Dialog Between Phospholipids and Water: Functional Groups Contributions to Sorption Isotherms

Abstract

The water sorption behavior of four phospholipids, dipalmitoylphosphatidyl choline (DPPC), dihexanoylphosphatidylcholine (DHPC), dipalmitoylphosphatidyl ethanolamine (DPPE), and Δ9-unsaturated phosphatidylcholine (D9PC), was investigated using Attenuated Total Reflectance Fourier transform infrared spectroscopy (FTIR–ATR). Sorption isotherms were obtained along a range of water activities to distinguish the distinct contributions of phosphate and carbonyl functional groups to water sorption in lipid membranes. The relative water content obtained by the spectroscopic approach was validated using gravimetric data from the literature. The importance of this study lies in its ability to provide a practical and systematic approach for determining the number of water molecules associated with each functional group in phospholipid membranes using vibrational spectroscopy. The classical model of Park describes the sorption process in terms of the energetics of hypothetical sites of adsorption without describing their molecular structure. In comparison to this model, the present analysis advances in the understanding of how specific functional groups and their combination conform the hydration sites in phospholipid membranes, by evaluating the number of water molecules in each of them. This insight provides the basis for further exploration of structuring and functional water properties in relation to membrane topology.