Studies of molecular diffusion in single-supported bilayer lipid membranes at low hydration by quasielastic neutron scattering*

We have extended our investigation of the quasielastic neutron scattering from single-supported bilayer lipid membranes to a sample of lower hydration using the backscattering spectrometer BASIS at the SNS of ORNL. To focus on the diffusive motion of the water, tail-deuterated DMPC membranes were deposited onto SiO$_{2}$-coated Si(100) substrates and characterized by AFM. Compared to a sample of higher hydration, the dryer sample does not have a step-like freezing transition at $\sim $267 K and shows less intensity at higher temperatures of a broad Lorentzian component representing bulk-like water. However, the broad component of the ``wet'' and ``dry'' samples behaves similarly at lower temperatures. The dryer sample also shows evidence of a narrow Lorentzian component that has a different temperature dependence than that attributed to conformational changes of the alkyl tails of the lipid molecules in the wet sample. We tentatively identify this slower diffusive motion (time scale $\sim $1 ns) with water more tightly bound to the membrane.