Trans-unsaturated lipid dynamics: modulation of dielaidoylphosphatidylcholine acyl chain motion by ethanol.

Acyl chain dynamics of the trans-unsaturated lipid, dielaidoylphosphatidylcholine (DEPC), were studied by conventional and saturation transfer electron paramagnetic resonance spectroscopy of aqueous dispersions of DEPC spin labeled with lecithins having doxyl groups at positions 5, 10, and 14 on the sn-2 chain. The gel to liquid crystalline transition is concerted with simultaneous increases in rotational motion about the long axis of the acyl chain (libration) and in gauche-trans conformational interconversions (wobble). Relative to saturated lecithins at similar reduced temperatures the double bond (a) slowed libration by an order of magnitude in both phases, while wobble motions were several times slower, and (b)-produced a pronounced stiffness of the acyl chain near the double bond. Ethanol (0-1.6 M), in addition to its well-known colligative effect on the phase transition, was found to decrease the bilayer order in a concentration-dependent manner. This effect was smaller in the gel than in the liquid crystalline phase, most pronounced next to the double bond, and weakest deep in the bilayer. Ethanol affected slow motions little in the gel phase but wobble and libration correlation times were markedly decreased in the liquid crystalline phase.