The pH dependence of the radical reaction differed significantly from that of the non-radical reaction (Figure ). The latter showed an optimum pH of approximately 8, and this 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 the 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 (Figure 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: Effect of alkali metals and alkaline earth metals was not significant in this work (Table , entries 2-5), while the use of buffer solutions or transition-metal sources inhibited the catalytic reactions (Table and. Structure-Activity Relationships: Unlike the brominebased catalytic system, the current organic molecule-based catalysts have structural diversity and flexibility in catalyst design. As such, we then investigated the catalytic ability of various thiol-based molecular catalysts (Figures and). The "chemical mutational" study revealed the importance of the thiol units as well as the adjacent amine or hydroxyl groups.