A series of CH2 wagging modes in the 1320-1370-cm-1 region of alkane and phospholipid IR spectra characteristic of nonplanar conformers has been used for quantitative evaluation of conformational states in disordered (phospholipid Lα and HII and alkane liquid) phases. A quantitative comparison of 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1,2-dipalmitoylphosphatidylethanolamine (DPPE) shows the former to have 0.4 double gauche (gg), 0.5 end gauche (eg), and about 1.0 (kink + gtg) conformers per chain just above the gel-liquid crystal phase transition, while the more highly ordered DPPE Lα phase shows about 0.2 gg, 0.1 eg, and 1.0 (kink + gtg) conformers. In all systems studied, the number of allowed gg and eg forms that occur is substantially reduced from those achieved in isotropic liquid alkanes. A study of the Lα → HII interconversion in two unsaturated phosphatidylethanolamines (1-palmitoyl-2-oleoyl- (POPE) and 1,2-dielaidoyl- (DEPE)) shows a substantial increase in both gg and eg conformers near temperatures leading to the inverted micellar state in each instance, with smaller percentage increases in (kink + gtg) states. Connections between these quantitative observations with those of 2H NMR and other spectroscopies are presented. Finally, a temperature dependence inconsistent with the prediction of the rotational isomeric state model is noted for the 1368-cm-1 band characteristic of (kink + gtg) conformers in alkanes.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry