Self-diffusion and cooperative diffusion of globular proteins in solution

The globular protein beta-lactoglobulin (beta-1g) was studied in aqueous solution at pH = 7.0 using pulsed field gradient nuclear magnetic resonance (PFG-NMR) and static (SLS) and dynamic light scattering (DLS). The concentration dependence of the self-diffusion coefficient (D-s) and the cooperative (or mutual) diffusion coefficient (D-c) were determined as a function of the concentration up to volume fraction phi = 0.15. The effect of electrostatic interactions was investigated by comparing systems with 0.003 and 0.1 M added salt. The concentration dependence of D-s with 0.1 M added salt is found to be the same as for other globular proteins reported in the literature. Directly measured values of D-c using DLS are consistent with values derived from a combination of PFG-NMR and SLS assuming that the protein-protein friction coefficient is small compared to the protein-solvent friction coefficient.