Monovalent cations differentially affect membrane surface properties and membrane curvature, as revealed by fluorescent probes and dynamic light scattering

Abstract

The effects of monovalent cations on the interfacial electrostatic potential ( ψ d), hydrodynamic shear boundary distance ( d s), and membrane curvature were studied in large unilamellar phospholipid and galacto/sulfolipid liposomes containing different fractions of negatively charged lipids. The differential effects of alkali metal ions on ψ d could be accurately determined at physiological surface charge densities with a surface-anchored fluorescent probe. Li + and Na + more effectively decrease ψ d and exhibit higher association constants ( K as) than K + and Cs +. These two groups of cations display qualitatively different perturbations of the interfacial structure. Combining K as values with the electrokinetic (ζ) potentials yielded the respective d s values. At low ionic strength d s more substantially increases with Li + or Na + than with K + or Cs +. Increasing surface charge density causes increased membrane curvature in the presence of K + or Cs +, but this is largely prevented by Li + or Na +. Membrane binding of the amphiphilic cation acridine orange decreases surface charge and membrane curvature more extensively than H 3O +, Li +, and Na +. The differential interface-perturbing behavior of monovalent cations is discussed with regard to their different hydration tendencies that will modulate the extent and stability of the hydrogen-bond network along the charged membrane surface.