Diffusion in protein solutions at high concentrations: A study by quasielastic light scattering spectroscopy

Quasielastic light scattering spectroscopy was used to study the mutual diffusion coefficient Dm of bovine serum albumin over the pH range 4.3–7.6 for concentrations ranging as high as 334 g/l. Using literature measurements of the tracer diffusion coefficient DT and the osmotic compressibility (∂π/∂c)P,T, we find that Dm depends on concentration and pH as predicted by the generalized Stokes–Einstein relation Dm=DT(∂π/∂c)P,T (1−φ)/KBT, φ being the volume fraction of solute and KB being Boltzmann’s constant. Our data show that Dm and DT depend quite differently on solute concentration; DT can be a factor of 3 smaller than Dm at protein concentrations exceeding 200 g/l.