Effect of titration charge on the diffusion of bovine serum albumin

Quasi-elastic light scattering studies were performed on purified bovine serum albumin sample under conditions of high and low ionic strength and pH. Two relaxation modes were observed in all cases. The apparent diffusion coefficient obtained in the asymptotic time window (T) limit T leads to 0-i.e., Dapp(T leads to 0)-was found to be approximately equal to 6.1 X 10(-7) cm2/sec under conditions that minimized electrical forces (100 mM KCl at pH 4.5). As the ionic strength was lowered to 0.1 mM KCl (pH 4.5) or the pH was raised to 10 (100 mM KCl), Dapp(T leads to 0) increased to 7.2-7.5 X 10(-7) cm2/sec. These observations for Dapp(T leads to 0) are interpretable in terms of small-ion-polyion coupled modes in accordance with the theory of Lin et al. regarding dynamic Donnan effects [Lin, S.-C., Lee, W. I. & Schurr, J. M. (1978) Biopolymers 17, 1041-1064] without having to invoke direct polyion-polyion interactions. These direct interactions may be important at extreme ionic strength and pH conditions (0.1 mM KCl at pH 10). Concomitant with an increase in Dapp(T leads to 0), under appropriate changes in solvent conditions, is an increase in the relative amplitude and relaxation time of the slow decay mode.