Unusual Disordering of Phospholipid Acyl Chains in Asymmetrical Lipid Bilayers

Cell membranes have an asymmetrical bilayer structure; lipid compositions of apposed leaflets are different. Lipids in each leaflet can form either liquid-ordered (Lo) or liquid-disordered (Ld) lateral domains. Experiments have shown that an Lo domain in one leaflet can induce an ordered domains in the other leaflet, and the two ordered domains in the apposing leaflets align perfectly in shape and size. Clearly, this alignment of domains must result in a low free energy for the bilayer. Here we study asymmetrical lipid bilayer on the nanometer scale using all-atom molecular dynamics simulations. One of the leaflet has the Lo phase, and the other has DOPC with varying cholesterol mole fraction. For cholesterol mole fraction below 30% (i.e., the order of two leaflet are not matched), we found unusual disordering of phospholipid acyl chains close to the bilayer interface. The disorder disappears at cholesterol mole fraction of 40%, when the orders of two leaflets are closely matched. Our results suggest that when orders of two apposing leaflets are not matched, the unusual disordering of chains can result in a high free energy, which explains the phenomenon of domain alignment