A new series of Cs+, K+ and Na+ chelators: Synthesis, kinetics, thermodynamics and modeling

The synthesis of two molecules, B1 and B2, based on elements of norbadione A, the natural Cs+ chelator in mushrooms, associated, in the case of B2, with an 18-crown-6 ether is reported. Thermodynamic and kinetic analyses performed in water, ethanol and ethanol/water 9/1 v/v (M1) show in M1 and ethanol that B1 and B2 form stable complexes with Na+, K+ and Cs+. Affinity constants, measured spectrophotometrically in ethanol and M1, by the use of the Specfit program, are in the 105 and 106 range for B1 and B2, respectively. The second-order rate constants are in the 106–107M−1s−1 range and the first-order rate constants about unity. The ratios of the second-order/first-order rate constants confirm the thermodynamic results in EtOH. The kinetic processes become much too fast to allow runs in M1. Molecular simulations in EtOH imply the existence of two isomers for each of the Cs+/B1 and Cs+/B2 complexes. With B1, the more stable one is that in which the two enolates are parallel and mimic the alkali-metal inclusion cavity already envisaged for norbadione A. With B2, two similar structures are extracted, in both of which Cs+ is included in the crown ether and capped by the enolate. The affinity of B1 for Cs+ is comparable to that of norbadione A, whereas that of B2 is higher. These results are encouraging as they introduce a new series of alkali chelators which can lead to molecules capable of complexing 137Cs+ for radioactive decontamination.