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.