Structure and packing of phosphatidylcholines in lamellar and hexagonal liquid-crystalline mixtures with a nonionic detergent: A wide-line deuterium and phosphorus-31 NMR study

Lamellar and hexagonal (H_I) liquid-crystalline mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) with the nonionic detergent octaethyleneglycol mono-n-dodecyl ether (C₁₂E₈) have been studied by solid-state ²H and ³¹P NMR spectral techniques. The H_I phase structure is considered as a useful model for small mixed detergent/phospholipid micelles. Formation of such mixed micelles is an obligatory step in the investigation of biological membranes and membrane proteins. Both the phospholipid ³¹P NMR chemical shift anisotropy and the ²H NMR quadrupole splittings can be observed in the hexagonal mesophase, in contrast to micelles where motional averaging precludes the direct observation of residual chemical shift or quadrupolar tensors. The ²H NMR spectra were evaluated in terms of carbon-deuterium bond order parameter profiles. The order profiles obtained in the lamellar detergent/phospholipid mixtures were significantly different from the corresponding profiles in the mixed detergent/phospholipid H_I phase. In the lamellar phase, an increasing proportion of C₁₂E₈ led to a significant reduction in the absolute magnitude of the individual order parameters, whereas in the H_I phase the order parameters remained relatively constant over a broad range of detergent/ phospholipid molar ratios. It was also shown by ³¹P and ²H NMR that the interfacial segments of the phospholipid (the glycerol backbone and the headgroup dipole) assume an almost identical conformation in the lamellar and in the H_I phase. Average chain lengths and mean cross-sectional areas were derived from the order parameter profiles and interpreted in terms of geometrical properties of the hexagonal and lamellar phase aggregates. Furthermore, measurements at different temperatures yielded an estimate of thermal expansion coefficients of the phospholipid acyl chains in the different phases. It is concluded that addition of detergent to lamellar phospholipids results in increasing packing constraints which are eventually relieved by a lamellar → H_I transition. These packing constraints are discussed in terms of the molecular shape concept and with reference to statistical mechanical models of chain packing in bilayers, H_I-structures, and small micelles.