Kinetics of glycolipid phase transitions: ms laser T‐jump synchrotron studies

Abstract

AbstractThe kinetics of the lamellar gel (Lβ) to inverse hexagonal (HII) phase transition as well as the LC→Lα phase change of aqueous suspensions of glycolipids were characterized using ms laser T‐jump techniques in combination with synchrotron X‐ray diffraction. The glycolipids employed were 1,2‐di‐O‐alkyl‐3‐O‐β‐D‐glucosyl‐snglycerols with identical alkyl chains of 16 and 18 carbon atoms, respectively, and 1,2‐di‐O‐alkyl‐3‐O‐β‐D‐galactosyl‐sn‐glycerol with C16 chains. The time course of the LC→Lα phase transition was probed using 1,2‐di‐O‐hexadecyl‐3‐O‐β‐D‐lactosyl‐sn‐glycerol, whose head group consists of the disaccharide β‐D‐galactopyranosyl‐(1→4)‐β‐D‐glucopyranoside. Surprisingly the phase change from the lamellar to the inverse hexagonal topology is very fast despite the major structural changes involved in the lipid‐water interface. The overall phase change of both glucolipids can be quantitatively described by a sequential mechanism of the type \documentclass{article}\pagestyle{empty}\usepackage{amsmath}\begin{document}$ {\rm L}_\beta \xrightarrow{{k_1}}{\rm I}\xrightarrow{{k_2}}{{\rm H}}_{\rm II} $\end{document}$\end{document}. The intermediate I is thinner than both the Lβ and the HII phases. It has a relatively broad distribution of d‐spacings and relaxes into the HII phase with half times of approximately 125 ms (16‐1,2‐Glc) and 244 ms (18‐1,2‐Glc), respectively. Both rate constants, k1 and k2, are chain length dependent.The galactolipid transforms into the HII phase following a sequential mechanism of the type \documentclass{article}\pagestyle{empty}\usepackage{amsmath}\begin{document}$ {{\rm L}}_\beta \xrightarrow{{k_1}}{{\rm I'}}\xrightarrow{{k_2}}{{\rm I}}\xrightarrow{{k_3}}{{\rm H}}_{\rm II} $\end{document}. The additional intermediate I' can be assigned to a lamellar phase bearing similarity to a thin Lα phase. The specific features of the galactose head group compared to glucose as head group that have been previously observed in equilibrium studies are also evident in the kinetic mechanism in that comparative structural changes in the intermediates are slower in the galactolipid than in the chain‐homologous glucolipid. There is evidence from the X‐ray data that the general transition mechanism is identical for gluco‐ and galacto‐lipids. However, the first intermediate I' disappears too fast in the glucolipid phase change to be detectable with the present time resolution of the X‐ray measurements.The kinetics of the phase change of the lactolipid is characterized within the resolution of the X‐ray sequence (5 ms) by a strict two‐state mechanism \documentclass{article}\pagestyle{empty}\usepackage{amsmath}\begin{document}$ {{\rm L}}_{{\rm C}} \xrightarrow{k}{{\rm L}}_{\alpha} $\end{document} with transient coexistence of the initial and final phase during the kinetic phase transformation. The relaxation time has been found to be highly temperature dependent, being 57 ms at 79.1 ° and 14 ms at 81.1 °.It is worth noting that there appears to be a principal difference in the phase change mechanism between glycolipids with mono‐ or disaccharide head groups in that the latter (16‐1,2‐Lac) shows no sign of any kinetic intermediate state.