Structure of Vitaminylated Lipids in Aqueous Dispersion: X-ray Diffraction and < sup > 31 < /sup > P NMR Studies of N-Biotinylphosphatidylethanolamines

Abstract

The structures of the phases formed in excess buffer (at pH 7.4) by a homologous series of saturated diacylphosphatidylethanolamines in which the headgroup is N-derivatized with biotin have been investigated for chain lengths of C(12:0) to C(20:0), using both 31P nuclear magnetic resonance (NMR) spectroscopy and small-angle X-ray diffraction. In 1 M NaCl, all lipids display 31P NMR spectra characteristic of a lamellar gel phase at low temperature. In the fluid phase, the lipids of C(12:0) and C(14:0) chain lengths display isotropic 31P NMR spectra, corresponding to aggregated phases with high surface curvature, whereas those with C(18:0) and C(20:0) chain lengths display sharp axial powder patterns characteristic of a lamellar (Lα) phase. The lipid of intermediate C (16:0) chain length displays a more complex temperature dependence of the 31P NMR spectra in the fluid phase. The spectra convert from an axial powder pattern of unusually low chemical shift anisotropy to one characteristic of a fluid lamellar (Lα) phase with increasing temperature. The small-angle X-ray diffraction patterns of the lipids in 1 M NaCl have lamellar repeat spacings in the gel phase 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 untilted, all-trans chains, indicating that the lipid chains are interdigitated in the gel phase (Lβi). The continuous X-ray scatter from those lipid aggregates with an isotropic 31P NMR spectrum [C(12:0) and C(14:0)], or with low chemical shift anisotropy [C(16:0)], in 1 M NaCl suggest that the fluid state for these lipids corresponds to a micellar-like aggregated phase, hitherto not identified for phospholipids. In the absence of salt, the 31P NMR spectra indicate that the lipids with chain lengths of C(16:0), C(18:0), and C(20:0) are in a lamellar gel phase at low temperature which on chain-melting convert to a fluid micellar phase. The small-angle X-ray diffraction patterns as a function of water content indicate that the lamellar gel phase for the lipid of C(18:0) chain length in the absence of salt is also one with interdigitated chains (Lβi). The lipids with chain lengths of C(12:0) and C(14:0) are found to be in a fluid micellar phase at temperatures above 0 °C, but on prolonged incubation at low temperature the C(14:0) lipid forms a gel phase. © 1993, American Chemical Society. All rights reserved.