CURVATURE, ORDER, AND DYNAMICS OF LIPID HEXAGONAL PHASES STUDIED BY DEUTERIUM NMR-SPECTROSCOPY

Solid-state deuterium (H-2) NMR spectroscopy enables one to study both equilibrium and dynamical properties of membrane constituents at the molecular level and can yield significant insights regarding the organ ization of non-bilayer lipid aggregates. We have investigated a repres entative unsaturated phosphatidylethanolamine, viz., toyl-2-linoleoyl- sn-glycero-3-phosphoethanolamine, PLPE-d31, in the lamellar, or L(alph a), phase and the reversed hexagonal, or H(II), phase. Phosphorus-31 ( P-31) NMR studies of PLPE-d31 in the H(II) phase revealed that the che mical shift anisotropy of the phosphoethanolamine head groups, DELTAsi gma was scaled by the expected geometrical factor of -1/2 relative to the lamellar state. However, we found the occurrence of a further redu ction in the H-2 NMR quadrupolar splittings, DELTAnu(Q), of the H-2-La beled palmitoyl acyl chain segments. These observations point toward t he role of interfacial curvature with regard to properties of reverse hexagonal phase lipids, and indicate that the pivotal position or neut ral surface of approximately constant area may lie near the glycerol o r polar head group region. Variations in the acyl chain packing due to curvature of the aqueous interface yield significant differences in t he segmental order profiles as determined by H-2 NMR spectroscopy. The latter reflect the local orientational order of the acyl chains and c an be used together with simple statistical theories to extract positi onal or structural information. Average projected acyl chain lengths a nd mean interfacial or cross-sectional areas for PLPE-d31 in the diffe rent phases have been calculated. In addition, we describe a new means of estimating the radius of curvature of H(II) phase lipid aggregates utilizing H-2 NMR spectroscopy, which is based on the difference betw een the lamellar and hexagonal phase order profiles. Here the radius o f curvature, R(c), is defined as the distance from the center of the w ater core to the lipid/water interface, near the carbonyl segments of the acyl chains, giving R(c) = 25.4-28.1 angstrom for PLPE-d31 in the H(II), phase at 60-degrees-C. This value is in good agreement with pre vious X-ray diffraction studies of 1,2-dioleoyl-sn-glycero-3-phosphoet hanolamine (DOPE). Alternatively, the data yield for the radius of the central water core that R(w) = 17.8-20.5 angstrom at 60-degrees-C. Th e differences in geometry also lead to higher quadrupolar echo relaxat ion rates (R2e) for the lipid acyl segments closest to the aqueous int erface in the H(II) versus the L(alpha) phase. We propose that this en hancement is due to an additional relaxation mechanism found in the he xagonal phases, namely, translational diffusion of lipids about the cy linder axes. For comparison, the normal hexagonal (H(I)) and lamellar (L(alpha)) phases of a lyotropic system comprising perdeuterated potas sium laurate were also studied. This research indicates clearly that t he packing and dynamical properties of the acyl chains of phospholipid s depend on the curvature of the aqueous interface and, thus, the aggr egate geometry. The latter is related to the average shape of lipids i n their respective phases and to the curvature free energy, which in t he planar state may influence protein-mediated functions of membranes.