Polyion character of globular proteins detected by translational and rotational diffusion

Photon correlation spectroscopy measurements are performed on a solution of lysozyme, a small, highly charged protein at pH=4, in glycerol-buffer mixtures. From the correlation functions of the polarized and depolarized scattered intensity, we derive an effective protein radius by inverting the Stokes Einstein relation for either the translational or the rotational diffusion, as a function of the protein concentration 0< or =C< or =40 g/L. The translational radius decreases with the protein concentration and extrapolates at C approximately 0 to a value slightly larger than the mean rotational radius, which is independent of the protein concentration. By employing recent calculations on electrolyte friction effects (eletrolyte-protein interactions) on both the translational and rotational radius and by keeping also into account the protein-protein electrostatic interactions, we are able to account for the observed differences, suggesting also that lysozyme is more highly hydrated in glycerol mixtures than in the pure buffer. These results indicate that depolarized photon correlation spectroscopy measurements can be used as a valuable tool to detect small changes in the overall protein size.