To clearly understand the difference in activity, we conducted the reaction with a lower concentration of thiols, a better initiator (Fig. ), and the optimized pH condition (Fig. ) for the limiting experiments. Fig. shows that cysteine (1) gave the best catalytic performance with a turnover number (TON) of 2.1×10 . The carboxylic acid-deleted mutant (8) also afforded comparable catalytic activity (TON = 2.0×10 3 ), whereas the lack of an amino group (7) led to lower performance (TON = 7.0×10 2 , Fig. ). The observed 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 considerable catalysis (TON = 1.9×10 3 ), indicating the potential neighboring group participation of the hydrophilic units. The use of bulky molecular catalysts is a common strategy 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.