Translational dynamics of water at the phospholipid interface.

The residual water-proton magnetic relaxation dispersion profile obtained from suspensions of phospholipid vesicles in deuterium oxide was found to be a logarithmic function of the proton Larmor frequency at high magnetic field strengths, and independent of Larmor frequency at low magnetic field strengths. The residual proton relaxation is caused by dipole-dipole coupling between the residual water proton in otherwise deuterated water and the phospholipid protons. The logarithmic dependence on magnetic field strength is the signature of water-proton diffusive exploration on the interface that is approximately two-dimensionally constrained. Application of relaxation theory for two-dimensional diffusion to the spin-lattice relaxation data yields a translational correlation time of approximately 70 ps for water diffusing in the interface of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) vesicles.