Water at Lipid Interfaces: Domains and Defects

Water organized at the lipid membrane extends into the cell interior and reconciles the limited view of membrane function in the cellular context under the accepted current models. In this regard, the properties of the hydrated interphase play a fundamental role in the propagation of events between the membrane and the cytoplasm.Phosphate (-PO₂^-) and carbonyl (-CO) phospholipid groups are specific hydration sites that modulate the arrangement of water molecules on the membrane surface. They form a network of H-bonds with specific thermodynamic, electrical, and mechanical properties. Its correlation gives rise place to cooperative and synergistic phenomena. Changes in water activity at the interphase can be produced by hydric, osmotic, saline stress, or metabolic events. Therefore, it is the thermodynamic link in a complex system that determines surface free energy and water-lipid interactions.The coordination of infrared spectroscopy coupled to attenuated total reflectance (FTIR/ATR) under controlled humidity and temperature conditions allowed to obtain information on the hydration state of lipid systems showing important details in the structure and properties that cannot be ignored to have a complete understanding of the thermodynamic behavior of lipid biomembranes.In this context, infrared spectroscopy permits to distinguish structural packing defects with special water arrangements where bioeffectors such as L-arginine (L-Arg) and L-phenylalanine (L-Phe) can be inserted.