MODELING OF COMPLEX BIOLOGICAL-SYSTEMS .2. EFFECT OF CHAIN-LENGTH ON THE PHASE-TRANSITIONS OBSERVED IN DIGLYCERIDE MONOLAYERS

We have performed molecular dynamics computer simulations to examine the effect of chain length on the phase transitions observed in 1,2-sn-diglyceride monolayers. Simulations were performed for three different diglycerides of chain length 10, 16, and 22. The monolayers composed of molecules of chain lengths 16 and 22 exhibit complex phase transitions as a function of surface pressure. These two Langmuir films undergo two phase transitions, where the transition at low surface density is associated with tilt. The high surface pressure transition is unique for the diglyceride molecules and is driven by a competition of hydrophobic/hydrophilic and intra-/intermolecular forces involving the two head groups. Increasing the chain length, on one hand, stabilizes the chains in the layer resulting in a lower percentage of gauche defects but, on the other hand, the low-pressure phase transition is shifted to higher surface density. No tilt transition is observed in the 1,2-sn-didecanoylglycerol film due to the short length of the alkyl chains. This film is more mobile than the C16 and C22 layers, and the melting of the layer proceeds by creating gauche defects in the alkyl chains.