A theoretical model for the association probabilities of saturated phospholipids from two-component bilayer lipid membranes

The non-random mixing of biomembrane components, especially saturated phosp holipids, exhibits important consequences in molecular biology. Particularl y, the distribution of lipids within natural and model membranes is strongl y determined by the selective association processes. These processes of pho spholipids take place due to the cooperative modes in multiparticle systems as well as the specific lipid-lipid interactions both in the hydrophobic c ore and in the region of the polar headgroups. We demonstrated that the inv estigation of the selective association processes of saturated phospholipid s might contribute to the insight of the lipid domains appearance inside th e bilayer membranes. The association probabilities of like-pairs and cross- pairs from a binary mixture of saturated phospholipids were tested for both parallel and anti-parallel alignments of the polar headgroups. The present model confirms the experimental evidence for saturated phospholipids to ha ve a high tendency for association in parallel configuration of the electri c dipole moments of the polar headgroups whether the cross-sectional area o f the polar headgroup is in an usual range of 25-55 Angstrom 2. There are t hree major lipid domains in a binary mixture of saturated phospholipids: (i ) lipid domains in non-mixed phase of the first mixture component, in paral lel alignment of the polar headgroups; (ii) lipid domains in non-mixed phas e of the second mixture component, in anti-parallel alignment of the polar headgroups; (iii) lipid domains in mixed phase. We think that the selective association processes of phospholipids are neither exclusively, nor only i nvolved in promoting the lipid domains appearance through bilayer phospholi pid membranes.