Effects of cis and trans unsaturation on the structure of phospholipid bilayers: a high-pressure infrared spectroscopic study.

In order to compare the effects of cis and trans unsaturation on the structure and packing of phospholipid bilayers, infrared spectra of aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dielaidoyl-sn-glycero-3-phosphocholine (DEPC) were measured in a diamond anvil cell at 28 degrees C as a function of pressure up to 36 kbar. The infrared spectra indicate that DEPC and DOPC undergo pressure-induced liquid-crystalline to gel phase transitions at critical pressures of 0.7 and 5.2 kbar, respectively. Below their respective critical pressures, the infrared spectra of DOPC and DEPC are essentially indistinguishable, whereas above these pressures, there are very pronounced differences in the barotropic behavior of these two lipids. Specifically, at the 5.2-kbar transition in DOPC, there are significant changes in the frequencies, intensities, and widths of bands associated with the interfacial C = O groups, the olefinic CH = CH groups, and the terminal CH3 groups, whereas the corresponding bands of DEPC are, by contrast, relatively insensitive to the pressure-induced phase transition. The unusual band shape changes in DOPC are attributed to a unique packing arrangement of the oleoyl acyl chains required to accommodate the bent geometries of adjacent cis double bonds. Moreover, above 5 kbar in DEPC, well-defined correlation field splittings of the CH2 scissoring and rocking modes are observed, with magnitudes very similar to those observed at comparable pressures in saturated lipid systems. The absence of correlation field splittings of the corresponding bands of DOPC up to 36 kbar suggests that the bent oleoyl acyl chains are closely packed with all chains oriented parallel to each other.