A study of phospholipid phosphate groups in model membranes by Fourier transform infrared spectroscopy

Abstract

The phosphate region (1000–1300 cm^–1) of the infrared spectrum of aqueous phospholipid dispersions has been studied by the infrared Fourier transform technique. The main features of this spectral region have been described for various phospholipids, including phosphatidylcholine, phosphatidylethanolamine, cardiolipin and phosphatidylglycerophosphate (alkyl ether), the major phospholipid of Halobacterium purple membranes. No changes in the phosphate region are observed due to lipid polymorphism or as a consequence of changes in fatty acyl chain structure. Shifts in the phosphoryl stretching bands are interpreted in terms of changes in hydrogen bonding, while a contribution from R—O—P vibration is considered to reflect changes in phospholipid headgroup conformation. When added in equimolar amounts to phosphatidylcholine or phosphatidylethanolamine bilayers, surfactants (Triton X-100, sodium cholate) modify the degree of hydration and/or the orientation of the headgroup with respect to the bilayer plane. Phospholipids containing two phosphate groups give rise to more complex spectral features. Infrared data suggest that the two phosphate groups of cardiolipin are conformationally non-identical when incorporated into a lipid bilayer. Controlled enzyme hydrolysis (e.g. with phospholipase D) may help in the study of these complex phospholipid headgroups.