Activation energies and kinetics of glycolipid phase transitions

Abstract

The kinetics of the L β→ H II and L β→ L α 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-β- d-glucosyl- sn-glycerol (16-1,2-Glc β), its α-anomer 1,2-di- O-hexadecyl-3- O-α- d-glucosyl- sn-glycerol (16-1,2-Glc α) and 1,2-di- O-alkyl-3- O-β- d-maltosyl- sn-glycerols with saturated identical C 14 and C 16 alkyl chains (14-1,2-Mal; 16-1,2-Mal). Measuremnets 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 β phase. The careful examination of the lamellar to non-lamellar transition of 16-1,2-Glc α 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.