STRUCTURE AND PACKING OF PHOSPHATIDYLCHOLINES IN LAMELLAR AND HEXAGONAL LIQUID-CRYSTALLINE MIXTURES WITH A NONIONIC DETERGENT - A WIDE-LINEDEUTERIUM AND P-31 NMR-STUDY

Lamellar and hexagonal (H-I) liquid-crystalline mixtures of 1,2-dimyri stoyl-sn-glycero-3-phosphocholine (DMPC) with the nonionic detergent o ctaethyleneglycol mono-n-dodecyl ether (C(12)E(8)) have been studied b y solid-state H-2 and P-31 NMR spectral techniques. The HI phase struc ture is considered as a useful model for small mixed detergent/phospho lipid micelles. Formation of such mixed micelles is an obligatory step in the investigation of biological membranes and membrane proteins. B oth the phospholipid P-31 NMR chemical shift anisotropy and the H-2 NM R quadrupole splittings can be observed in the hexagonal mesophase, in contrast to micelles where motional averaging precludes the direct ob servation of residual chemical shift or quadrupolar tensors. The H-2 N MR spectra were evaluated in terms of carbon-deuterium bond order para meter profiles. The order profiles obtained in the lamellar detergent/ phospholipid mixtures were significantly different from the correspond ing profiles in the mixed detergent/phospholipid H-I phase. In the lam ellar phase; an increasing proportion of C(12)E(8) led to a significan t reduction in the absolute magnitude of the individual order paramete rs, whereas in the H-I phase the order parameters remained relatively constant Over abroad range of detergent/phospholipid molar ratios. It was also shown by P-31 and H-2 NMR that the interfacial segments of th e 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 fr om the order parameter profiles and interpreted in terms of geometrica l properties of the hexagonal and lamellar phase aggregates. Furthermo re, measurements at different temperatures yielded an estimate of ther mal expansion coefficients of the phospholipid acyl chains in the diff erent phases. It is concluded that addition of detergent to lamellar p hospholipids results in increasing packing constraints which are event ually relieved by a lamellar --> H-I transition. These packing constra ints are discussed in terms of the molecular shape concept and with re ference to statistical mechanical models of chain packing in bilayers, H-I-structures, and small micelles.