Ip. Sugar et al., Geometrical properties of gel and fluid clusters in DMPC/DSPC bilayers: Monte Carlo simulation approach using a two-state model, BIOPHYS J, 81(5), 2001, pp. 2425-2441
In this paper the geometrical properties of gel and fluid clusters of equim
olar dimyristoylphosphatidylcholine/ distearoylphosphatidylcholine (DMPC/DS
PC) lipid bilayers are calculated by using an Ising-type model (Sugar, I. P
., T. E. Thompson, and R. L. Biltonen. 1999. Biophys. J. 76:2099-2110). The
model is able to predict the following properties in agreement with the re
spective experimental data: the excess heat capacity curves, fluorescence r
ecovery after photobleaching (FRAP) threshold temperatures at different mix
ing ratios, the most frequent center-to -center distance between DSPC clust
ers, and the fractal dimension of gel clusters. In agreement with the neutr
on diffraction and fluorescence microscopy data, the simulations show that
below the percolation threshold temperature of gel clusters many nanometer-
size gel clusters co-exist with one large gel cluster of size comparable wi
th the membrane surface area. With increasing temperature the calculated ef
fective fractal dimension and capacity dimension of gel and fluid clusters
decrease and increase, respectively, within the (0, 2) interval. In the reg
ion of the gel-to-fluid transition the following geometrical properties are
independent from the temperature and the state of the cluster: 1) the clus
ter perimeter linearly increases with the number of cluster arms at a rate
of 8.2 nm/arm; 2) the average number of inner islands in a cluster increase
s with increasing cluster size, S, according to a power function of 0.00427
X S-1.3; 3) the following exponential function describes the average size
of an inner island versus the size of the host cluster, S: 1 + 1.09(1 - e(-
0.0072XS)). By means of the equations describing the average geometry of th
e clusters the process of the association of clusters is investigated.