Molecular dynamics simulations of unsaturated lipid bilayers:: effects of varying the numbers of double bonds

The importance of unsaturated, and especially polyunsaturated phosphatidylcholine molecules for the functional properties of biological membranes is widely accepted. Here, the effects of unsaturation on the nanosecond-scale structural and dynamic properties of the phosphatidylcholine bilayer were elucidated by performance of multinanosecond molecular dynamics simulations of all-atom bilayer models. Bilayers of dipalmitoylphosphatidylcholine and its mono-, di-, and tetraunsaturated counterparts were simulated, containing, respectively, oleoyl, linoleoyl, or arachidonoyl chains in the sn-2 position. Analysis of the simulations focused on comparison of the structural properties, especially the ordering of the chains in the membranes. Although the results suggest some problems in the CHARMM force field of the lipids when applied in a constant pressure ensemble, the features appearing in the ordering of the unsaturated chains are consistent with the behaviour known from H-2 NMR experiments. The rigidity of the double bonds is compensated by the flexibility of skew state single bonds juxtaposed with double bonds. The presence of double bonds in the sn-2 chains considerably reduces the order parameters of the CH bonds. Moreover, the double bond region of tetraunsaturated chains is shown to span all the way from the bilayer centre to the head group region.