STATIC STRUCTURE FACTOR AND COLLECTIVE DIFFUSION OF GLOBULAR-PROTEINSIN CONCENTRATED AQUEOUS-SOLUTION

We report our measurement of the time average and the temporal autocor relation function of the intensity of light scattered by the highly mo nomeric globular protein, bovine gamma(II)-crystallin, in aqueous solu tion 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 p hi and T. We show that S(phi,T) may be well characterized by modeling the proteins as interacting through the Baxter adhesive hard sphere pa ir interaction potential. The temporal autocorrelation function data i s used to determine the collective diffusion coefficient (D) over tild e(phi,T) of the proteins as a function of phi and T. We then obtain th e 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 o f 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 equ al 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 cons istent 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 autocorrelati on functions, and an unexpected, large change in the slope of the (H) over tilde vs phi relation. This crossover at such low phi suggests th e existence of a heretofore unappreciated length scale in the dynamics of colloid solutions. Clearly, further theoretical insights are requi red to understand the origin of this crossover behavior. (C) 1996 Amer ican Institute of Physics.