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 spectros copy. The temperature dependence of the CH2 and CD2 stretching frequen cies showed that P-d(31)OPC displayed a gel --> liquid crystal phase t ransition between -6 and -3 degrees C, with parameter changes similar to, but slightly reduced from, those found in disaturated PCs. In cont rast, the oleoyl chain of OP-d(31)PC showed a broadened (nearly abolis hed) transition with substantial conformational disorder in the gel ph ase. The palmitoyl chain of this species also revealed a broadened tra nsition, with, however, the retention of an ordered gel phase. Examina tion of the CH2 wagging re ion, which reveals the occurrence of partic ular two- and three-bond conformational states in disordered phospholi pid phases, shows that the sn-l oleoyl chains of OP-d(31)PC chains hav e 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)OP C monolayers in situ at the air/water interface by IR reflection-absor ption spectroscopy (IRRAS) showed the oleoyl chains of the latter to b e somewhat more ordered at surface pressures > approx. 25 mN m(-1), an d the palmitoyl-d(31) chains to possess a similar level of conformatio nal order. The conformational order in these monolayers at high pressu re, 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 th an at their tails. Overall, these studies confirm the utility of parti ally deuterated derivatives for structural investigations of phospholi pids both in monolayer and bulk phases.