Effect of Trans Unsaturation on Molecular Organization in a Phospholipid Membrane

We employed solid state 2 H NMR, complemented by computer simulations, to compare molecular organization in model membranes composed of 1-elaidoyl-2-stearoylphosphatidylcholine ( t 18:1-18:0PC), 1-oleoyl-2-stearoylphosphatidylcholine ( c 18:1-18:0PC), and 1,2-distearoylphosphatidylcholine (18:0-18:0PC). These phospholipids have elaidic acid (EA) containing a trans double bond, oleic acid (OA) containing a cis double bond, and saturated stearic acid (SA), respectively, at the sn -1 position and were synthesized with perdeuterated SA at the sn -2 position. The temperature of the chain melting transition is depressed less for t 18:1-18:0PC (31.5 °C) than c 18:1-18:0PC (7 °C) relative to 18:0-18:0PC (53 °C), reflecting the smaller deviation from the linear conformation produced by a trans as opposed to cis double bond. Acyl chain order in t 18:1-18:0PC ( S̅ CD = 0.135) in the liquid crystalline state is much closer to that of c 18:1-18:0PC ( S̅ CD = 0.128) than that of the substantially more ordered 18:0-18:0PC ( S̅ CD > 0.156), which is attributed to the reduced energy barrier to rotation about the C−C single bonds next to either a trans or cis carbon double bond. A conformation that somewhat resembles a saturated chain and an intrinsic disorder approaching that of a cis unsaturated chain characterize EA and, we speculate, may play a role in the adverse impact dietary trans fatty acids (TFA) have on biological function.