Nitroxide radical induced solvent proton relaxation: Measurement of localized translational diffusion

Abstract

The field dependence of solvent proton nuclear magnetic relaxation is shown to provide a test of whether the relaxation is dominated by translational or rotational motions. When a paramagnetic center is present with a sufficiently long electron relaxation time, the dominance of translational contributions to the correlation time for the electron–nucleus interaction permits characterization of the translational diffusion of the solvent in local environment of the paramagnetic center. For bovine serum albumin covalently labeled with nitroxide radicals the approach yields a value of (3±1)×10−6 cm2 s−1 for water within the first 10 Å of the protein surface. The method is general for the measurement of localized solvent diffusion coefficients.