Activation energies and kinetics of glycolipid phase transitions

The kinetics of the L-beta --> H-Pi and L-beta --> L-alpha phase transitions of various glycolipids have been determined in aqueous suspensions by laser T-jump techniques in combination with synchroton X-ray diffraction. The glycolipids employed in the present study were 1,2-di-O-hexadecyl-3-O-beta-D-glucosyl-sn-glycerol (16-1,2-Glc beta), its alpha-anomer 1,2-di-O-hexadecyl-3-O-alpha-D-glucosyl-sn-glycerol (16-1,2-Glc alpha) and 1,2-di-O-alkyl-3-O-beta-D-maltosyl-sn-glycerols with saturated identical C-14 and C-16 alkyl chains (14-1,2-Mal; 16-1,2-Mal). Measurements at different temperatures permitted activation energies to be determined.. In addition to the kinetic studies equilibrium measurements at slow scan rates were performed to distinguish unambigously between kinetic and equilibrium intermediates. 14-1,2-Mal and 16-1,2-Mal show both a small subtransition in the L-beta phase. The careful examination of the lamellar to non-lamellar transition of 16-1,2-Glc alpha indicates that the formation of the hexagonal phase starts with tubes whose d 10 reflection is smaller by 1.1 nm than that of the tubes observed after the transition at the final temperature. Estimates for the cooperative units for kinetic and equilibrium studies are presented. A comparison of these values shows that the cooperative units determined from the kinetic studies are much smaller than those calculated from equilibrium measurements. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.