Differential scanning calorimetry study of the influence of phospholipid analogs with a carbonyl-terminated sn-2 chain on the interdigitated phases formed by 1-stearoyl-2-capryl-sn-glycero-3-phosphatidylcholine (C18:C10-PC).

Synthetic glycerophosphocholines with highly asymmetric chain lengths form interdigitated bilayers in the gel phase. In nature, phospholipids with one hydrocarbon chain approximately twice as long as the other can arise from the autooxidation of unsaturated linkages in the acyl chain, and thus the oxidation products would contain a carbonyl group at the chain terminus. In this study, we have investigated the thermotropic behavior of bilayers prepared from mixtures of the well-studied, mixed-chain phospholipid, 1-stearoyl-2-capryl-sn-glycero-3-phosphocholine (C18:C10-PC, 1), with synthetic 1-stearoyl-2-acyl-sn-glycero-3-phosphocholines in which the sn-2 chain is approximately one-half the length of the sn-1 chain and contains a C==O group near the omega terminus. Phase diagrams of binary mixtures of 1 with a chain-terminal ketone-PC analog (2) or with a chain-terminal ester-PC analog (3) in excess water exhibited gel-phase immiscibility over a wide compositional range, but miscibility in the liquid-crystalline phase. However, 1 was completely miscible with C18:C10:1 delta 10-PC (compound 4), which bears a chain-terminal carbon-carbon double bond, in both the gel and liquid-crystalline phases. The calorimetric data suggest that phosphatidylcholines (PC) with carbonyl-terminated chains, which can be produced by autooxidation of naturally abundant 1-saturated-2-unsaturated phospholipids such as 1-stearoyl-2-oleoyl-PC, may not form the normal triple-chain mixed interdigitated structure characteristic of hydrocarbon-terminated PCs in gel-phase bilayers.