Kinetics and mechanism of the interconversion of inverse bicontinuous cubic mesophases

Abstract

This paper describes time-resolved x-ray diffraction data monitoring the transformation of one inverse bicontinuous cubic mesophase into another, in a hydrated lipid system. The first section of the paper describes a mechanism for the transformation that conserves the topology of the bilayer, based on the work of Charvolin and Sadoc, Fogden and Hyde, and Benedicto and O'Brien in this area. We show a pictorial representation of this mechanism, in terms of both the water channels and the lipid bilayer. The second section describes the experimental results obtained. The system under investigation was 2:1 lauric acid: dilauroylphosphatidylcholine at a hydration of 50% water by weight. A pressure-jump was used to induce a phase transition from the gyroid ͑Q II G ͒ to the diamond ͑Q II D ͒ bicontinuous cubic mesophase, which was monitored by time-resolved x-ray diffraction. The lattice parameter of both mesophases was found to decrease slightly throughout the transformation, but at the stage where the Q II D phase first appeared, the ratio of lattice parameters of the two phases was found to be approximately constant for all pressure-jump experiments. The value is consistent with a topologypreserving mechanism. However, the polydomain nature of our sample prevents us from confirming that the specific pathway is that described in the first section of the paper. Our data also reveal signals from two different intermediate structures, one of which we have identified as the inverse hexagonal ͑H II ͒ mesophase. We suggest that it plays a role in the transfer of water during the transformation. The rate of the phase transition was found to increase with both temperature and pressure-jump amplitude, and its time scale varied from the order of seconds to minutes, depending on the conditions employed.

Pressure effects on lipid membrane structure and dynamics

“…However, for most transitions, this change in lattice parameter is sufficiently small to be ignored. The formation and destruction kinetics of lipid transitions have largely been fitted empirically (Conn et al, 2008;Squires et al, 2000Squires et al, , 2002Squires et al, , 2005 to obtain rate constants which have been compared as a function of transition conditions. Changes in layer spacing during lamellar to bicontinuous cubic lipid structures have been qualitatively attributed to a proposed stalk transition model (Kozlovsky and Kozlov, 2002) and recently a quantitative kinetic model has been developed (Squires et al, 2009) to describe phase transitions between different bicontinuous cubic phases in lipid systems.…”

Hydrophilic nanoparticles stabilising mesophase curvature at low concentration but disrupting mesophase order at higher concentrations

“…52,54,55 In the reported p-T diagram for MO in excess water, the inverse cubic double diamond phase (Q DD II ) persisted over a large temperature and pressure range, with the H II only observed at the high temperature and low pressure region and the L c further at lower temperatures and higher pressures. 57 However, no such H II intermediate phases were observed in the Q DD II -Q G II transition for MO. 56 In an HP-SAXS kinetics study, a 2 : 1 lauric acid : dilauroylphosphatidylcholine (DLPC) lipid system hydrated to 50 wt% showed a rapid Q DD II -Q G II transition, with an H II phase implicated as the transient structure.…”

High Pressure X-ray Studies of Lipid Membranes and Lipid Phase Transitions

“…However, for most transitions, this change in lattice parameter is sufficiently small to be ignored. The formation and destruction kinetics of lipid transitions have largely been fitted empirically [29,33,34,78] to obtain rate constants which have been compared as a function of transition conditions. Changes in layer spacing during lamellar to bicontinuous cubic lipid structures [28] have been qualitatively attributed to a proposed stalk transition model [79].…”