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Recent research has shown that nitroxyl (HNO) has important and unique biological activity, especially as a potential alternative to current treatments of cardiac failure. HNO is a reactive molecule that spontaneously dimerizes and subsequently dehydrates to form nitrous oxide (N2O), making its detection in solution or biologically relevant preparations difficult. The prefluorescent probe 4‐((9‐acridinecarbonyl)amino)‐2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO‐9‐AC) has been used to detect HNO in aqueous solution and to differentiate it from nitric oxide (NO). TEMPO‐9‐AC reacts with HNO via a net hydrogen abstraction to produce the highly fluorescent TEMPO‐9‐AC‐H and NO. The utility of TEMPO‐9‐AC as a probe for HNO has been shown using the common HNO donors Angeli's salt and Piloty's acid (PA) along with a recently reported HNO donor, 2‐bromo‐PA. The use of TEMPO‐9‐AC is complicated by intermolecular fluorescence quenching and competitive HNO trapping by the NO produced, but nonetheless, it can be used to study HNO reactivity in aqueous solution. Copyright © 2011 John Wiley & Sons, Ltd.

Photolysis of p-azidoacetophenone (1a) or 2,3,5,6-tetrafluoro-p-azidoacetophenone (1b) releases the corresponding singlet nitrenes 2a and 2b. In aqueous solutions singlet nitrenes relax (1.1 ps and 43 ns, respectively) to the lower energy triplet nitrenes 3a and 3b, intermediates which do not react to form cross-links or adducts with typical amino acids and nucleic acids. In a hydrophobic environment singlet nitrene 2a partitions between forming triplet nitrene 3a and an acyl-substituted didehydroazepine 4a, which can be detected by LFP and time-resolved IR spectroscopy. The absolute rate constant of reaction of didehydroazepine 4a with water, in acetonitrile, was determined (3.5 x 10(4) M-1 s-1) by laser flash photolysis (LFP) techniques with IR detection at ambient temperature. Photolysis of tetrafluoro azide 1b releases singlet nitrene 2b, which has a lifetime of 172 ns in benzene and can readily be intercepted by pyridine to form ylide 10b (lambdamax = 415 nm). Singlet nitrene 2b reacts with the unactivated CH bonds of cyclohexane to form adduct 8b in 46% yield. Absolute rate constants of reaction of 1b with N-methylimidazole, phenol, dibutyl sulfide, indole, methanol, and dimethyl sulfoxide were determined using the pyridine ylide probe method. It is concluded that photolysis of p-azidoacetophenone (1a) will not lead to cross-link formation but that tetrafluorinated azide 1b can form useful singlet nitrene derived adducts upon photolysis.

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