein and the solvent through perturbations of the surface tension of water. The model proposed can be used for interpretation and correlation of thermal unfolding data and, as a diagnostic tool, to assess whether the surface tension mechanism provides the overwhelming contribution to protein unfolding.