Conformation of the acyl chains in diacylphospholipid gels by IR spectroscopy

An IR study of the conformational disorder in the part of the hydrocarbon chain nearest the ester group in gel-phase lipid bilayers is presented. The infrared method for determining the conformation of the CH2-CH2 bond attached to the ester group is discussed and applied to a number of representative gel-phase 1,2-diacylphosphatidylcholine fully hydrated dispersions, including symmetric and mixed-chain PCs in their gel and subgel phases. The effect on chain conformation of the headgroups PC, PE, and PA(-) was explored. Only two conformations of the alkyl chains were found: a planar (all-trans) chain and a nonplanar chain that is all-trans except for the CH2-CH2 bond nearest the ester group. Concentrations of the two conformers were also determined. The overall concentration of the planar form always exceeds 50% but varies significantly depending on the lipid involved. In two cases, the concentration of the planar conformer was estimated for both the SN1 and SN2 chains.