SEIRAS studies of water structure at the gold electrode surface in the presence of supported lipid bilayer

• Hydration water of lipids polar heads predominates at zero charge.
• Monomers and multimers of water molecules are at intermediate charges.
• Liquid like water is in the space separating bilayer from the metal at very negative charges.

Surface-enhanced infrared absorption spectroscopy (SEIRAS) was employed to study structure of water in a phospholipid bilayer deposited at a gold electrode surface. The technique employs attenuated total reflection (ATR) and an enhancement of the electric field of the IR photon that decays steeply with distance from the metal surface. These conditions allow one to subtract the background from the bulk water and to determine spectra of water in the bilayer or in a confined space between the metal surface and the bilayer. The IR data demonstrated that three types of water are present in the supported bilayer. At potentials close to zero charge the polar heads of the phospholipid molecules retain hydration water. At intermediate charge densities water penetrating deeply into the bilayer and multimers of water molecules were detected in the bilayer. At charge densities more negative than −20 μC/cm2 the bilayer is lifted from the metal surface and liquid like water appears in the space separating the bilayer from the metal.