Component and State Separation in DMPC/DSPC Lipid Bilayers:A Monte Carlo Simulation Study

Abstract

In this paper a two-state, two-component, Ising-type model is used to simulate the lateral distribution of the components and gel/fluid state acyl chains in dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) lipid bilayers. The same model has been successful in calculating the excess heat capacity curves, the fluorescence recovery after photobleaching (FRAP) threshold temperatures, the most frequent center-to-center distances between DSPC clusters, and the fractal dimensions of gel clusters (Sugar, I. P., T. E. Thompson, and R. L. Biltonen, 1999. Biophys. J. 76:2099-2110). Depending on the temperature and mole fraction the population of the cluster size is either homogeneous or inhomogeneous. In the inhomogeneous population the size of the largest cluster scales with the size of the system, while the rest of the clusters remain small with increasing system size. In a homogeneous population, however, every cluster remains small with increasing system size. For both compositional and fluid/gel state clusters, threshold temperatures-the so-called percolation threshold temperatures-are determined where change in the type of the population takes place. At a given mole fraction, the number of percolation threshold temperatures can be 0, 1, 2, or 3. By plotting these percolation threshold temperatures on the temperature/mole fraction plane, the diagrams of component and state separation of DMPC/DSPC bilayers are constructed. In agreement with the small-angle neutron scattering measurements, the component separation diagram shows nonrandom lateral distribution of the components not only in the gel-fluid mixed phase region, but also in the pure gel and pure fluid regions. A combined diagram of component and state separation is constructed to characterize the lateral distribution of lipid components and gel/fluid state acyl chains in DMPC/DSPC mixtures. While theoretical phase diagrams of two component mixtures can be constructed only in the case of first-order transitions, state and component separation diagrams can be constructed whether or not the system is involved in first-order transition. The effects of interchain interactions on the component and state separation diagrams are demonstrated on three different models. The influences of state and component separation on the in-plane and off-plane membrane reactions are discussed.

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

“…In all these studies, domains between tens and hundreds of nanometers were obtained. In some cases, the small nanometer scale domains were found to coexist with a large micrometer scale domain (Michonova-Alexova and Sugár, , 2002Sugár et al, 1999). When a large domain was found, it was seen to decrease in size as the temperature was increased (Michonova-Alexova and as is observed for monolayers in the present work.…”

“…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).…”

Tuning Membrane Thickness Fluctuations in Model Lipid Bilayers

“…In this region, we conjecture the possible existence of small transient DSPC clusters that reduce the area of fluid domains that can support enhanced thickness fluctuations, and may even anchor the surrounding molecules, limiting their extensions or compressions and consequently lowering the average amplitude of the membrane thickness fluctuations. Such clusters have been reported in contrast-matched SANS experiments near the upper gel-fluid transition of the mixture (22) and in Monte Carlo simulations on fluid DMPC/DSPC bilayers (21).…”

“…The two molecules experience distinct transition temperatures, T m , at which the tails change from a stiff stretched configuration (gel phase) to a flexible, more coiled configuration (fluid phase). For tail-deuterated (dt) lipids, the transition temperatures are T m (dtDSPC) ¼ 50.5 C and T m (dtDMPC) ¼ 20.5 C (12), yielding a broad thermal range of gel-fluid coexistence in DMPC/DSPC mixtures (21,22). Additionally, the two lipid molecules experience dramatic changes in their tail-length mismatch of z1 nm in the fluid and gel phases and z2 nm in the gel-fluid coexistence phase (12).…”

Karhunen-Loeve analysis for pattern description in phase separated lipid bilayer systems

“…With the use of simplified force fields, on the other hand, it becomes possible to predict the phase diagram of phospholipidlike molecules. Domain formation and qualitatively accurate domain sizes have only been predicted using simple lattice models, [16][17][18] where the coexistence of domains of different sizes has been shown. In this state, a high two-dimensional fluidity is observed.…”