Specific binding of chloride ions to lipid vesicles and implications at molecular scale

Abstract

Biological membranes composed of lipids and proteins are in contact with electrolytes like aqueous NaCl solutions. Based on molecular dynamics studies it is widely believed that Na(+) ions specifically bind to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes, whereas Cl(-) ions stay in solution. Here, we present a careful comparison of recent data from electrophoresis and isothermal titration calorimetry experiments as well as molecular dynamics simulations suggesting that in fact both ions show very similar affinities. The corresponding binding constants are 0.44(±0.05) M(-1) for Na(+) and 0.40(±0.04) M(-1) for Cl(-) ions. This is highlighted by our observation that a widely used simulation setup showing asymmetric affinities of Na(+) and Cl(-) for POPC bilayers overestimates the effect of NaCl on the electrophoretic mobility of a POPC membrane by an order of magnitude. Implications for previous simulation results on the effect of NaCl on polarization of interfacial water, transmembrane potentials, and mechanisms for ion transport through bilayers are discussed. Our findings suggest that a range of published simulations results on the interaction of NaCl with phosphocholine bilayers have to be reconsidered and revised and that force field refinements are necessary for reliable simulation studies of membranes at physiological conditions on a molecular level.

Figure 1

Fraction of lipids binding Na⁺ or Cl⁻ ions as a function of the NaCl concentration from joint analysis of ITC and electrophoretic data via Eq. 16. The lines are fits of Eq. 17 to the data, yielding binding constants of $0.44\left(\pm 0.05\right)$ M⁻¹ for Na⁺ ions and $0.40\left(\pm 0.04\right)$ M⁻¹ for Cl⁻ ions.