Molecular Dynamics Study on the Effect of Chain Branching on the Physical Properties of Lipid Bilayers: Structural Stability

In general, bilayers composed of branch-chained lipid molecules are known to have high stability and low ion permeability. To understand how chain branching affects bilayer properties on a molecular level, two molecular dynamics (MD) simulations of lipid bilayers have been undertaken in the isothermal-isobaric ensemble. The first MD simulation was carried out on the straight-chained DPPC bilayer, and the second was carried out on the branch-chained DPhPC bilayer. Chain branching reduced segmental order of the lipid chain. This would be closely related to a high gauche probability at the dihedrals in the vicinity of tert-carbons; these dihedrals brought about the chain bending at the branched segments. Due to the characteristic conformation of the branched chain, some different effects were observed: First, the probability of parallel orientation of the two chains in a lipid was reduced; second, a chain caught between the two chains of the neighboring lipid in the same leaflet of the bilayer was frequently observed. As a consequence, the branched chain showed a much lower overall rate of trans-gauche isomerization than its straight counterpart. In conclusion, the high structural stability of the branched DPhPC bilayer is attributable mainly to the slow conformational motion of the hydrophobic chain, which is clearly correlated with the observed chain "entrapment" between the lateral neighboring lipid molecules.