Excited singlet molecular O(2) ((1)Δ(g)) is generated enzymatically from excited carbonyls in the dark

In mammalian tissues, ultraweak chemiluminescence arising from biomolecule oxidation has been attributed to the radiative deactivation of singlet molecular oxygen [O(2) ((1)Δ(g))] and electronically excited triplet carbonyl products involving dioxetane intermediates. Herein, we describe evidence of the generation of O(2) ((1)Δ(g)) in aqueous solution via energy transfer from excited triplet acetone. This involves thermolysis of 3,3,4,4-tetramethyl-1,2-dioxetane, a chemical source, and horseradish peroxidase-catalyzed oxidation of 2-methylpropanal, as an enzymatic source. Both sources of excited carbonyls showed characteristic light emission at 1,270 nm, directly indicative of the monomolecular decay of O(2) ((1)Δ(g)). Indirect analysis of O(2) ((1)Δ(g)) by electron paramagnetic resonance using the chemical trap 2,2,6,6-tetramethylpiperidine showed the formation of 2,2,6,6-tetramethylpiperidine-1-oxyl. Using [(18)O]-labeled triplet, ground state molecular oxygen [(18)O(2) ((3)Σ(g)(-))], chemical trapping of (18)O(2) ((1)Δ(g)) with disodium salt of anthracene-9,10-diyldiethane-2,1-diyl disulfate yielding the corresponding double-[(18)O]-labeled 9,10-endoperoxide, was detected through mass spectrometry. This corroborates formation of O(2) ((1)Δ(g)). Altogether, photoemission and chemical trapping studies clearly demonstrate that chemically and enzymatically nascent excited carbonyl generates (18)O(2) ((1)Δ(g)) by triplet-triplet energy transfer to ground state oxygen O(2) ((3)Σ(g)(−)), and supports the long formulated hypothesis of O(2) ((1)Δ(g)) involvement in physiological and pathophysiological events that might take place in tissues in the absence of light.