Water–proton nuclear magnetic relaxation in heterogeneous systems: Hydrated lysozyme results

Abstract

Spin‐lattice relaxation rates of water protons in hydrated immobilized lysozyme are measured as a function of magnetic field strength. The dependence of water relaxation versus hydration is presented from 35 to 55% by weight water content. The water‐proton relaxation is directly coupled to that of the protein and the coupling exists in the absence of chemical exchange. A model is applied where relaxation within the two proton phases is coupled through a dipolar cross‐relaxation mechanism as well as chemical exchange. The observed amplitudes of the water‐proton relaxation profiles scale with the ratio of protein to water protons as well as the protein‐proton relaxation rate. The field dependence of the protein‐proton spin‐lattice relaxation is presented in the presence of D₂O where a cross‐relaxation coupling is absent. The coupled relaxation model accounts well for the NMR relaxation data as a function of magnetic field strength which is similar to measurements on other heterogeneous systems such as tissues. © 1991 Academic Press, Inc.