Mechansim of the Early Stages of the Lamellar/Bicontinuous Cubic Phase Transition

Abstract

The transition between lamellar and bicontinuous inverted cubic (QII) phases is mediated by catenoidal bilayer channels, which are also the structures that are produced by membrane fusion. The QII phase forms by production of these channels in an array of initially flat bilayers. As the number of channels increase with increasing temperature, they initially form a disordered, metastable array in the lamellar phase, which has been referred to as the “isotropic phase.” The isotropic phase accomplishes the topological change between the lamellar and QII phases. The disordered lattice subsequently transforms into the QII phase by local bending of the bilayers. There are discordant observations concerning the rate of formation and temperature interval of existence for the “isotopic phase.” Here, the expected dimensions of channels are predicted as a function of temperature, as well as the extent of “isotropic phase” formation as a function of temperature and sample water content. The predictions are made using the fourth order curvature energy model previously used to rationalize the stability of the QII phase in phospholipids (Langmuir, 2010, 26:8673). The extent of channel and “isotropic phase” formation is sensitive to the interaction energy of the flat bilayers in the lamellar phase, the lateral dimensions of the lamellar phase bilayers and the sample water volume fraction, as well as the temperature and curvature elastic parameters of the lipid. The theory, as well as the hysteretic nature of the transition process, accounts for the apparent conflict between early observations, made mostly by phosphorus NMR.