Small-ion friction, hydrodynamic interaction, and charge fluctuation effects in solutions of a globular macroion

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Abstract
The initial slope of the quasielastic light‐scattering (QELS) autocorrelation function taken on solutions of charged bovine serum albumin is shown not to scale inversely with the scattering intensity as the interaction strength is varied by changing the ionic strength. This effect is shown to be nearly independent of scattering q vector over a factor of 3 variation in q2. Molecular expansion is shown not to occur. The apparent increase in the friction factor is shown to be partly due to counterion friction. A measure of the nonexponentiality of the autocorrelation functions is used together with thermodynamic parameters to calculate an upper limit for the protein charge heterogeneity.