Computational modeling of cationic lipid bilayers in saline solutions

Based on computer simulations performed at single-molecule resolution, the effects of monovalent NaCl salt on cationic DMTAP/DMPC (dimyristoyltrimethylammoniumpropane/dimyristoylphosphatidylcholine) lipid bilayer systems are discussed. The monograph reviews, revises and expands the previously published work on how NaCl affects the structural and electrostatic [1] and the dynamic [2] properties of these systems. The effects of NaCl depended qualitatively on the cationic DMTAP lipid fraction. When the fraction was low, NaCl had a notable effect of the structural properties of the bilayer, decreasing the area per lipid, increasing the tail order, reorienting the DMPC head groups, and increasing the average electrostatic potential difference over the head group region. At high DMTAP fraction there was scarcely an effect when NaCl was added. The reason for this dichotomy was the ability of the Na+ ions to bind with the DMPC lipid carbonyl oxygens at low DMTAP fraction and to tie 2 to 4 DMPCs into a dynamic complex. At high DMTAP fraction the binding of Na+ was prevented by the high positive surface charge of the bilayer. The lateral diffusion of Na+ ions within the carbonyl region had two qualitatively different modes. Na+ ions bound to a DMPC diffused very slowly, whereas the free Na+ ions traveled rapidly within the carbonyl region. The combined effect of the two motions appeared as Na+ ions hopping from one DMPC carbonyl oxygen to the next.