Relaxation of the Electronic Spin Moment of Copper(II) Macromolecular Complexes in Solution

Abstract

The magnetic field dependence of the longitudinal relaxation rate (1/T1 of solvent protons (NMRD profile) was measured for solutions of Cu2+-substituted transferrin and native (copper-zinc) superoxide dismutase as a function of solvent viscosity, the latter adjusted with sucrose. Similar measurements were made on demetalated transferrin and reduced superoxide dismutase to obtain the diamagnetic background. Both sets of data are found to be dependent on the viscosity of the solvent, as expected. Subtraction of the two sets of data gives the paramagnetic contribution to the NMRD profiles, which is insensitive to solvent viscosity. This indicates that the correlation time for the magnetic interaction of protons with the paramagnetic Cu2+ centers is insensitive to thermal (Brownian) rotational motion of the protein. From this it is argued that the longitudinal relaxation time of the electronic spin moment of the Cu2+ ions, including the correlation time for its coupling to the thermal motions of the protein, is also insensitive to the rotational thermal motion of the protein. Possible implications for the mechanism of electron relaxation in copper systems in solution are discussed. © 1989, American Chemical Society. All rights reserved.