The mutual diffusion coefficientDmwas measured by means of photon correlation spectroscopy (PCS) for bovine serum albumin (BSA) atpH 4.7 and ionic strengthIsime;0.12 M. The BSA concentrations ranged from 1.00 to 10.7 g/dland the magnitude of the scattering wave vector K varied from 3.21times;104to 3.51times;105cmminus;1. It was found thatDm=D0lsqb;1minus;(0.0166plusmn;0.0009)Crsqb;, whereD0=(5.92plusmn;0.04)times;10minus;7cm2/s corrected to water at 20thinsp;deg;C and theCmeasured in units of g/dl. The quantityCn/K3, whereCnis the concentration in number of solute particles per cm3, ranged from 2 to 27 000, but this had no influence on the measured values ofDm. This work does not support the idea that the small size ofKminus;1or the small number of molecules contained in the volumeKminus;3affects the magnitudes of diffusion coefficients determined by PCS for proteins at moderate ionic strengths.
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