Benzylic Monooxygenation Catalyzed by Toluene Dioxygenase from Pseudomonas putida

Toluene dioxygenase, a multicomponent enzyme system known to oxidize mononuclear aromatic hydrocarbons to cis-dihydrodiols, oxidized indene and indan to 1-indenol and 1-indanol, respectively. In addition, the enzyme catalyzed dioxygen addition to the nonaromatic double bond of indene to form cis-l,2-indandiol. The oxygen atoms in 1-indenol and cis-l,2-indandiol were shown to be derived from molecular oxygen, whereas 70% of the oxygen in 1-indanol was derived from water. All of the isolated products were optically active as demonstrated by¹⁹F NMR and HPLC discrimination of diastereomeric esters and by chiroptic methods. The high optical purity of (-)-(lR)-indanol (84% enantiomeric excess) and the failure of scavengers of reactive oxygen species to inhibit the monooxygenation reaction supported the contention that the monooxygen insertion is mediated by an active-site process. Experiments with 3-[²H]indene indicated that equilibration between C-l and C-3 occurred prior to the formation of the carbon-oxygen bond to yield 1-indenol. Naphthalene dioxygenase also oxidized indan to 1-indanol, which suggested that benzylic monoxygenation may be typical of this group of dioxygenases.