The sulfur-containing antioxidant molecule ergothioneine with an ability to protect metalloenzymes from reactive oxygen species (ROS) has attracted significant interest in both chemistry and biology. Herein, we demonstrated the importance of hydrogen bonding in S-oxygenation reactions between various thiones and H 2 O 2 and its significance in protecting the metal ion from H 2 O 2 -mediated oxidation. Among all imidazole- and benzimidazole-based thiones (1-10), Im Me S H (2) showed the highest reactivity toward H 2 O 2 - almost 10 and 75 times more reactive than N,N′-disubstituted Im Me S Me (5) and Bz Me S Me (10), respectively. Moreover, metal-bound Im Me S H (2) of [TpmCu(2)] + (13) was found to be 51 and 1571 times more reactive toward H 2 O 2 than the metal-bound Im Me S Me (5) of [TpmCu(5)] + (16), and Bz Me S Me (10) of [TpmCu(10)] + (21), respectively. The electron-donating N-Me substituent and the free N-H group at the imidazole ring played a very crucial role in the high reactivity of Im Me S H toward H 2 O 2 . The initial adduct formation between Im Me S H and H 2 O 2 (Im Me S H ·H 2 O 2 ) was highly facilitated (-23.28 kcal mol -1 ) due to the presence of a free N-H group, which leads to its faster oxygenation than N,N′-disubstituted Im Me S Me (5) or Bz Me S Me (10). As a result, Im Me S H (2) showed a promising effect in protecting the metal ion from H 2 O 2 -mediated oxidation. It protected biomolecules from Cu(I)-mediated oxidative damage of through coordination to the Cu(I) center of [TpmCu(CH 3 CN)] + (11), whereas metal-bound Im Me S Me or Bz Me S Me failed to protect biomolecules under identical reaction conditions. © 2019 American Chemical Society.