Diffusivity of protein in aqueous solutions

The diffusion coefficient of lysozyme, a globular protein, was measured at various conditions as functions of lysozyme concentration, salt concentration, and solution ‘age’ in concentrated, saturated, and supersaturated solutions, employing Gouy interferometry. Distilled water, 0.05 M potassium phosphate buffer, and 0.1 M sodium acetate buffer solutions with 0, 2, 4, and 5 wt% NaCl were used as solvents. The pH of lysozyme solutions in distilled water was 4.75 due to the self-buffering capacity of lysozyme. The pH’s of the lysozyme solutions in the potassium phosphate and sodium acetate buffers were adjusted to 6.8 and 4.0, respectively. The experimental temperature was 25°C. In a salt-free system, the concentration dependent diffusion of lysozyme showed typical electrolyte diffusion behavior, while a salt-polyelectrolyte system exhibited the behavior of a non-electrolyte. Diffusion results in the supersaturated region showed a little effect of concentration or solution ‘age’ al a fixed NaCl concentration. A rapid decline in diffusion coefficient with increasing NaCl concentration in the supersaturated region, however, was observed.