Studying the mechanism of Lα/HII and Lα/QII phase transmissions in phospholipid liquid crystals with temperature-jump cryo-TEM

Abstract

The transitions between lamellar (Lα), inverted cubic (QII), and inverted hexagonal (HII) liquid crystalline phases can be very rapid, despite drastic changes in system topology. The Lα phase is a stack of flat lipid bilayers, the HII phase is a hexagonal array of inverted rod micelles, and the QII phase is a lipid bilayer labyrinth with the geometry of an infinite periodic minimal surface. The mechanism of the transitions from the Lα a phase are of particular interest: they start with interbilayer events that are related to the process of biomembrane fusion, a ubiquitous and essential process in cellular function. Previous theoretical work suggested two possible mechanisms for these transitions. We have studied these transitions using time-resolved cryo-TEM. Earlier, we used on-the-grid mixing techniques to trigger the transition and observe some of the intermediates that form within seconds. Here we use a novel temperature-jump cryo-TEM technique which triggers the phase transition several milliseconds before vitrification. We extend the initial results obtained with this apparatus and reinterpret them in terms of a new theory of the transition mechanism.