FTIR studies of phospholipid membranes containing monoacetylenic acyl chains

The conformational behaviour of phosphatidylcholine and phosphatidyl acid m embranes 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 p hase the conformational changes of the acyl chains are followed by the anal ysis of the CH2 stretching bands and CH2 wagging band progressions. In the liquid crystalline phase a quantitative determination of the amounts of gau che-trans-gauche, double gauche, and end gauche conformers is achieved by t he analysis of the CH2 wagging band region. In this connection, several mod el compounds (hex-2-yne, and hex-3-yne, oct-4-yne) have been examined to as sist the assignment of a special vibration band at 1328 cm(-1) due to the C H2-C=C unit. The results for the phospholipids studied here clearly demonst rate that the conformational properties critically depend on their actual l ipid structure, sample composition and sample temperature. The derived data are discussed in conjunction with earlier investigations of more "conventi onal" lipid systems-mainly containing saturated acyl chains-to evaluate the specific influence of the incorporation of the C-C triple bond into the fa tty acid chains.