Insights on Hydrogen Bond Network of Water in Phospholipid Membranes: an Infrared Study at Varying Hydration

Water at membrane interfaces is vital for the cellular biological functions, but despite its importance, the structure and function of biological water remain elusive. Here, by studying the OH stretching mode in partially hydrated lipid multilayers by FTIR measurements, relevant information on the water structure at the surface with lipid membranes has been gathered. The water hydrogen-bond network is highly perturbed in the first layers that are in contact with the lipid membrane, exhibiting strong deviations from tetrahedral symmetry and a significant number of defects, such as isolated water molecules and a large number of hydrogen-bonded water dimers in the interfacial region. These findings supports the hypothesis that in phospholipid membranes water chains are formed that are involved in the proton transfer across lipid bilayers by phosphate groups of opposing lipids. Furthermore, we have determined that even at very low hydration levels, a small amount of water is embedded within the confined spaces of the hydrocarbon region of phospholipid bilayers, which could potentially contribute to the structural stability of the lipid membrane.