Horseradish Peroxidase‐Catalyzed Generation of Acetophenone and Benzophenone in the Triplet State*

Abstract

ABSTRACTHorseradish peroxidase catalyzes the aerobic oxidation of 2‐phenylpropanal and diphenylacetaldehyde at physiological pH to yield acetophenone and benzophenone partly in the triplet state, respectively. These excited products plus formic acid are expected from the thermolysis of dioxetane intermediates. The presence of acetophenone was demonstrated spectrophotometr‐ically and the chemiluminescence spectrum (δmax ‐ 430 nm) was consistent with its triplet state. Energy transfer to 9,10‐dibromoanthracene‐2‐sulfonate ion, a triplet carbonyl counter, but not to anthracene‐2‐sulfonate ion, a singlet carbonyl acceptor, occurred, confirming the triplet nature of the main emitter. In the case of the diphenylacetaldehydelperoxidase system, benzophenone could also be detected spectrophotometrically but the corresponding chemiluminescence spectrum showed only red emission (δmax ‐ 630 nm), which was tentatively attributed to singlet oxygen formed by triplet‐triplet energy transfer to ground state oxygen. Horseradish peroxidase can be replaced by other he‐meproteins such as myoglobin, hemoglobin and micro‐peroxidase as catalyst of the chemiluminescent reaction. The distinct emission spectra achieved with different hemeproteins suggest a role of the microenvi‐ronment in totally or partly protecting the excited species from oxygen collisions, resulting in emission maxima around 430 nm, 630 nm or both.