Lipid Bilayer Pre-Transition as the Beginning of the Melting Process: a Periodic Melting

Abstract

We investigate the bilayer pre-ransition, which some lipids present at temperatures below their main phase transition, and which is generally associated to the formation of periodic ripples. Experimentally, we focus on the ubiquitous dipalmytoylphosphatidylcholine (DPPC) and on its charged analog dipalmytoylphosphatidylglycerol (DPPG) at different ionic strengths. Analysis of the excess heat capacity of DPPC and DPPG shows that both pre- and main transitions are part of the melting process. The cooperativeness of DPPG is lower at low ionic strength. Electron spin resonance of spin labels located at the bilayer center reveals the coexistence of gel and fluid domains at temperatures between the pre- and main transitions. Excitation generalized polarization of Laurdan also suggests microphase coexistence in the ripple phase of both lipids. To broaden the knowledge on the ripple phase, we introduce a new statistical model where a next-nearest-neighbor competing interaction is added to the usual two-state model. For the first time, modulated phases, with ordered and disordered lipids naturally appearing in a periodic fashion, are obtained between the homogeneous gel and fluid phases. A better understanding of the different interactions among lipids in a bilayer is of fundamental importance to the full knowledge of the biophysics of natural membranes.