Conformational disorder in unsaturated phospholipids by FTIR spectroscopy

Conformational disorder in liquid alkenes and in the L(alpha) and H-parallel to phases of some unsaturated phospholipids has been monitored by FTIR spectroscopy. The CH2 wagging region (1330-1390 cm(-1)) in saturated chains contains vibrations of particular 2- and 3-bond conformational states as follows: 1341 cm(-1), end-gauche leg); 1352 cm(-1), double gauche (gg); 1368 cm(-1), the sum of kink and gtg states. In unsaturated chains, this spectral region revealed an additional band at 1362 cm(-1) and(occasionally) a feature near 1348 cm(-1). The 1362 cm(-1) band is tentatively assigned to the wagging of CH2 groups adjacent to the C = C bond. Substantial populations of both gg and (kink + gtg) states are evident in the L(alpha), phases of unsaturated phosphatidylcholines (PC's). Unsaturated phosphatidylethanolamines (PE's) are more ordered than their PC counterparts, and possess fewer gg and eg states. Chain disorder in the H-parallel to phase of PE's approaches that in L(alpha) phases of unsaturated PC's. Changes in conformer distributions during the L(alpha) --> H-parallel to transition in 1,2-dioleoylphosphatidylethanolamine (DOPE), 1-palmitoyl,2-oleoylphosphatidylethanolamine (POPE), 1,2-dielaidoylphosphatidylethanolamine (DEPE), and N-methyl-DOPE(N-MeDOPE) were semi-quantitatively estimated. For DOPE and DEPE, slight cooperative increases in both gg and (kink + gtg) states occur, for POPE only the gg population increases and for N-MeDOPE only the kink + gtg populations increase. These disorder increases are consistent with the small calorimetric Delta H for this transition. Difficulties in quantitative determination of conformational disorder in unsaturated chains are discussed.