Biophysical properties of dimeric phospholipids

Abstract

A series of unusual bipolar and bis-phospholipids (dimeric phospholipids) have been studied. The structure, conformation, morphology and biophysical properties of the resulting phospholipid aggregates were investigated.Deuterium magnetic resonance spectroscopy ($sp2$H NMR) was used to study and characterize the conformation and acyl chain order in oriented bipolar lipid membranes. The $sp2$H-NMR studies indicated a large and constant value for the order parameter (S$rm sb{mol})$ for all positions along the bipolar lipid diacyl chain for mechanically oriented, magnetically oriented and unoriented samples. This indicates that the great majority ($>$90%) of the bipolar lipid exists in a highly ordered spanning conformation.Dimeric phospholipid aggregate morphologies were studied using $sp{31}$P NMR, small angle X-ray scattering, electron microscopy, differential scanning calorimetry, and the Langmuir film balance technique in order to study the relationship between lipid structure and aggregate morphology. Dimeric phospholipids favour a lamellar morphology. A number of lipid structure-dependent features have been observed including tri-lamellar structures, extended ripple phases and hexagonal phases.Dimeric and non-hydrolyzable phospholipids were used to study the phenomenon of interfacial activation of extracellular phospholipase A$sb2$ (EC. 3.1.1.4) (PLA$sb2)$ in relation to lipid phase, substrate conformation and mobility. Kinetic results and product analyses are consistent with a situation where the spanning conformer of bipolar phospholipids is resistant to PLA$sb2$-catalyzed hydrolysis but the hairpin conformer is readily hydrolyzed. Finally, an analysis of interfacial kinetics in non-hydrolyzable matrices indicated varying degrees of interfacial inhibition and hydrolysis product activation. This has not been explicitly recognized before and affects the choice of assay conditions for PLA$ sb2.