Effect of cholesterol on the chain-ordering transition of 1-palmitoyl-2-arachidonoyl phosphatidylcholine

Deuterium nuclear magnetic resonance and differential scanning calorimetery were used to study the phase behavior, chain order, and dynamics of bilayers consisting of 1-palmitoyl-2-arachidonoyl phosphatidylcholine perdeuterated on the saturated acyl chain (16:0-20:4 PC-d(31)) alone and in mixed bilayers containing 15 and 30 mol % cholesterol. In the liquid crystalline phase, saturated chain orientational order increases with increasing cholesterol concentration. The transition to a highly ordered phase is centered on -25 degrees C regardless of cholesterol concentration. The observation of coexisting ordered and liquid crystalline phase spectral contributions near the transition in the absence of cholesterol implies a discontinuous transition. In the presence of cholesterol, the phase change is continuous and no evidence of coexisting domains of ordered and fluid phase lipid is observed. The observation of spectral intensity near +/-63 kHz in the ordered phase suggests near-immobilization of the methylene deuterons and is reminiscent of the subgel or L-C phase of disaturated phospholipids. Cholesterol does not alter the apparent splitting of methylene deuterons in the ordered phase but does narrow the spectral contribution from methyl deuterons on the saturated chain. Quadrupole echo decay time measurements are used to gain some insight into long-range and more localized lipid motions that modulate the orientation-dependent quadrupole interaction on the time scale of the quadrupole echo experiment. The observed temperature dependence of the echo decay times suggests that echo decay in the ordered phase of 16:0-20:4 PC-d(31) bilayers is dominated by different motions than in disaturated phospholipids. These observations are compared with results from an earlier study on bilayers containing 16:0-18:2 PC-d(31) While the difference in unsaturated chain has little effect on the chain-ordering transition temperature, small but potentially significant differences are observed in the response of the bilayer transition and dynamics to cholesterol.