Bilayers of Monomeric and Polymeric 4,N-POMECY and Dimyristoylphosphatidylcholine

We have modeled mixed bilayers of dimyristoylphosphatidylcholine (DMPC) and the polymerizable amphiphile 4,N-POMECY monomers as well as bilayers 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^up_m ≃ 29 °C? (ii) Why then do the pure (polymerized) 4, 16-POMECY macrolipids possess T^up_m ≈ 45 °C? (iii) What does the measured phase diagram imply about macrolipid structure. Using analytic methods and computer simulation to obtain phase diagrams, we conclude that the answrr to (i) is that the entropy of the relatively long polar group contributes to reducing T_m from ~41 to ~29 °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 which yields T_m ≈ 41 °C. We modeled the macrolipids as (a) flexible chains, (b) compact hexagons, or (c) rigid rods of fixed length. We found that none of these models described the experimental data. We simulated the polymerization process in the bilayer and found that the average length of the polymers formed increased with initial monomer concentration. Using these results we constructed a phase diagram using model b and found good agreement with experiment. We deduced that 4, 16-POMECY macrolipids formed in a bilayer are probably rigid with compact segments connected, possibly by rigid rods. We make predictions of the transition temperatures of monomeric and polymeric 4,N-POMECY for N = 14 and 18.