CHARACTERIZATION OF LIPID-MEMBRANE DYNAMICS BY SIMULATION .1. TORSIONANGLE MOTIONS OF THE LINEAR-CHAINS

The torsion angle motions, generated from molecular dynamics (MD) simu lations, of the two aliphatic chains of 1,2-dimyristoyl-sn-glycero-3-p hosphatidylcholine (DMPC) in its lipid monolayer were evaluated by com paring these motions to those of an equivalent isolated (free) n-alkan e chain, and the same n-alkane chain in its crystal lattice. The time- dependent autocorrelation and (1,2)-, (1,3)-, (1,4)-, and (1,5)-cross- correlation functions were constructed to analyze the torsion angle mo tions. It was found that the torsion angle motions of the DMPC lipid m onolayer aliphatic chains are intermediate to those of the free n-alka ne chain and the same n-alkane chain in its crystal lattice, particula rly for short correlation times. The torsion angle motions of the alip hatic chains of DMPC are also found to be essentially independent of t he charge state on the head group. The linear aliphatic chains of a DM PC lipid monolayer behave most like the isolated n-alkane chains with respect to torsion angle flexibility, even though the pairs of aliphat ic chains of each DMPC are part of an ordered monolayer assembly. The aliphatic chains of the DMPC molecules in their monolayer exhibit at l east two types of wave motions. One of the wave motions is the same in form, though somewhat more diffuse, as a traveling wave found in n-alk ane crystals. The other wave motion involves major torsion angle trans itions, and has some characteristics of the soliton properties observe d in n-alkane crystals near their respective melt transition temperatu res. (C) 1997 John Wiley & Sons.