An IR study of the conformational disorder in the part of the hydrocarbon c
hain nearest the ester group in gel-phase lipid bilayers is presented. The
infrared method for determining the conformation of the CH2-CH2 bond attach
ed to the ester group is discussed and applied to a number of representativ
e gel-phase 1,2-diacylphosphatidylcholine fully hydrated dispersions, inclu
ding symmetric and mixed-chain PCs in their gel and subgel phases. The effe
ct 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 det
ermined. The overall concentration of the planar form always exceeds 50% bu
t varies significantly depending on the lipid involved. In two cases, the c
oncentration of the planar conformer was estimated for both the SN1 and SN2
chains.