Size Distribution of Gel and Fluid Clusters in DMPC/DSPC Lipid Bilayers. A Monte Carlo Simulation Study

Abstract

Proteins and lipid components are organized into domains in many biological membranes. With different experimental techniques vastly different cluster sizes have been measured in an equimolar mixture of a DMPC/ DSPC two-component lipid bilayer: very small ones in the nanometer range and very large ones of size comparable with the size of the bilayer. In this paper the lateral distribution of gel and fluid lipid molecules in a DMPC/DSPC bilayer is simulated by using a two-state, Ising type model with the application of Monte Carlo methods. The same model has been able to predict the excess heat capacity curves, FRAP threshold temperatures, average coherence length between DSPC clusters, and the fractal dimension of gel clusters, in agreement with the respective experimental data. In this work, similarly to the experimental results, the calculated equilibrium distributions of cluster size show that, between the onset temperature of the gel-tofluid transition and the percolation threshold temperature of the gel clusters, nanometer size gel clusters coexist with a gel cluster of size comparable to the bilayer's size itself. The calculated upper bound for the size of the small clusters, 8 ( 1.5 nm is very close to the experimental estimate of 10 nm. More than one large gel clusters might be present in the case of nonequilibrium lateral distributions. By means of the calculated temperature dependence of three different cluster size averages we can get insight into the process of cluster growth.

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

“…The analysis of the bilayer energy distribution function revealed that the gel-fluid transition is a continuous transition through equilibrium states for DMPC, DSPC, and DMPC/DSPC mixtures; i.e., the system is above the critical point. The same model successfully predicted the excess heat capacity curves and the FRAP threshold temperatures at different mole fractions, the most frequent center-to-center distance between DSPC clusters at different temperatures, the fractal dimensions of the gel clusters, and the upper bound for the size of the small, nonpercolated gel clusters, in good agreement with the respective experimental data (Sugar and Biltonen, 2000;Michonova-Alexova and Sugar, 2001;Sugar et al, 2001). Recently, the geometrical properties of the gel and fluid clusters, such as cluster perimeter, cluster size, number of arms along the cluster perimeter, and number and size of inner islands in a host cluster were characterized in an equimolar mixture of DMPC/DSPC (Sugar et al, 2001).…”

Geometrical Properties of Gel and Fluid Clusters in DMPC/DSPC Bilayers: Monte Carlo Simulation Approach Using a Two-State Model

“…the calculated, bimodal distribution of the cluster size (see above), and more than one large cluster can be observed if the lateral distribution of the lipid molecules in the giant vesicle did not attain the equilibrium (Michonova-Alexova and Sugar, 2001). According to our experience one can accelerate the attainment of the equilibrium distribution by means of freezing and thawing cycles (Liu et al, 1997).…”

Computationally efficient prediction of area per lipid

“…Computer simulation, in particular molecular dynamics (MD), provides an important contribution to these investigations as an atomistic-precision tool. 8,18,[20][21][22][23][24][25] MD simulations of model bilayers have provided comprehensive insights into structure and function of these fascinating systems. In turn, the area per lipid (APL) of a bilayer provides important information about a bilayer or a membrane, because of its high sensitivity to hydrophilic attraction between head groups and hydrophobic repulsion between non-polar hydrocarbon tails.…”