Kinetics of Lyotropic Phase Transitions Involving the Inverse Bicontinuous Cubic Phases

Abstract

Time-resolved X-ray diffraction was employed to monitor transitions, induced by pressure jumps, between inverse lyotropic liquid crystalline phases of the mixed lipid system lauric acid/dilauroylphosphatidylcholine (2:1 molar ratio) in water. We studied transitions between the gyroid, , and the double diamond, , inverse bicontinuous cubic phases and between and the inverse hexagonal, HII, phase at a fixed water composition of 50 wt %. The transition → occurred within the 0.5 s time resolution of our X-ray measurements. This transition occurs without any change in water content within the mesophase but with a small change in the total topology. The transition → ( + HII + excess water) was observed at 59.3, 62.8, and 66.9 °C. The process appeared to follow first-order kinetics, with rate constants of 0.11, 0.17, and 0.26 s-1, respectively. The reverse transition occurred much more quickly, being completed in <1 s. In both cases, intermediate structures were observed, including the appearance of the inverse bicontinuous cubic phase based on Schwarz's P minimal surface, . We found that the latter intermediate had, to first order, the same curvature elastic energy as the phase but a greater water content. Building on earlier work (Erbes, J.; Winter, R.; Rapp, G. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1996, 100, 1713−1722), we consider the rate to depend on the difference between the final pressure and the pressure at the phase transition boundary and suggest that this can explain both the temperature dependence of the transition → ( + HII + excess water) and the higher rate of the reverse process.

Pressure effects on lipid membrane structure and dynamics

“…A further innovation of this system is the use of a high pressure reservoir on the pump side of the jumping valves; this allows larger amplitude pressure jumps to be performed. The specifications of this pressure system have set the bench mark for more recent soft condensed matter pressure cells and this system has facilitated some extremely exciting lipid experiments (Conn et al, , 2008Eisenblatter, 2006;Jeworrek et al, 2008;Kraineva et al, 2005;Squires et al, 2000Squires et al, , 2002.…”

“…This is achieved by affecting a rapid pressure jump across a previously determined phase boundary. A wide range of lyotropic lipid phase transitions have been investigated including: gel -L ␣ (Caffrey et al, 1991), L ␣ -hexagonal (Erbes et al, 2000), L ␣ -bicontinuous cubic , hexagonal -bicontinuous cubic (Squires et al, 2000) and inter bicontinuous cubic (Conn et al, 2008;Kraineva et al, 2005). SAXS has proved extremely useful in identifying relatively short lived structural intermediates in 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 (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.…”

Pressure-jump X-ray studies of liquid crystal transitions in lipids

“…We have used time-resolved X-ray diffraction to monitor transitions, induced by pressure-jumps, between various inverse lyotropic phases of a 1 : 2 dilauroylphosphatidylcholine/lauric acid (DLPC/LA) system in 50 wt% water (Squires et al 2000(Squires et al , 2002(Squires et al , 2005Seddon et al 2003). The pressure-temperature (p-T ) phase diagram (figure 7) of this sample was first assessed by static X-ray diffraction measurements.…”

High pressure cell for neutron reflectivity measurements up to 2500 bar

“…de. have also been employed to investigate proteins and lipids under high pressure conditions up to several kbar. [10][11][12][13][14][15][16][17][18][19] SAXS and SANS can be used to probe overall molecular structures and aggregation processes in solution. However, in recent years, biochemical interfacial structures and biomolecular interactions at interfaces, such as lipid membranes, have received increased interest.…”