FTIR studies of phospholipid membranes containing monoacetylenic acyl chains

Abstract

The conformational behaviour of phosphatidylcholine and phosphatidyl acid membranes derived from octadec-14-ynoic and octadec-9-ynoic acid is studied by FTIR spectroscopy comprising both the liquid crystalline and gel phases. In the vicinity of the transition from the gel to the liquid crystalline phase the conformational changes of the acyl chains are followed by the analysis of the CH₂ stretching bands and CH₂ wagging band progressions. In the liquid crystalline phase a quantitative determination of the amounts of gauche-trans-gauche, double gauche, and end gauche conformers is achieved by the analysis of the CH₂ wagging band region. In this connection, several model compounds (hex-2-yne, and hex-3-yne, oct-4-yne) have been examined to assist the assignment of a special vibration band at 1328 cm^-1 due to the CH₂–C3C unit. The results for the phospholipids studied here clearly demonstrate that the conformational properties critically depend on their actual lipid structure, sample composition and sample temperature. The derived data are discussed in conjunction with earlier investigations of more “conventional’' lipid systems—mainly containing saturated acyl chains—to evaluate the specific influence of the incorporation of the C–C triple bond into the fatty acid chains.