Magnetization transfer, cross-relaxation, and chemical exchange in rotationally immobilized protein gels

Water proton spin-lattice relaxation rates are reported as a function of the magnetic field strength for cross-linked bovine serum albumin samples. The relaxation dispersion profile is analyzed using a relaxation model where the solid components have the magnetic field dependence proportional to v-0.5 which may result from a defect diffusion model with two degrees of freedom. If the cross-linking agent concentration is not sufficiently high, the relaxation dispersion curve may have significant contributions from freely rotating protein. The magnetic field dependence of the relaxation rates studied as a function of the proton mole fraction in the sample show that approximately 30% of the magnetization transfer rate is directly proportional to the proton mole fraction. This contribution is identified with the magnetization transfer from exchange of whole water molecules with buried binding sites on the protein. The second order magnetization transfer rate constant is 388 s-1 assuming unit water spin concentration. The solid component relaxation obeys an Arrhenius activation law, but the overall temperature dependence of the cross-relaxation is complicated by chemical exchange processes which enter with opposite sign.