Characterization of the L.lambda. phase in trehalose-stabilized dry membranes by solid-state NMR and x-ray diffraction

Abstract

Solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray powder diffraction were used to investigate the mechanism of trehalose (TRE) stabilization of lipid bilayers. Calorimetric investigation of dry TRE-stabilized bilayers reveals a first-order phase transition (L.kappa. .fwdarw. L.lambda.) at temperatures similar to the L.beta.'' .fwdarw. (P.beta.'') .fwdarw. L.alpha. transition of hydrated lipid bilayers. X-ray diffraction studies show that dry mixtures of TRE and 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC) have a lamellar structure with excess crystalline TRE being present. The L.kappa. phase shows typical gel-phase X-ray diffraction patterns. In contrast, the L.kappa.-phase diffraction patterns indicate disordered hydrocarbon chains. 2H NMR of specifically 2H chain-labeled DPPC confirmed that the acyl chains are disordered in the L.lambda. phase over their entire lengths. 2H spectra of the choline headgroup show hindered molecular motions as compared to dry DPPC alone, and 13C spectra of the sn-2-carbonyl show rigid lattice powder patterns indicating very little motion at the headgroup and interfacial regions. Thus, the sugar interacts extensively with the hydrophilic regions of the lipid, from the choline and the phosphate moieties in the headgroup to the glycerol and carbonyls in the interfacial region. We postulate that the sugar and the lipid form an extensive hydrogen-bonded network with the sugar acting as a spacer to expand the distance between lipids in the bilayer. The fluidity of the hydrophobic region in the L.lambda. phase together with the bilayer stabilization at the headgroup contributes to membrane viability in anhydrobiotic organisms.