Prediction of proton relaxation rates from measurements of deuterium relaxation in aqueous systems

Spin-lattice relaxation for deuterium in aqueous solutions of macromolecules is dominated by quadrupolar interactions so that the rotational correlation time for D₂O and HDO may be determined by measuring the deuteron spin-lattice relaxation time T₁. This correlation time may then be used to predict the dipole-dipole contribution to proton relaxation from rotational intramolecular water interactions. By subtracting these calculated values from the directly measured proton relaxation rates, the contribution to proton relaxation due to translation and intermolecular dipolar effects may be found. In solutions of polyethylene glycol the translational contribution was estimated to be a constant 40% of the overall relaxation rate when the deuteron relaxation rate varied four-fold, consistent with the view that the translation correlation time varies in direct proportion with the rotational time. It is proposed that deuteron relaxation measurements thus offer a novel method for estimating the hydrodynamic contribution to relaxation due to water-water interactions only in solutions of macromolecules and tissues, separate from the effects of cross relaxation, and these measurements are thus useful for understanding the relative importance of different proton relaxation mechanisms in such media.