| Abstract | In order to compare the effect of one and two mono-unsaturated (oleoyl) acyl chains on the structure and packing of phospholipid bilayers, infrared spectra of aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and several monoenoic lecithins, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC), and 1-O-hexadecyl-2-oleoyl-sn-glycero-3-phosphocholine (HOPC), were measured at 28°C as a function of pressure up to 47 kbar. Whereas no pressure-induced correlation-field splitting of the CH₂ scissoring or rocking modes is observed in the spectra of the dimonounsaturated system (DOPC), both of these modes split at high pressure (>8 kbar) in the spectra of the mono-unsaturated systems (POPC, OPPC and HOPC). The observation of this splitting rules out a parallel orientation of the acyl chains in the mono-unsaturated systems. The very weak CH₂ band progression observed in the gel phase spectra of the monoenoic lecithins indicates that the oleoyl chain in these systems is significantly disordered, whereas the strong band progression observed in the gel phase of DOPC suggests that the polymethylene chain segments of the oleoyl chains are in their trans conformation. The absence in the POPC, OPPC or HOPC gel phase spectra of the 2820 and 630 cm⁻¹ bands associated in DOPC with the C−H stretching mode of CH₂ groups adjacent to the double bond, and with the out-of-plane = CH deformation mode, respectively, indicates that the conformation of the oleoyl chain in the region of the cis double bond is different in the mono-unsaturated systems. |
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