Further Studies on the Rotation of Globular Proteins in Polymer Solutions

The rotational diffusion of pyrene‐butyric‐labelled human serum albumin and dansyl‐labelled bovine fetuin, β‐lactoglobulin, α‐lactalbumin and pancreas ribonuclease has been studied by fluorescence polarization in concentrated dextran solutions (up to 0.4 g/ml). The relaxation times of the proteins increased only moderately even at very high polysaccharide concentrations. The results confirmed the conclusions drawn from studies on dansyl‐labelled serum albumin that the friction exerted by the polysaccharide on the rotation of a protein is low. The very strong inhibition of translational diffusion in polymer media can therefore be ascribed to steric hindrance. The effect of dextran on the rotational diffusion of different proteins varied and the reason for this is discussed. The measurements indicate that conformational changes may occur in some proteins at high polymer concentrations.