Partially deuterated phospholipids as IR structure probes of conformational order in bulk and monolayer phases

Acyl chain conformations of the positional isomers 1-palmitoyl-d(31), 2-oleoylphosphatidylcholine (P-d(31)OPC) and 1-oleoyl-2-palmitoyl-d(31)-phosphatidylcholine (OP-d(31)PC) were investigated by FT-IR spectroscopy. The temperature dependence of the CH2 and CD2 stretching frequencies showed that P-d(31)OPC displayed a gel --> liquid crystal phase transition between -6 and -3 degrees C, with parameter changes similar to, but slightly reduced from, those found in disaturated PCs. In contrast, the oleoyl chain of OP-d(31)PC showed a broadened (nearly abolished) transition with substantial conformational disorder in the gel phase. The palmitoyl chain of this species also revealed a broadened transition, with, however, the retention of an ordered gel phase. Examination of the CH2 wagging re ion, which reveals the occurrence of particular two- and three-bond conformational states in disordered phospholipid phases, shows that the sn-l oleoyl chains of OP-d(31)PC chains have a somewhat different set of these conformations than the sn-2 oleoyl chains of P-d(31)OPC. A simple structural model based on the putative existence of a bend at the 2' position of the sn-2 chain is proposed to explain these observations. Examination of OP-d(31)PC and P-d(31)OPC monolayers in situ at the air/water interface by IR reflection-absorption spectroscopy (IRRAS) showed the oleoyl chains of the latter to be somewhat more ordered at surface pressures > approx. 25 mN m(-1), and the palmitoyl-d(31) chains to possess a similar level of conformational order. The conformational order in these monolayers at high pressure, as judged by the CH2 or CD2 stretching frequencies, is similar to that in the bulk phase. Finally, IRRAS studies of tail-end deuterated (both chains) 1,2-dipalmitoylphosphatidylcholine-d(34) showed that the chains possess more conformational order adjacent to the interface than at their tails. Overall, these studies confirm the utility of partially deuterated derivatives for structural investigations of phospholipids both in monolayer and bulk phases.