Size distribution of gel and fluid clusters in DMPC/DSPC lipid bilayers.: A Monte Carlo simulation study

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-to-fluid 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.