Short-range interactions of globular proteins at high ionic strengths

We compare the interactions of betalactoglobulin and lysozyme under different salt concentrations of ammonium sulfate by means of photon correlation spectroscopy measurements. The diffusion coefficient depends linearly upon protein concentration (1-25 g/L), and no evident aggregation occurs even at the highest salt concentrations. The slope of the diffusion coefficient is larger for lysozyme than for betalactoglobulin, especially at high salt concentration. The analysis of the interaction coefficients by means of the DLVO theory does not offer an unified picture of the protein interactions for the two systems. We discuss the possibility that specific (hydrogen bonding, salt bridges, etc.) or nonspecific (hydration forces) interactions, that go beyond the DLVO approximation, might be the origin of the larger van der Waals attraction shown by lysozyme. The data are consistent with a contribution of repulsive hydration forces that are smaller for lysozyme than for betalactoglobulin. This effect is in agreement with the difference in the protein size and charge.