Interestingly, the addition of a water-soluble radical initiator 2,2'-azobis(2-amidinopropane) (AAPH) to the neutral aqueous solution greatly enhanced the reaction (Figure , and Table entry 4), while the use of an organic solvent (Table ), the addition of radical inhibitors (Table ), or the reaction under an oxygen atmosphere (Table ) significantly retarded the isomerization. Simple irradiation did not affect the thermal catalysis (Table , entry 8), while photochemical reactions using Ru(bpy)3 2+ as a photosensitizer also realized the catalytic reaction upon visible-light irradiation (Table , entries 9-11, and Figure ); in this case, thiols (E < 0.2 V vs SCE) could be oxidized by the photoexcited ruthenium complex ( 3 [Ru(bpy)3 2+ ]*, E = 0.82 V vs SCE in acetonitrile) to give the thiyl radicals. In fact, the use of alanine (12) as a thioldeleted mutant of cysteine resulted in almost no reaction to take place under the otherwise optimized conditions with AAPH initiator . Control experiments also confirmed that no reaction occurred without thiols (Table ), indicating that both a radical initiator and a thiol are indispensable for the catalytic reaction to take place. These results indicate that the reaction could be accelerated by cysteine thiyl radicals either in the initial insertion to form the adducts, or at the final step of hydrogen abstraction from the formed adducts to close the catalytic cycle (vide infra).