Influence of ionic strength on the diffusion of polystyrene latex spheres, bovine serum albumin, and polynucleosomes

Abstract

Quasielastic light scattering methods were used to determine apparent diffusion coefficients (Dapp) for polystyrene latex spheres, bovine serum albumin, and polynucleosomes under conditions of neutral pH and low ionic strength (0.1–50 mM). Data were collected at several time intervals (Δt) at each of several scattering angles (θ) in the range 35<θ<120 and analyzed by the asymptotic analysis method (Dapp vs T, where T=NΔt is the time window for the N+1 point autocorrelation function). These data suggest that the asymptotic limit Dapp(T → 0) is independent of θ for the three systems examined, whereas the asymptotic limit Dapp(T → ∞) appears to be proportional to the reciprocal of the total light scattering intensity. These results are interpreted in terms of ‘‘free’’ and ‘‘temporal aggregate’’ domains within the polyelectrolyte solution. The critical size, hence stability, of the temporal aggregate is proposed to result from a balance between attractive forces arising from fluctuations in the small ion and polyion distributions and the disruptive Brownian forces. Under conditions of extremely low ionic strength, direct interactions between polyions tend to dominate as inferred from data in the literature. These observations suggest at least four regimes in the description of the ionic strength dependence of Dapp.