The structure of lipids and proteins studied by attenuated total reflection (ATR) infrared spectroscopy. II. Oriented layers of a homologous series: phosphatidylethanolamine to phosphatidylcholine.

Polarized infrared ATR spectra of dry oriented multilayers of dipalmitoylphosphatidylethanolamine, sheep brain phosphatidylethanolamine, dipalmitoylphosphatidyl-N-methylethanolamine, dipalmitoylphosphatidyl-N-N-dimethylethanolamine, dipalmitoylphosphatidylcholine and egg phosphatidylcholine are reported. Structural features of hydrocarbon chains and polar headgroups are discussed. The average deviation fo hydrocarbon chains from the normal to the plane of the bilayer was found to be 20-30 degrees. However it was not possible to decide whether the chains are oriented parallel to each other. The fatty acid ester groups in beta- and gamma-position have different conformations. The phosphate group of dipalmitoylphosphatidylethanolamine exists probably in the protonated (formula: see text) and not in the ionized (greater than PO2-) state. However, the latter state is expected for all other phospholipids of this series. The deviation of the bisector of (formula: see text) of the greater than PO2- group from the normal to the bilayer is less than 45 degrees and the mean orientation of all polar head groups is rather parallel than perpendicular to the plane of the bilayer. The polar headgroup of phosphatidylcholine assumes at least two different conformations of the O-C-C-N moiety, i.e. gauche and trans. A variety of conformers has to be expected also for the polar head groups of most of the other phospholipids investigated in this work.