Phosphorus-31 and deuterium solid-state nuclear magnetic resonance studies of the headgroup conformation of phosphonolipids in biological and model membranes.

Abstract

Solid-state nuclear magnetic resonance (NMR) techniques were applied to the study of the phase behavior and to the determination of the headgroup conformation of phosphonolipids, both natural and synthetic. The results are compared with those for analogous phospholipids in biological and model membranes. $\sp{31}$P NMR was used to characterize the phase behavior of phospho- and phosphonolipids present in polar and total lipid extracts of the protozoa Tetrahymena thermophila. The $\sp{31}$P NMR spectra of aqueous dispersions of polar and total lipids consist in the partial superposition of two powder patterns, one for each phosphorus-containing lipid class. At low temperature, both lipid extracts give rise to lineshapes characteristic of the lamellar structure. Spectra of the polar lipids show that between 15 and 40$\sp\circ$C, a broad, reversible transition from bilayer to hexagonal phase takes place. On the other hand, the phase behavior for total lipids is different: no hexagonal phase is formed, the lipids remain in the bilayer phase at a higher temperature, and a transition to an isotropic phase occurs between 35 and 40$\sp\circ$C, which is not easily reversible. A large proportion of ethanolamine-containing lipids, both phosphate and phosphonate analogs, is responsible for the hexagonal phase formation observed with the polar lipids. When neutral lipids are present with the polar lipids, the bilayer is stabilized up to a higher temperature. One of the neutral lipid components, tetrahymanol, a pentacyclic triterpenoid, is believed to be responsible for this stabilization. The results show that the conformation of the headgroup up to the phosphorus atom is very similar in phospho- and phosphonolipids, i.e. the C2-C3-O-P torsion angle is nearly trans and the C3-O-P-C1 angle variable over the range of values investigated, with a bend at the phosphorus atom. Contrary to the phosphodiester moiety in analogous phospholipids which adopts a gauche-gauche conformation, two solutions for the next torsion angle O-P-C1-C2 were found around 100 and 130$\sp\circ$. The last segment P-C1-C2-N is nearly trans, whereas the corresponding O-C1-C2-N segment in phospholipids is gauche. Despite the differences in the torsion angle values, the overall appearance of the headgroup is similar for both phospho- and phosphonolipids. The headgroup first extends out of the bilayer plane, bending at the phosphorus atom and the terminal portion lies nearly parallel to the membrane surface. (Abstract shortened by UMI.)