Laser-Raman study of valinomycin-doped and ω-CD 3-labeled phosphatidylcholine liposomes

Abstract

The relative intensities of the CH stretching vibrations are used to study the interaction of lecithin liposomes with valinomycin, a mobile carrier for alkali ions. In the case of dipalmitoyl lecithin liposomes, the lipid phase transition is not significantly affected by valinomycin. However, in dimyristoylphosphatidylcholine liposomes, the phase transition is broadened by the addition of 1 mol% valinomycin even at low K + concentrations. This indicates that the carrier interacts with the hydrophobic core of the bilayer. In addition, these experiments showed that the lipid phase transitions which are reflected by the methylene groups and the terminal methyl groups are nearly equivalent. Therefore a reevaluation of the assignment of the CH stretching bands seemed necessary. Our Raman spectroscopic investigation of ω-deuterated dipalmitoyl lecithin liposomes improves the assignment of CH stretch vibrations to methylene and methyl groups. The deuteration displaces the methyl group vibrations to the 2050–2250 cm −1 region and produces gross intensity changes of the bands at 2883 and 2936 cm −1. These changes lead to the conclusion that both bands arise from vibrations which can be attributed simultaneously to the methylene and methyl groups of the fatty acid chains. The displacement of the CH 3 group vibrations from their original positions enhances the intensity ratios (per centimeter), 2883 2847 and 2936 2847 , for the CH 2- groups which are used to monitor the lipid phase transition, and implies that the contributions of the CH 3 groups to the phase transition curves are unimportant. Our finding that the -CD 3 groups reflect no phase transition supports this statement.