The Structure of Lipids and Proteins Studied by Attenuated Total Reflection (ATR) Infrared Spectroscopy II. Oriented Layers of a Homologous Series: Phosphatidylethanolamine to Phosphatidylcholine

Abstract

Polarized infrared ATR spectra of dry oriented multilayers of dipalmitoylphosphatidylethanol- amine, 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 of hydrocarbon chains from the normal to the plane of the bi­layer was found to be 20-30°. However it was not possible to decide whether the chains are oriented parallel to each other. The fatty acid ester groups in β- and γ-position have different conformations. The phosphate group of dipalmitoylphosphatidylethanolamine exists probably in the I protonated and not in the ionized (>PO2 - ) state. However, the latter state is ex­pected for all other phospholipids of this series. The deviation of the bisector of ∢ (OPO) of the >PO2 - group from the normal to the bilayer is less than 45° 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.