The wave vector dependence of diffusion coefficients in photon correlation spectroscopy of protein solutions

The mutual diffusion coefficient Dm was measured by means of photon correlation spectroscopy (PCS) for bovine serum albumin (BSA) at pH 4.7 and ionic strength I≃0.12 M. The BSA concentrations ranged from 1.00 to 10.7 g/dℓ and the magnitude of the scattering wave vector K varied from 3.21×104 to 3.51×105 cm−1. It was found that Dm=D0[1−(0.0166±0.0009)C], where D0=(5.92±0.04)×10−7 cm2/s corrected to water at 20 °C and the C measured in units of g/dℓ. The quantity Cn/K3, where Cn is the concentration in number of solute particles per cm3, ranged from 2 to 27 000, but this had no influence on the measured values of Dm. This work does not support the idea that the small size of K−1 or the small number of molecules contained in the volume K−3 affects the magnitudes of diffusion coefficients determined by PCS for proteins at moderate ionic strengths.