Structural features and functional properties of water in model DMPC membranes: Thermally stimulated depolarization currents (TSDCs) and Fourier transform infrared (FTIR) studies

摘要

Thermally stimulated depolarization currents (TSDCs) and Fourier transform infrared (FTIR) spectroscopies were employed to investigate the state of water incorporated in a model DMPC (dimyristoyl-phosphatidylcholine) membrane. The lipid multilayers, highly inhomogeneous from the dielectric point of view, originate complex TSDC spectra critically dependent on the sample water content and thermal history. Different temperature ranges were chosen to polarize the sample, i.e. 100-300 K (type I) and 100-285 K (type II). The purpose of the latter choice was to avoid any sample heating above the DMPC phase transition temperature (295 K) along the sample polarization. According to the results, water in a fully hydrated system (a_w = 0.92) (1) is ordered around the hydrophilic head molecular groups, (2) is layered in the interbilayer space and (3) penetrates among the hydrocarbon chains. It can assume different local structural configurations depending on the lipid packing. Irreversible conformational transitions in the lipid array system were monitored as a consequence of different dehydration treatments. FTIR absorption measurements were performed to study the water sorption kinetics into a DMPC thin film. The water related OH band was decomposed into three components, describing three water states, with different propensity to the H-bond formation. The changes of the lipid characteristic groups (CH_2/CH_3, and C=O) absorption bands as a function of increasing hydration level were monitored and discussed.