TRITIUM PROTON MAGNETIZATION TRANSFER AS A PROBE OF CROSS RELAXATION IN AQUEOUS LIPID BILAYER SUSPENSIONS

Saturation-transfer experiments were used to characterize relaxation coupling between macromolecular and water hydrogen nuclei in a suspension of egg phosphatidylcholine/cholesterol in tritium-doped water. Saturation of the proton resonance from the restricted lipid environment (1Hr) results in a decrease of the bulk-water proton (1Hf) spin-lattice relaxation time and negative magnetization transfer to the water protons, whereas no effect on the 3Hf relaxation and a positive enhancement of the equilibrium 3Hf magnetization was observed. The positive 3Hf (enhancement can be accounted for by intramolecular1Hf-3Hf NOE with indirect 3Hf (saturation through magnetization transfer with the lipid proton pool. These results are consistent with lipid proton-water proton relaxation coupling via dipolar magnetization transfer in the spin-diffusion limit. Dynamic- and rigid-lattice spin-diffusion models are considered. Both models can account for the observed relaxation and magnetization transfer with limits imposed on the water-macromolecular dynamics on the basis of the absence of 1Hr-3Hf coupling. © 1991.