Conformations, orientations and time scales characterising dimyristoylphosphatidylcholine bilayer membrane. Molecular dynamics simulation studies

The results of molecular dynamics simulation of fully hydrated dimyristoylphosphatidylcholine (DMPC) bilayer membrane in the liquid-crystalline phase are presented. They show that the probability of a gauche conformation varies periodically along the chain with only a slight increase towards the end of the chain. However, the frequency of transition between conformations increases, due to a decrease in the lifetime of the trans conformation, along the chain. The average lifetimes for trans conformations are in the range of 1-2 x 10(-10) s and for gauche conformations in the range of 4-7 x 10(-11) s. The alpha-chain of the DMPC head group has mainly an extended conformation, due to predominantly trans conformation of alpha 5 torsion. The rotational correlation time for the P-N vector is 3.7 ns. The C2-C1-O11-P fragment of the DMPC head group (theta 1, alpha 1, alpha 2 torsions) is rigid while the P-O12-C11-C12 fragment (alpha 3, alpha 4, alpha 5 torsions) is flexible. The lateral diffusion coefficient for DMPC self-diffusion in the membrane is 2 x 10(-7) cm(2)/s; the rate of transverse diffusion is the same. Large differences in the calculated rotational correlation times for the alpha-, beta-, gamma-chains and for the O21-C1 vector indicate that in the liquid-crystalline bilayer each segment of the DMPC molecule exhibits its own rotational freedom, in addition to its internal flexibility resulting from rotational isomerism. The results obtained in these calculations, although in general agreement with some experimental data, shed new light on the dynamical behaviour of phosphatidylcholine molecules in the bilayer membrane in the liquid-crystalline phase.