Hydration Water Dynamics in Zwitterionic Phospholipid Membranes Mixed with Charged Phospholipids

The effect of adding charged lipids on the hydration water dynamics near lipid membranes was investigated using quasi-elastic neutron scattering (QENS) experiments and molecular dynamics (MD) simulations. This study focused on the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) mixed with the anionic lipid 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine (DMPS). QENS profiles were analyzed based on four different types of hydration water dynamics. Three slower modes were identified as tightly bound, loosely bound, and free water, as previously reported, while the fastest mode was attributed to the rotational motion of the water molecules. The diffusion coefficients of the three types of hydration water increased with the addition of charged lipids, and the rotational motion of water molecules was also accelerated. The numbers of loosely and tightly bound water molecules changed slightly, whereas free water remained largely unchanged. These findings suggest that the charged headgroups of the DMPS disrupted the hydrogen-bonding network among water molecules, as supported by MD results.