A COMBINED SAXS WAXS INVESTIGATION OF THE PHASE-BEHAVIOR OF DI-POLYENOIC MEMBRANE-LIPIDS/

Real-time measurements of the SAXS/WAXS diffraction patterns of aqueou s dispersions (1:1 wt/wt) of the di-polyenoic lipids di-18:2 PC, di-18 :3 PC, di-18:2 PE and di-18:3 PE were made over the temperature range 10 degrees to about -80 degrees C. The results of these measurements w ere compared to similar measurements performed on the corresponding di -18:0 and di-18:1 derivatives. SAXS measurements of the temperature de pendence of lamellar repeat distances show that the di-polyenoic lipid s undergo broad second-order transitions between their gel and liquid- crystal lamellar phases spanning 30-40 degrees C. The di-18:1 and di-1 8:0 derivatives, in contrast, undergo abrupt first-order transitions. The gel phases of the di-18:0 derivatives are characterised by two-com ponent WAXS patterns with a sharp component close to 0.42 nm and a bro ader component at narrower spacings. On cooling, these lipids appear t o undergo an initial transition L(beta') phase followed by a conversio n to an L(c) phase. The gel phases of the di-18:1 derivatives also sho w two-component patterns but with the sharp component centred closer t o 0.44 nm. The di-polyenoic lipids, in contrast, are characterised by a single broad peak centred at a spacing of about 0.42 nm, close to th at of conventional L(beta) phases. The changes in lamellar repeat dist ance accompanying the transitions in the di-monoenoic and di-polyenoic lipids, all of which occur in the frozen state, are very similar, ind icating that the acyl chains of the polyenoic lipids are close to thei r maximum extension in the gel state. The WAXS patterns of the polyeno ic lipids suggest that the saturated upper parts of the aryl chains ar e packed on a regular hexagonal lattice while their polyunsaturated te rmini remain relatively disordered.