Complexation of Cs⁺, K⁺ and Na⁺ by norbadione A triggered by the release of a strong hydrogen bond: nature and stability of the complexes

Norbadione A (NBA) is a pigment present in edible mushrooms which is presumed to selectively complex Cs⁺ cations. Due to a very uncommon complexation mechanism, we used a combination of several experimental techniques, including ¹H-NMR, ¹³³Cs-NMR, isothermal calorimetric, potentiometric titrations and molecular dynamics MD simulations to determine the nature of the complexed species, as well as their stability constants for the NBA-M⁺ systems (M⁺ = Cs⁺, K⁺, Na⁺) in methanol:water 80:20 solutions at 25.0 °C. We show that almost no complexation occurs below pH 7.5, as long as a proton, involved in a strong hydrogen bond, bridges both carboxylic and enolic groups of each pulvinic moiety of NBA. Thus, neutralization of that proton is necessary to both set free potential coordination sites and to trigger a conformational change, two conditions needed to bind successively a first, then a second metallic cation. The stability constants determined in this study are in good agreement with each other, leading to the stability order Cs⁺ > K⁺ > Na⁺ for both mono- and bimetallic complexes, which is the reversed order to the one generally observed for low molecular weight carboxylic ligands in water. According to MD simulations in solution, complexation involves a mixture of Z/E isomers and conformers of NBA with a broad diversity of binding modes. Some pH and environment dependent aggregation phenomena are considered to also contribute to the binding process, and to possibly explain the accumulation of radionuclides in mushrooms.