CH2 WAGGING MODES OF UNSATURATED ACYL CHAINS AS IR PROBES OF CONFORMATIONAL ORDER IN METHYL ALKENOATES AND PHOSPHOLIPID-BILAYERS

CH2 wagging progressions have been monitored in the IR spectra of a series of methyl alkenoate solid phases and diunsaturated phosphatidylcholine (PC) aqueous gel phases. The acyl chain structures are of the form CH3(CH2)mC=C(CH2)nCO(=O)CH3. Band assignments for each series are verified from dispersion curves constructed with the assumption that the n CH2 groups between the C=C and ester moieties are solely responsible for the observed progressions. The utility of the progression intensities as a marker of acyl chain conformational order in the phospholipids is established through studies of thermotropic phase transitions in 1,2-dinervonoyl phosphatidylcholine (DNPC), 1,2-dierucoylPC (DErPC), and 1,2-dielaidoylPC (DEPC). Finally, the thermotropic behavior of the wagging progression intensities in diunsaturated PC/cholesterol mixtures is compared with that in 1,2-dipalmitoylPC (DPPC)/cholesterol. The unsaturated PC's exhibit a response at 33 mol % cholesterol that differs from DPPC. In particular, the "liquid-ordered" phase present in DPPC/cholesterol is absent for the unsaturated phospholipids under similar conditions of temperature and sterol concentration and is replaced by a substantially more disordered state. Implications of these results for the structure of biological membranes are discussed.