Effects of Alkali Cations and Halide Anions on the DOPC Lipid Membrane

Abstract

By means of molecular dynamics simulations with an all-atom force field, we investigated the affinities of alkali cations and halide anions for the dioleoylphosphatidylcholine lipid membrane in aqueous salt solutions. In addition, changes in phospholipid lateral diffusion and in headgroup mobility upon adding NaCl were observed using fluorescence spectroscopy. The simulations revealed that sodium is attracted to the headgroup region with its concentration being maximal in the vicinity of the phosphate groups. Potassium and cesium, however, do not preferentially adsorb to the membrane. Similarly, halide anions do not exhibit a strong affinity for the lipid headgroups but merely compensate for the positive charge of the sodium countercations. Nevertheless, larger halides such as bromide and iodide penetrate deeper into the headgroup region toward the boundary with the hydrophobic alkyl chain, this effect being likely underestimated within the present nonpolarizable force field. Addition of alkali halide salts modifies physical properties of the bilayer including the electronic density profiles, the electrostatic potential, and the area per lipid headgroup.

Free energy landscapes of sodium ions bound to DMPC–cholesterol membrane surfaces at infinite dilution

“…Therefore, a bound state can be characterized by the ion's coordination number with lipid binding sites and its simultaneous coordination number with water molecules. A number of experiments [3][4][5][6] and simulation works [6][7][8][9][10][11][12][13] have indicated that metal cations bind directly to the oxygen atoms of the negative charged phosphate (PO − 4 ) and carbonyl (C=O) groups of lipid molecules. Accordingly, we defined two collective variables (CVs) to describe the ion's bound states: the coordination number between a Na + ion and lipid (including cholesterol) oxygens (CLP), and the coordination number between a Na + ion and water oxygens (CWT).…”

“…Several experiments revealed that metal cations are bound to the negative charged head groups of phospholipid membranes [3][4][5][6] . Meanwhile, numerous molecular dynamics (MD) simulations have been performed to study the binding of metal cations at phospholipid membranes from an atomic point of view [6][7][8][9][10][11][12][13] . However, most of the experiments and simulation works on ionic binding to phospholipid membranes have been devoted to cholesterol-free environments.…”

Fluorescence Techniques to Study Lipid Dynamics

“…There have been many studies on phase separated model membranes, supported or free-standing, to determine the diffusion characteristics of lipids in different phases (Chiantia et al 2008(Chiantia et al , 2009Lingwood et al 2008;. It has been shown that the diffusion coefficient is influenced by environmental conditions such as ionic strength or sugar content of the medium (Bockmann et al 2003;Sum et al 2003;Doeven et al 2005;van den Bogaart et al 2007;Guo et al 2008;Vacha et al 2009). The role of cholesterol in membrane organization, a big issue in lipid biology, has been intensively addressed by FCS Bacia et al 2004Bacia et al , 2005Kahya and Schwile 2006b).…”

Molecular electrometer and binding of cations to phospholipid bilayers

“…The relative ion binding affinities are generally agreed to follow the Hofmeister series [1][2][3][4][5][6][7][8][9] , however, consen-sus on the quantitative affinities is currently lacking. Until 1990, the consensus (documented in two extensive reviews 2,3 ) was that while multivalent cations interact significantly with phospholipid bilayers, for monovalent cations (with the exception of Li + ) the interactions are weak.…”

“…This conclusion has since been strengthened by further studies showing that bilayer properties remain unaltered upon the addition of sub-molar concentrations of monovalent salt 4,10,11 . Since 2000, however, another view has emerged, suggesting much stronger interactions between phospholipids and monovalent cations, and strong Na + binding in particular [6][7][8][9][12][13][14][15][16][17][18] .…”