A time-resolved synchrotron X-ray study of a crystalline phase bilayer transition and packing in a saturated monogalactosyldiacylglycerol-water system

Abstract

The structural changes associated with a phase transition between the gel-phase bilayer (L β) in which the acyl chains pack in a hexagonal subcell, and a crystalline bilayer phase (L C1) where the acyl chains are packed in an orthorhombic subcell in a saturated monogalactosyldiacylglycero-water system are reported. The phase change is cooperative and takes place isothermally after the lamellar-gel phase has been held at 20°C for about 8 min. The transformation of the acyl chain subcell from hexagonal to orthorhombic induces a change in diffraction maxima observed in the region 0.6–0.7 nm which is interpreted as a change in packing of the galactose residues from an orthorhombic to hexagonal subcell. We conclude that the rearrangement of the acyl chains into a more closely packed subcell requires the head groups to reorient to reduce the steric hindrance between the bulky galactose residues.