Ultrasensitive detection of interfacial water diffusion on lipid vesicle surfaces at molecular length scales

Measurements of the interfacial diffusion coefficient of the surface hydration layer of lipid vesicles in dilute solutions are presented. This was made possible by the greatly enhanced sensitivity and unique contrast provided by the site-specific and selective Overhauser dynamic nuclear polarization of solvent molecules that approach nitroxide radical-based spin labels within <5-10 A. All experiments were carried out using minute microliter sample volumes of lipid vesicle solutions, using low spin label concentrations (<2 mol %) and under physiological conditions. This presents unprecedented sensitivity for analyzing interfacial solvent diffusion of macromolecules and their assemblies in solutions and highlights the feasibility of investigating precious samples. Interfacial diffusion on DOTAP (1,2-DiOleoyl-3-TrimethylAmmonium-Propane) and DPPC (1,2-DiPalmitoyl-sn-glycero-3-PhosphoCholine) surfaces are further analyzed as a function of temperature to determine the activation energy of their hydration layer dynamics. The temperature-dependent analysis across the phase transition of DPPC concludes that the hydration water with 100-200 ps dynamics displays Arrhenius behavior and does not undergo a phase transition unlike the lipid chains. We also discuss the advantages of determining the activation energy of diffusion as a general approach to comparing interfacial diffusivity on surfaces that have vastly different charge topologies and, thus, may display different distances of closest approach between the spin label placed at the surface and the protons of hydration water. The further development and application of this technique is expected to facilitate the study of membrane dynamics and their phase behavior, including the formation of lipid rafts, with lipid-specific resolution.