SMOOTHED ACYL-CHAIN ORIENTATIONAL ORDER-PARAMETER PROFILES IN LPHOSPHATIDYLCHOLINE-DISTEAROYLPHOSPHATIDYLCHOLINE MIXTURES - A H-2-NMR STUDY

The accommodation of chain-length mismatch in liquid crystal phase bil ayers was examined by using deuterium nuclear magnetic resonance to ob tain smoothed orientational order parameter profiles for acyl chains o f both components in binary lipid mixture bilayers. Mixtures of dimyri stoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSP C) covering a range of compositions were prepared with either DSPC acy l chains or DMPC acyl chains perdeuterated. Orientational order parame ters in the plateau regions of the smoothed profiles for both componen ts were found to increase smoothly with increasing DSPC concentration. The orientational order parameters in the DSPC-smoothed profile were found to be slightly higher than corresponding values for DMPC over a wide range of bilayer composition. The shapes of the smoothed profiles for both components were found to be sensitive to bilayer composition . At low DSPC concentration, DSPC methylene deuterons near the bilayer center display a secondary plateau at low orientational order. At hig h DSPC concentration, the plateau of the DMPC-smoothed profile is stre tched slightly. The concentration dependence of the smoothed profiles at low DSPC concentration appears to be consistent with a picture in w hich the last few segments of the DSPC chain cross the bilayer midplan e, on average, but remain very disordered.