Quaternary Diffusion Coefficients in a Protein-Polymer-Salt-Water System Determined by Rayleigh Interferometry.

The authors have exptl. investigated multicomponent diffusion in a protein-polymer-salt-water quaternary system. Specifically, the authors have measured the nine multicomponent diffusion coeffs., Dij, for the lysozyme-poly(ethylene glycol)-NaCl-water system at pH 4.5 and 25 using precision Rayleigh interferometry. Lysozyme is a model protein for protein-crystn. and enzymol. studies. The authors find that the protein diffusion coeff., D11, decreases as polymer concn. increases at a given salt concn. This behavior can be quant. related to the corresponding increase in fluid viscosity only at low polymer concn. However, at high polymer concn. (250 g/L), protein diffusion is enhanced compared to the corresponding viscosity prediction. The authors also find that a protein concn. gradient induces salt diffusion from high to low protein concn. This effect increases in the presence of poly(ethylene glycol). Finally, the authors have evaluated systematic errors assocd. with measurements of protein diffusion coeffs. by dynamic light scattering. This work overall helps characterize protein diffusion in crowded environments and may provide guidance for further theor. developments in the field of protein crystn. and protein diffusion in such crowded systems, such as the cytoplasm of living cells.