STRUCTURE OF VITAMINYLATED LIPIDS IN AQUEOUS DISPERSION - X-RAY-DIFFRACTION AND P-31 NMR-STUDIES OF N-BIOTINYLPHOSPHATIDYLETHANOLAMINES

The structures of the phases formed in excess buffer (at pH 7.4) by a homologous series of saturated diacylphosphatidylethanolamines in whic h the headgroup is N-derivatized with biotin have been investigated fo r chain lengths of C(12:0) to C(20:0), using both P-31 nuclear magneti c resonance (NMR) spectroscopy and small-angle X-ray diffraction. In 1 M NaCl, all lipids display P-31 NMR spectra characteristic of a lamel lar gel phase at low temperature. In the fluid phase, the lipids of C( 12:0) and C(14:0) chain lengths display isotropic P-31 NMR spectra, co rresponding to aggregated phases with high surface curvature, whereas those with C(18:0) and C(20:0) chain lengths display sharp axial powde r patterns characteristic of a lamellar (L(alpha)) phase. The lipid of intermediate C(16:0) chain length displays a more complex temperature dependence of the P-31 NMR spectra in the fluid phase. The spectra co nvert from an axial powder pattern of unusually low chemical shift ani sotropy to one characteristic of a fluid lamellar (L(alpha)) phase wit h increasing temperature. The small-angle X-ray diffraction patterns o f the lipids in 1 M NaCl have lamellar repeat spacings in the gel phas e which increase linearly with chain length and are consistently lower than those in the fluid phase [for chain lengths of C(16:0) to C(20:0 )]. In addition, the gradient in long spacing with chain length in the gel phase is approximately half that expected for a gel phase with un tilted, all-trans chains, indicating that the lipid chains are interdi gitated in the gel phase (L(beta)i). The continuous X-ray scatter from those lipid aggregates with an isotropic P-31 NMR spectrum [C(12:0) a nd C(14:0)], or with low chemical shift anisotropy [C(16:0)], in 1 M N aCl suggest that the fluid state for these lipids corresponds to a mic ellar-like aggregated phase, hitherto not identified for phospholipids . In the absence of salt, the P-31 NMR spectra indicate that the lipid s with chain lengths of C(16:0), C(18:0), and C(20:0) are in a lamella r gel phase at low temperature which on chain-melting convert to a flu id micellar phase. The small-angle X-ray diffraction patterns as a fun ction of water content indicate that the lamellar gel phase for the li pid of C(18:0) chain length in the absence of salt is also one with in terdigitated chains (L(beta)i). The lipids with chain lengths of C(12: 0) and C(14:0) are found to be in a fluid micellar phase at temperatur es above 0-degrees-C, but on prolonged incubation at low temperature t he C(14:0) lipid forms a gel phase.