The pH dependence of the radical reaction differed significantly from that of the non-radical reaction (Fig. ). The latter showed an optimum pH of approximately 8, which is consistent with the enzymatic reaction catalyzed by maleate isomerase (~8.5) with sulfanyl groups (thiols) having pKa values of 8-9. In contrast, the catalytic reaction with the radical initiator showed much greater dependence on pH, with the summit of the volcano at pH ~ 6. At a lower pH, the deprotonation of thiol units is not favored, whereas catalyst deactivation can be expected at a higher pH via the dimerization of thiyl radicals, as previously observed by transient absorption analysis. The present catalytic reaction is also terminated in the presence of oxygen (Fig. and Table ), probably by the oxidative coupling of thiols to disulfides, which is a central chemical reaction observed in many natural systems. Consideration of inhibition effects will be essential for controlling the experimental reproducibility of the catalysis: The effects of alkali metals and alkaline earth metals were not significant in this work (Table , entries 2-5), while the use of buffer solutions, organic solvents, or transition-metal sources inhibited the catalytic reactions (Table , entries 6-12, 14 and 15).