Interestingly, the addition of a water-soluble radical initiator, 2,2'-azobis(2-amidinopropane) (AAPH), to the neutral aqueous solution greatly enhanced the reaction (Fig. 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 entry 7, Fig. ) significantly retarded the isomerization. Photochemical experiments revealed that simple irradiation did not affect thermal catalysis (Table , entry 8), while visible-light irradiation with Ru(bpy)3 2+ as a photosensitizer also realized the catalytic reaction (Table , entries 9-11, and Fig. ); in this case, thiols (Eº ~ -0.17 V versus SCE) could be oxidized by the photoexcited ruthenium complex ( 3 [Ru(bpy)3 2+ ]*, E = 0.82 V versus 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 an AAPH initiator (Table , entry 12). No reaction occurred in the absence of thiols (Table , entry 13), 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).