Deuterium NMR study of intermolecular interactions in lamellar phases containing palmitoyllysophosphatidylcholine

Theoretical models of phospholipid systems have indicated that both intramolecular and intermolecular forces are important in governing their acyl chain order. Knowledge of the nature and magnitude of these interactions is central to understanding the balance of forces present in lipid lamellar phases, which in turn is related to their microscopic and macroscopic behavior. It is possible to explore the contribution of iniermolecular interactions using lipid systems with the same headgroup and acyl chain identity by variation of the ratio of the headgroups to acyl chains. In this paper, deuterium (²H) NMR spectroscopy has been used to gain information on the orientational order of an acyl chain perdeuterated lipid, 1-perdeuteriopalmitoyl-sn-glycero-3-phosphocholine (PaLPC-d₃₁), in various molecular environments. The orientational order of PaLPC-d₃₁, was studied in four different lamellar phases, including pure PaLPC-d₃₁ (containing 10 wt % H₂O), dipalmitoylphosphatidylcholine/PaLPC-d₃₁ (3:1), palmitic acid/PaLPC-d₃₁ (1:1), and cholesterol/PaLPC-d₃₁ (1:1) (each containing 50 wt % H₂O) ²H NMR spectra were obtained for the low-temperature and liquid-crystalline (L_α) states of each of these mixtures. In the low-temperature state, the first three systems yielded ²H 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 presence of spectral discontinuities and moment analysis, was found to be almost identical in the low-temperature phases. A different behavior was observed for the cholesterol/PaLPC-d₃₁ (1:1) sample in that the maximum splitting was close to the all-trans rotating value, with a profile of quadrupolar splittings due to increased disorder near the chain ends. The first three systems underwent order-disorder phase transitions near the same midpoint temperature (range of T_m values 40-48°C), whereas the chotesterol/PaLPC-d₃₁ (1:1) sample did not display a transition over the temperature range studied. In the L_α phase, where order profiles were determined as a function of acyl chain segment 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 PaLPC-d₃₁ the curvature stress is potentially large, and interdigitation of the acyl chains of the apposed monolayers may occur. However, in mixtures of PaLPC-d₃₁ with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), the curvature elastic stress is apparently relieved by an increase in the cross-sectional acyl chain area, 〈A〉, i.e. corresponding to an increase in configurational freedom. 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 lute/molecular interactions within a given lamellar phase. Average properties of the entire system such as average cross-sectional area accessible to each acyl chain relative to the headgroup area can be modulated by these interactions. Such mfermolecular interactions may be related to the presence of lipid diversity in biological membranes.