SIMULATION OF THE GEL-FLUID TRANSITION IN A MEMBRANE COMPOSED OF LIPIDS WITH 2 CONNECTED ACYL CHAINS - APPLICATION OF A DIMER-MOVE STEP

Phospholipids have been treated as dimers on a hexagonal lattice, and a move has been introduced that allows the dimers to move and change t heir orientation on the lattice. Simulations have been performed in wh ich phospholipid chains have been treated as being either independent or infinitely coupled thermodynamically with regard to their conformat ional state. Both types of simulation have reproduced well experimenta l heat-capacity curves of dipalmitoyl phosphatidylcholine small unilam ellar vesicles. Apart from a different gel-fluid interaction parameter and a different number of unlike nearest-neighbor contacts, most of t he averages and thermodynamic quantities were essentially the same in the two types of simulation. These results indicate that the transitio n is not first order and validate those of previous Monte Carlo simula tions that have neglected the dimeric nature of phospholipids in the s ense that they show that for the thermotropic transition the approxima tion of phospholipids as monomers is valid.