We have modeled mixed bilayers of dimyristoylphosphatidylcholine (DMPC
) and the polymerizable amphiphile 4,N-POMECY monomers as well as bila
yers of DMPC and 4,16-POMECY polymers (macrolipids). Our intention was
to answer three questions: (i) Why, with two C-16 hydrocarbon chains,
does pure monomeric 4,16-POMECY exhibit a phase transition at T-m(up)
similar or equal to 29 degrees C? (ii) Why then do the pure (polymeri
zed) 4,16-POMECY macrolipids possess T-m(p) approximate to 45 degrees
C? (iii) What does the measured phase diagram imply about macrolipid s
tructure. Using analytic methods and computer simulation to obtain pha
se diagrams, we conclude that the answer to (i) is that the entropy of
the relatively long polar group contributes to reducing T-m from simi
lar to 41 to similar to 29 degrees C. When the macrolipids are formed,
via polymerization in their polar groups, this entropy is lost and T-
m is then determined by the interaction between hydrocarbon chains whi
ch yields T-m approximate to 41 degrees C. We modeled the macrolipids
as (a) flexible chains, (b) compact hexagons, or (c) rigid rods of fix
ed length. We found that none of these models described the experiment
al data. We simulated the polymerization process in the bilayer and fo
und that the average length of the polymers formed increased with init
ial monomer concentration. Using these results we constructed a phase
diagram using model b and found good agreement with experiment. We ded
uced that 4,16-POMECY macrolipids formed in a bilayer are probably rig
id with compact segments connected, possibly by rigid rods. We make pr
edictions of the transition temperatures of monomeric and polymeric 4,
N-POMECY for N = 14 and 18.