To clearly understand the difference in activity, we conducted the reaction with a lower concentration of thiols, a better initiator (Figure ), and the optimized pH condition (Figure ) for the limiting experiments. Figure shows that cysteine (1) gave the best catalytic performance with a turnover number (TON) of 2.1×10 3 . The carboxylic acid-deleted mutant ( ) also afforded comparable catalytic activity (TON = 2.0×10 3 ), whereas the lack of an amino group ( ) led to lower performance (TON = 7.0×10 2 , Figure ). This order of reactivity (1 ≈ 8 > 7 ≈ 2) for the limiting conditions indicates the importance of the adjacent amino unit in the thiyl radical-based molecular catalysis. A similar thiol molecule with a hydroxyl group, amphiphilic 2-mercaptoethanol (9) also showed a considerable catalysis (TON = 1.9×10 3 ), indicating a potential neighboring group participation of the hydrophilic units. The use of bulky molecular catalysts is one of the strategies to improve catalysis and achieve higher durability both in transition-metal-catalyzed and organoradical-catalyzed systems. However, the use of bulkier analog such as penicillamine (13) showed a slower reaction as compared with pristine cysteine (1), with no improvement in catalyst durability, indicating that steric hindrance rather than steric protection is dominant in this study. . (a) (b) The dependence of the anoxic reaction rate on the thiol and radical initiator concentrations showed that the catalytic TON of the reaction reached 2.5×10 3 after optimization (Figure and Table ), with a turnover frequency (TOF) was 1.1 s -1 . Although the TOF was one order of magnitude smaller than that of the enzymatic reactions, the catalytic TON and TOF are higher than those of other radical systems (Table ), including Br -/S2O8 2- at higher reaction temperatures. This shows the potential of the current thiyl systems at lower reaction temperatures.