MODEL OF A SUB-MAIN TRANSITION IN PHOSPHOLIPID-BILAYERS

A recently discovered submain phase transition in multi-lamellar bilay ers of long-chain saturated diacyl phosphatidylcholines (Jorgensen, K. (1995) Biochim. Biophys. Acta 1240, 111-114) is discussed In terms of a theoretical molecular interaction model using computer simulation t echniques. The model interprets the transition to be due to a decoupli ng of the acyl-chain melting from the melting of the pseudo-two-dimens ional crystalline lattice of the P-beta' phase. A two-stage melting pr ocess is predicted by the calculations suggesting that the sub-main tr ansition involves a lattice melting whereas the acyl-chain melting tak es place at a higher temperature at the main transition. The calculate d heat contents of the two transitions as well as the chain-length dep endence compare favorably with experimental data for multi-lamellar ph osphatidylcholine lipid bilayers.