Deuterium NMR study of intermolecular interactions in lamellar phases containing palmitoyllysophosphatidylcholine

Abstract

50 wt % HzO). 'H NMR spectra were obtained for the low-temperature and liquid-crystalline (L,) states of each of these mixtures. In the low-temperature state, the fit three systems yielded ZH NMR spectra characteristic of all-trans chains undergoing axial diffusion, with the methyl groups rotating about their C3 axes. The molecular order, as judged by the presenoe of spectral discontinuities and moment analysis, was found to be almost identical in the low-tempratwe phases. A different behavior was observed for the ch~lesterol/PaLPC-d~~ (1:l) sample in that the maximum splitting was close to the all-trans rotating value, with a profile of quadrupolar splittings due to inmased disorder near the chain ends. The fit three systems underwent orderdisorder phase transitions near the same midpoint temperature (range of T, values 40-48 C), whereas the ch~lesterol/PaLPC-d~~ (1:l) sample did not display a transition over the temperature range studied. In the L, phase, where order profdm were determined as a function of acyl chain segmglt position, the segmental ordering differed significantly among the samples. The differences were interpreted using a simple diamond lattice model for the acyl chain configurational statistics, as a means of comparing the effective lengths, (L), projected along the bilayer normal and estimated chain cross-sectional areas, (A), of PaLPC-d31 in the various mixtures. The derived values of (L) and (A) can be understood qualitatively in terms of average packing parameters related to the balance of forces in the headgroup and acyl chain regions, or alternatively the curvature free energy of the membrane lipid-water interface. In lamellar phases of pure P~LPC-C~~~ the curvature stnap is potentially large, and interdigitation of the acyl chains of the appoaed monolayers may occur. However, in mixtures of PaLPC-d31 with 1,2-dipalmitoyl- sn-Bly~~3-phosp~ ~~e (DPPC), the curvature elaatic stress is apparently relieved by an increase in the cross-sectional acyl chain area, (A), Le. corresponding to an increase in configurational fdom. The data were also compared to the results of statistical theories to yield additional knowledge of the intermolecular forces. These studies indicate how the segmental ordering reflects intermolecular interactions within a given lamellar phase. Avmge properties of the entire system such as average cross-sectional arm accessible to each acyl chain relative to the headgroup area can be modulated by these interactions. Such intermolecular interactions may be related to the prcaence of lipid diversity in biological membranes.