Second virial coefficients as a measure of protein-osmolyte interactions

The cytoplasm contains high concentrations of cosolutes. These cosolutes in clude macromolecules and small organic molecules called osmolytes. However, most biophysical studies of proteins are conducted in dilute solutions. Tw o broad classes of models have been used to describe the interaction betwee n osmolytes and proteins. One class focuses on excluded volume effects, whi le the other focuses on binding between the protein and the osmolyte. To be tter understand protein-smolyte interactions, we have conducted sedimentati on equilibrium analytical ultracentrifugation experiments using fenicytochr ome c as a model protein. From these experiments, we determined the second virial coefficients for a series of osmolytes. We have interpreted the seco nd virial coefficient as a measure of both excluded volume and protein-osmo lyte binding. We conclude that simple models are not sufficient to understa nd the interactions between osmolytes and proteins.