Diffusion of water on supported bilayer lipid membranes*

We compare the temperature dependence of quasielastic incoherent neutron scattering from water associated with fully hydrated single bilayers of the charge-neutral DMPC (dimyristoylphosphocholine) lipid supported on a SiO$_{\mathrm{2}}$-coated silicon substrate to that of water in proximity to a similarly supported \textit{anionic} DMPG (dimyristoylphosphoglycerol) bilayer. The diffusion constant of water near the DMPC membrane decreases on cooling in two step-like transitions: 1) at the freezing point of bulk-like water (267 K); and 2) at a second transition of unknown origin at 261 K [2]. In contrast, we observe on cooling only a continuous decrease in the diffusion constant of water in proximity to the DMPG membrane. Water remains mobile to lower temperature on the anionic membrane; however, its diffusion is systematically slower than on DMPC in the temperature range above 255 K where water is mobile in both systems. [2] M. Bai \textit{et al}., Europhys. Lett. \textbf{98}, 48006 (2012).