Buffer-specific effects arise from ionic dispersion forces

Buffer solutions do not simply regulate pH, but also change the properties of protein molecules. The zeta potential of lysozyme varies significantly at the same buffer concentration, in the order Tris > phosphate > citrate, with citrate even inverting the zeta potential, usually positive at pH 7.15, to a negative value. This buffer-specific effect is a special case of the Hofmeister effect. Here we present a theoretical model of these buffer-specific effects using a Poisson-Boltzmann description of the buffer solution, modified to include dispersion forces of all ions interacting with the lysozyme surface. Dispersion coefficients are determined from quantum chemical polarizabilites calculated for each ion for tris, phosphate, and citrate buffer solutions. The lysozyme surface charge is controlled by charge regulation of carboxylate and amine sites of the component amino acids. The theoretical model satisfactorily reproduces experimental zeta potentials, including change of sign with citrate, when hydration of small cosmotropic ions (Na+, H+, OH-) is included.