In this paper, we describe a relatively simple lattice model of a two-compo
nent, 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)/distearoylphosp
hatidylcholine (DSPC) mixtures as well as the lateral distributions of the
components and properties related to these distributions. The analysis of t
he bilayer energy distribution functions reveals that the gel-fluid transit
ion 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 temperature
s 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 th
e gel clusters is 0.36. At every composition, the calculated mole fraction
of gel state molecules at the fluorescence recovery after photobleaching th
reshold 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 o
n the packing geometry of the molecules and the interchain interactions. Ho
wever, it is independent of temperature, system composition, and state of t
he percolating cluster.