THERMODYNAMIC ANALYSIS OF THE EFFECTS OF SMALL INERT COSOLUTES IN THEULTRACENTRIFUGATION OF NONINTERACTING PROTEINS

Considerations of the effect of a small cosolute on the sedimentation equilibrium distribution for a noninteracting protein have led to the development of a simple procedure for evaluating both the molecular we ight of the protein and the second virial coefficient describing the e xcluded volume interaction between protein and cosolute. Its applicati on is illustrated by analysis of sedimentation equilibrium distributio ns for bovine thyroglobulin and horse liver alcohol dehydrogenase in t he presence of a range of sucrose concentrations, and also of those fo r aldolase in the presence of urea to obtain the subunit molecular wei ght of this tetrameric enzyme. The effects of sucrose concentration on the sedimentation coefficients of thyroglobulin, catalase, and horse liver alcohol dehydrogenase are also examined to demonstrate that the second virial coefficients for protein-cosolute excluded volume intera ction may be determined, albeit with less precision, from the cosolute concentration required to render the sedimentation coefficient zero b y virtue of its effect on the buoyancy term. These findings serve to r einforce the fact that the effects of small cosolutes usually ascribed to changes in ''protein solvation'' are envisaged more realistically in terms of excluded volume.