Monte Carlo Simulation of Two-Component Bilayers: DMPC/DSPC Mixtures

Abstract

In this paper, we describe a relatively simple lattice model of a two-component, two-state phospholipid bilayer. Application of Monte Carlo methods to this model permits simulation of the observed excess heat capacity versus temperature curves of dimyristoylphosphatidylcholine (DMPC)/distearoylphosphatidylcholine (DSPC) mixtures as well as the lateral distributions of the components and properties related to these distributions. The analysis of the bilayer energy distribution functions reveals that the gel-fluid transition is a continuous transition for DMPC, DSPC, and all DMPC/DSPC mixtures. A comparison of the thermodynamic properties of DMPC/DSPC mixtures with the configurational properties shows that the temperatures characteristics of the configurational properties correlate well with the maxima in the excess heat capacity curves rather than with the onset and completion temperatures of the gel-fluid transition. In the gel-fluid coexistence region, we also found excellent agreement between the threshold temperatures at different system compositions detected in fluorescence recovery after photobleaching experiments and the temperatures at which the percolation probability of the gel clusters is 0.36. At every composition, the calculated mole fraction of gel state molecules at the fluorescence recovery after photobleaching threshold is 0.34 and, at the percolation threshold of gel clusters, it is 0.24. The percolation threshold mole fraction of gel or fluid lipid depends on the packing geometry of the molecules and the interchain interactions. However, it is independent of temperature, system composition, and state of the percolating cluster.

Temperature and pressure dependent growth and morphology of DMPC/DSPC domains studied by Brewster angle microscopy

“…At this surface pressure, the areas per lipid molecule in the pure monolayers are similar, considering the experimental error, to the ones reported for bilayers (34Å 2 molecule −1 versus 45Å 2 molecule −1 and 71Å 2 molecule −1 versus 65Å 2 molecule −1 for DSPC and DMPC monolayers and bilayers, respectively) (Marsh, 1990). There are however two striking differences between the lateral structure seen on the BAM images and the one which can be seen or deduced from other techniques (Bagatolli and Gratton, 2000a;Giocondi et al, 2001;Sugár et al, 1999;Vaz et al, 1989). The first difference concerns the topology.…”

“…The second difference concerns the size of the domains. Whereas AFM studies have provided evidence for submicron sized domains (Giocondi et al, 2001;Kaasgaard et al, 2003;Leidy et al, 2002), as expected from indirect techniques (Arnold et al, 2004;Gliss et al, 1998;Sankaram et al, 1992) and Monte Carlo simulations (Michonova-Alexova and Sugár et al, 1999), domains with sizes between 5 and 20 m were observed in this work, a size which is close to the ones imaged by two-photon fluorescence microscopy (Bagatolli and Gratton, 2000a).…”

“…The first difference concerns the topology. Whereas, FRAP (Vaz et al, 1989) and Monte Carlo simulations (Michonova-Alexova and Sugár, 2002;Sugár et al, 1999) predict that the fluid phase becomes discontinuous at some point within the two-phase coexistence region, the monolayers observed by BAM were composed of condensed domains separated by an expanded phase, except for the (25:75) mixture in which the domains joined although their merging was not observed. The discontinuous nature of the fluid phase was also observed in single bilayers deposited on mica by AFM (Giocondi et al, 2001) but is not mentioned in the two-photon fluorescence microscopy work (Bagatolli and Gratton, 2000a).…”

“…DMPC/DSPC bilayers have also been studied by Monte Carlo simulations Sugár, 2001, 2002;Sugár et al, 1999). In all these studies, domains between tens and hundreds of nanometers were obtained.…”

“…It is believed to play a key role in important biological processes such as membrane protein binding, insertion and function (Brown and London, 2000;Mouritsen (Jørgensen and Mouritsen, 1995;Sugár et al, 1999). Among these model membranes, the binary mixture of dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) stands out as one of the most studied Morrow et al, 1991;Sankaram et al, 1992;Sugár et al, 1999;Vaz et al, 1989). The temperature-composition phase diagram of this system displays a broad biphasic region in which gel and liquid crystalline domains coexist (Foster and Yguerabide, 1979;Knoll et al, 1981;Shimshick and McConnell, 1973).…”