SOLUTE EXCLUDED-VOLUME EFFECTS ON THE STABILITY OF GLOBULAR-PROTEINS - A STATISTICAL THERMODYNAMIC THEORY

A statistical thermodynamic theory, is developed to investigate the ef fects of solute excluded volume on the stability of globular proteins. Proteins ale modeled as two states in chemical equilibrium: the denat ured state is modeled as a flexible chain of tangent hard spheres (pea rl necklace chain) while the native state is modeled as a single hard sphere. Study of model proteins bovine pancreatic trypsin inhibitor. a nd lysozyme in a McMillan-Mayer model solution of hard spheres indicat es that the excluded volume of solutes has three distinct types of eff ects on protein stability: (1) small-size solutes strongly denature pr oteins, (2) medium-size solutes stabilize proteins at low solute conce ntrations and destabilize them at high concentrations, and (3) large-s ize solutes stabilize native-state proteins across the whole liquid re gion. The study also finds that increasing the chain length of hard-ch ain polymer solutes has an effect On protein stability that is similar to increasing the diameter of spherical solutes. This work qualitativ ely explains why stabilizers tend to be large size molecules such as s ugars, polymers, polynols, nonionic, and anionic surfactants while den aturants tend to be small size molecules such as alcohols, glycols, am ides, formamides, ureas, and guanidium salts. Quantitative comparison between theoretical predictions and experimental results for folding f ree energy changes shows that the excluded-volume effect is at least a s important as the binding and/or electrostatic effects on solute-assi sted protein-denaturation processes. Our theory may also be able to ex plain the effect of excluded volume on the Phi, condensation of DNA. ( C) 1996 John Wiley & Sons, Inc.