Water translational motion at the bilayer interface: an NMR relaxation dispersion measurement

Nuclear magnetic relaxation rates for water protons in aqueous palmitoyloleoylphosphatidylcholine vesicle suspensions containing different nitroxide free radical spin labels are reported as a function of magnetic field strength corresponding to proton Larmor frequencies from 10 kHz to 30 MHz. Under these conditions the water proton relaxation rate is determined by the magnetic coupling between the water protons and the paramagnetic nitroxide fixed on the phospholipid. This coupling is made time-dependent by the relative translational motion of the water proton spins past the nitroxide radical. Using theories developed by Freed and others, we interpret the NMR relaxation data in terms of localized water translational motion and find that the translational diffusion constant for water within approximately 10 A of the phospholipid surface is 6 x 10(-10) m2 s(-1) at 298 K. Similar results are obtained for three different nitroxide labels positioned at different points on the lipid. The diffusion is a thermally activated process with an activation energy only slightly higher than that for bulk water.