Effect of trans unsaturation on molecular organization in a phospholipid membrane.

ABSTRACT

We employed solid state (2)H NMR, complemented by computer simulations, to compare molecular organization in model membranes composed of 1-elaidoyl-2-stearoylphosphatidylcholine (t181-180PC), 1-oleoyl-2-stearoylphosphatidylcholine (c181-180PC), and 1,2-distearoylphosphatidylcholine (180-180PC). 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 t181-180PC (31.5 degrees C) than c181-180PC (7 degrees C) relative to 180-180PC (53 degrees C), reflecting the smaller deviation from the linear conformation produced by a trans as opposed to cis double bond. Acyl chain order in t181-180PC (S(CD) = 0.135) in the liquid crystalline state is much closer to that of c181-180PC (S(CD) = 0.128) than that of the substantially more ordered 180-180PC (S(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.