Effect of methanol on the phase-transition properties of glycerol-monopalmitate lipid bilayers investigated using molecular dynamics simulations: In quest of the biphasic effect

摘要

The effect of methanol on the phase and phase-transition properties of a 2 x 8 x 8 glycerol-1-monopalmitate bilayer patch is investigated using a series of 239 molecular dynamics simulations on the 180 ns timescale, considering methanol concentrations C-M and temperatures Tin the ranges 0-12.3 M and 302-338 K, respectively. The results in the form of hysteresis-corrected transition temperatures T-m are compatible with the expected features of the biphasic effect, with a reversal concentration c(rev), of about 5.2 M. Below this concentration, the main transition is between the liquid crystal (LC) and gel (GL) phases, and T-m decreases upon increasing c(M). Above this concentration, the interdigitated (ID) phase is the stable ordered phase instead, and T-m slightly increases upon increasing T up to about 10 M. The analysis of the structural and dynamical properties also reveals very different sensitivities and responses of the three phases to changes in c(M). In particular, the properties of the GL phase are insensitive to c(M), whereas those of the LC and ID phases are altered via an increase of the area per lipid. For the LC phase, increasing c(M) promotes disorder and fluidity. For the ID phase, in contrast, increasing c(M) up to about 10 M slightly increases the ordering and rigidity. Two side issues are also addressed, concerning: (i) the occurrence tilt-precession motions in the GL and ID phases; (ii) the influence of the pressure coupling scheme employed in the simulations, semi- or fully-anisotropic, on the simulation results. (C) 2014 Elsevier Inc. All rights reserved.