No abstract

The accommodation of chain-length mismatch in liquid crystal phase bilayers was examined by using deuterium nuclear magnetic resonance to obtain smoothed orientational order parameter profiles for acyl chains of both components in binary lipid mixture bilayers. Mixtures of dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) covering a range of compositions were prepared with either DSPC acyl chains or DMPC acyl chains perdeuterated. Orientational order parameters in the plateau regions of the smoothed profiles for both components 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 high DSPC concentration, the plateau of the DMPC-smoothed profile is stretched slightly. The concentration dependence of the smoothed profiles at low DSPC concentration appears to be consistent with a picture in which the last few segments of the DSPC chain cross the bilayer midplane, on average, but remain very disordered.

The temperature dependence of the mean orientational order parameter in the vicinity of the liquid crystal to gel phase transition is obtained from the first moment M1 of deuterium nuclear magnetic resonance spectra for bilayers of chain perdeuterated phosphatidylcholines with acyl chains of 12, 14, 16, and 18 carbons. The data clearly show an increasing temperature dependence of the orientational order parameter in the vicinity of the transition, with the effect becoming more pronounced with decreasing chain length. Assuming a linear relationship between the mean orientational order parameter and the extension of the acyl chain, estimates of the change in area of the membrane at the transition are shown to be consistent with those obtained from other measurements. It is shown that the transition may be modeled in terms of a Landau expansion of the free energy involving a small number of phenomenological parameters. From this it is shown that the behavior of these systems in the temperature range of interest is, in large part, controlled by the close proximity of a spinodal to the transition temperature.

Under magic-angle spinning (MAS), the 2D-MELODRAMA pulse sequence reintroduces homonuclear dipolar couplings, thereby allowing internuclear correlations to be determined. The evolution of the density matrix under the average Hamiltonian established under the 2D-MELODRAMA pulse sequence has been evaluated, and closed-form expressions for the dependence of cross-peak intensity as a function of the dipolar mixing time are presented. In the case of sodium proprionate-2-3-13 C 2 (>90% 13 C) it is shown that analysis of cross-peak intensity vs mixing times allows estimates of dipolar coupling with an accuracy comparable with the 1D-MELODRAMA experiment. The potential for simultaneous multiple internuclear correlations and distance measurements in spin systems for which there is not a large difference in resonance frequencies is illustrated with glucose-13 C 6 (36% uniformly labeled). It is shown that multiple correlations for each ring carbon can be detected and good estimates of the corresponding internuclear distances can be extracted from analysis of cross-peak intensities as a function of the dipolar recoupling time.