Water hydrogen-bonding structure and dynamics near lipid multibilayer surface: Molecular dynamics simulation study with direct experimental comparison.

Lipid multibilayers are excellent model systems for studying water structures and dynamics near biological membrane surfaces. In particular, the orientational distribution and rotational dynamics of water molecules near hydrophilic lipid groups are found to be sensitive to the chemical nature and charge distributions of the amphiphilic lipids. To elucidate how different parts of these lipids affect the water hydrogen-bonding structure and dynamics and to directly compare with recent experimental results, we carried out molecular dynamics (MD) simulations of lipid multibilayer systems. We found that the water molecules close to positively charged choline groups have a broad distribution of orientations due to the clathratelike shell formation around the choline groups but that those associated with phosphate groups, even in the second hydration shell, are orientationally restricted due to their strong hydrogen bonding with the phosphate group. These MD simulation results are in excellent agreement with our time-resolved infrared pump-probe anisotropy measurements, and we believe that they provide valuable insights into the role of water molecules in maintaining lipid bilayer integrity.