Static structure factor and collective diffusion of globular proteins in concentrated aqueous solution

We report our measurement of the time average and the temporal autocorrelation function of the intensity of light scattered by the highly monomeric globular protein, bovine gamma(II)-crystallin, in aqueous solution as a function of wave number q, protein volume fraction phi, and temperature T. The time average intensity data is used to obtain the q-->0 limit of the static structure factor S(phi,T), as a function of phi and T. We show that S(phi,T) may be well characterized by modeling the proteins as interacting through the Baxter adhesive hard sphere pair interaction potential. The temporal autocorrelation function data is used to determine the collective diffusion coefficient (D) over tilde(phi,T) of the proteins as a function of phi and T. We then obtain the experimental hydrodynamic factor (H) over tilde(phi,T)=S(phi,T)[(D) over tilde(phi,T)/D-0(T)], where D-0(T) is the diffusion coefficient of the individual proteins in the phi-->0 limit. We find that (H) over tilde exhibits a different phi-dependence at low (phi less than or equal to 0.016) and high (greater than or similar to 0.02) protein volume fractions. In the low phi domain our data for (H) over tilde are consistent with the theoretical result for the collective diffusion in the q-->0, t-->0 limit. However, for phi greater than or similar to 0.02 we find a deviation from single exponential decay in the autocorrelation functions, and an unexpected, large change in the slope of the (H) over tilde vs phi relation. This crossover at such low phi suggests the existence of a heretofore unappreciated length scale in the dynamics of colloid solutions. Clearly, further theoretical insights are required to understand the origin of this crossover behavior. (C) 1996 American Institute of Physics.