Inequivalence of single CHa and CHb methylene bonds in the interior of a diunsaturated lipid bilayer from a molecular dynamics simulation

Orientational order parameters for individual CHa and CHb bonds are local measures for the alignment of the bonds in a membrane interior. Experimental values exist for some lipid systems but no results are available from molecular dynamics (MD) simulations, although they are increasingly used to study biomembranes. We present such detailed analysis of a one nanosecond MD simulation for a PLPC (16:0/18:2Δ9,12) bilayer. The results show marked inequivalence for the CHa and CHb bonds of the methylene segments in the beginning and in the double bond region of the diunsaturated sn-2 chain. They also suggest slight inequivalences in the saturated chain.