A scale of metal ion binding strengths correlating with ionic charge, Pauling electronegativity, toxicity, and other physiological effects

Equilibrium constants for binding to plant plasma membranes have been reported for several metal ions, based upon adsorption studies and zeta-potential measurements. Log K values for the ions are these: Al3+, 4.30; La3+, 3.34; Cu2+, 2.60; Ca2+ and Mg2+, 1.48; Na+ and K+, 0 M−1. These values correlate well with log K values for ion binding to many organic and inorganic ligands. Log K values for metal ion binding to 12 ligands were normalized and averaged to produce a scale for the binding of 49 ions. The scale correlates well with the values presented above (R2 = 0.998) and with ion binding to cell walls and other biomass. The scale is closely related to the charge (Z) and Pauling electronegativity (PE) of 48 ions (all but Hg2+); R2 = 0.969 for the equation (Scale values) = −1.68 + Z(1.22 + 0.444PE). Minimum rhizotoxicity of metal ions appears to be determined by binding strengths: log aPM,M = 1.60 − 2.41 exp [0.238(Scale values)] determines the value of ion activities at the plasma membrane surface (aPM,M) that will ensure inhibition of root elongation. Additional toxicity appears to be related to softness, accounting for the great toxicity of Ag+, for example. These binding-strength values correlate with additional physiological effects and are suitable for the computation of cell-surface electrical potentials.