An infrared spectroscopic study of specifically deuterated fatty-acyl methyl groups in phosphatidylcholine liposomes

The region of the infrared spectrum corresponding to C-2H stretching vibrations (2050-2250 cm-1) has been examined for liposomes composed of dimyristoylphosphatidylcholine deuterated specifically at the methyl ends of either one (sn-2) or both the fatty acyl chains. This label is intended to provide information on lipid dynamics in the contact region between monolayers. The two most prominent bands observed correspond, respectively, to antisymmetric (2212 cm-1) and symmetric (2075 cm-1) C-2H stretching vibration. The antisymmetric band consists of two overlapping peaks, whose positions vary with the gel or liquid-crystalline state of the lipid. The separation between the peaks making up the antisymmetric band increases with temperature, and is maximum above the Tc transition temperature; this rules out the previously proposed assignment of these two peaks to different rotational modes of the methyl group relative to the adjacent methylene. The position and width of the symmetric band at 2075 cm-1 are also sensitive to the physical state of the lipid. The presence of cholesterol at an equimolar ratio with the phospholipid abolishes all the phase-dependent changes observed. The intrinsic polypeptide gramicidin A, at a 5:1 lipid/peptide mol ratio, is seen to enlarge the lipid thermotropic transition, with small effects above Tc. Cytochrome c, an extrinsic protein, at a 10:1 mole ratio, does not modify the phase-dependent behaviour of the terminal methyl groups, but consistently shifts all the observed bands to lower-frequency positions, which suggests a long-range effect of the protein along the phospholipid fatty acyl chains.