Activation energies and kinetics of glycolipid phase transitions

The kinetics of the L-beta --> H-Pi and L-beta --> L-alpha phase transition s of various glycolipids have been determined in aqueous suspensions by las er 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-hexadec yl-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 alky l chains (14-1,2-Mal; 16-1,2-Mal). Measurements at different temperatures p ermitted activation energies to be determined.. In addition to the kinetic studies equilibrium measurements at slow scan rates were performed to disti nguish unambigously between kinetic and equilibrium intermediates. 14-1,2-M al 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-Gl c alpha indicates that the formation of the hexagonal phase starts with tub es whose d 10 reflection is smaller by 1.1 nm than that of the tubes observ ed after the transition at the final temperature. Estimates for the coopera tive units for kinetic and equilibrium studies are presented. A comparison of these values shows that the cooperative units determined from the kineti c studies are much smaller than those calculated from equilibrium measureme nts. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.