Measurement of the local translational diffusion rates of proteins by saturation transfer EPR spectroscopy.

The concentration dependence of the normalized integral of the saturation-transfer EPR spectrum of human serum albumin spin-labelled on amino groups is found to be sensitive to viscosity and to temperature in aqueous solution and in 60% glycerol. The dependence on protein concentration is consistent with theoretical predictions for a diffusion-controlled Heisenberg spin-exchange interaction and the experimentally derived bimolecular collision rate constants are in reasonable agreement with those calculated theoretically for a diffusion-controlled process. The method is therefore applicable to the determination of translational diffusion coefficients of proteins in solution and, because of the nature of the saturation transfer EPR method, should be ideally suited to the study of local translational diffusion of proteins in membranes.