title: Horseradish peroxidase-mediated aerobic and anaerobic oxidations of 3-alkylindoles.
abstract: Little is known about the HRP-mediated oxidations of 3-alkylindoles. Whereas 3-methylindole and 3-ethylindole were found to be turned over smoothly by HRP, reactions of tryptophol and N-acetyltryptamine were inefficient. Oxidations of the former two indoles by HRP under aerobic conditions led to the corresponding ring-opened o-acylformanilides and oxindoles, whereas anaerobic oxidations resulted in oxidative dimerization. The major products of anaerobic oxidation of 3-methylindole were shown to be two hydrated dimers, while anhydrodimers were obtained in the 3-ethyl case. The proposed mechanism involves HRP-mediated one-electron oxidation to give an indole radical that either dimerizes (anaerobic conditions) or reacts with O2 (aerobic conditions). Measured kinetics of indole oxidation by HRP compounds I and II mirrored their relative reactivities under turnover conditions. The observed comparable binding affinities for all four indole substrates investigated suggest that the low reactivity of tryptophol and N-acetyltryptamine reflect binding to HRP in an orientation that is disadvantageous to electron transfer oxidation of the indole ring.
authors.affiliation: Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA.
authors.forename: Ke-Qing
authors.initials: KQ
authors.lastname: Ling
authors.forename: Lawrence M
authors.initials: LM
authors.lastname: Sayre
mesh_terms: D019281:Dimerization; D004583:Electrons; D006735:Horseradish Peroxidase / Q000378:metabolism*; D006861:Hydrogen Peroxide / Q000378:metabolism; D007211:Indoles / Q000138:chemical synthesis / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism*
chemical_list: D007211:Indoles; D006861:Hydrogen Peroxide; D006735:Horseradish Peroxidase; D010100:Oxygen