| Abstract
| - The hydration of two peculiar unsaturated lipids with phosphatidylethanolamine (PE) and phosphatidylcholine(PC) headgroups, namely dioctadecadienoyl PC (DODPC) and dioleoyl PE (DOPE), has been studied atroom temperature by infrared (IR) spectroscopy including the linear dichroism. Oriented films of these lipidssubjected to an atmosphere with definite values of relative humidity were investigated. The choice of DODPCand DOPE was motivated by their common property of undergoing a lyotropic phase transition, which, ineach case, nearly exclusively affects the polar part of the lipid aggregates and is, thus, termed “solvation-induced” transition. The temperature−RH phase diagrams of both lipids were determined. The headgroupsof both lipids form quasi-crystalline structures at small RH. The long axes of the PE and PC moieties orientessentially parallel with respect to the polar interface of the lipid aggregates. Model-based analyses of IRlinear-dichroism data are consistent with the idea that about 8−12 DOPE molecules with disordered chainsassemble within one cross-section layer of a ribbonlike phase, Pα. Dry DODPC forms a subgel phase (SGI)with all-trans acyl chains. At a critical water activity near 0.6, the molecular disorder in the polar region ofthe lipid aggregates increases distinctly. This “headgroup melting” reflects the solvation-induced transitionsleading to the SGII and HII phases in DODPC and DOPE, respectively. Hydrogen-bonding specifics due tothe polar parts of both lipids were probed by means of H → D exchange. Particularly stable H bonds areindicated to exist in dried DOPE.
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