1H magnetic cross-relaxation between multiple solvent components and rotationally immobilized protein.

Magnetic cross-relaxation spectra or Z-spectra are presented for water, acetone, methanol, dimethylsulfoxide, and acetonitrile in cross-linked bovine serum albumin gels. Each solvent studied, reports the same Z-spectrum linewidth and shape for the solid component that follows from solutions of the coupled relaxation equations. The Z-spectra demonstrate competition among solvents for specific protein binding sites. The rate of magnetization transfer in the rotationally immobilized protein environment is approximated by 1/T2 for the solid component, which is shown to account for the observed magnetization transfer rates in the systems studied. The temperature dependence of the Z-spectra are different for water compared with the organic solvents. The cross-relaxation efficiency in the organic solvents decreases with increasing temperature because molecules bind less well at high temperature. For water, the hydrogen exchange path becomes increasingly important relative to the whole molecule path with increasing temperature, which improves the net cross-relaxation efficiency.