Alkaline-earth cations enhance ortho-quinone-catalyzed ascorbate oxidation

ortho-Quinones 1,10-phenanthroquinone and β-lapachone but not para-quinones naphthazarin (NZQ) and 1,4-naphthoquinone enhance ascorbate oxidation in the presence of MgCl2 and CaCl2 at constant ionic strength. Alkaline-earth cation chelation is observed for the ortho-semiquinones but not for the para-semiquinones, while no interaction between these quinones (with the exception of NZQ) or ascorbate and these salts was detected, suggesting that semiquinone–metal complexes are responsible for the catalytic action on ascorbate oxidation of these metal salts in the presence of these ortho-quinones. Thus, redox cycling efficiency of the quinones under study here, in the presence of ascorbate, depends not only on the quinone redox potential but also on the semiquinone ability to chelate alkaline-earth cations.