Diacetyl, methylglyoxal, and glyoxal are alpha-dicarbonyl catabolites prone to nucleophilic additions of amino groups of proteins and nucleobases, thereby triggering adverse biological responses. Because of their electrophilicity, in aqueous medium, they exist in a phosphate-catalyzed dynamic equilibrium with their hydrate forms. Diacetyl and methylglyoxal can be attacked by peroxynitrite (k(2),approximate to 1.0 x 10(4) M(-1) s(-1) and K(2) approximate to 1.0 X 10(5) M(-1) s(-1), respectively), a potent biological nucleophile and oxidant, yielding the acetyl radical from the homolysis of peroxynitrosocarbonyl adducts, and acetate or formate ions, respectively. We report here that glyoxal also reacts with peroxynitrite, yielding formate ion at rates at least 1 order of magnitude greater than does methylglyoxal. A triplet EPR signal (1:2:1; a(H) = 0.78 mT) attributable to hydrated formyl radical was detected by direct flow experiments. In the presence of the spin trap 2-methyl-2-nitrosopropane, the EPR spectrum displays the di-tert-butyl nitroxide signal, another signal assignable to the spin trapping adduct with hydrogen radical (a(N) = a(H) = 1.44 mT), probably formed from formyl radical decarbonylation, and a third EPR signal assignable to the formyl radical adduct of the spin trap (a(N) = 0.71 mT and a(H) = 0.14 mT). The novelty here is the detection of singlet oxygen ((1)Delta(g)) monomol light emission at 1270 nm during the reaction, probably formed by subsequent dioxygen addition to formyl radical and a Russell reaction of nascent formylperoxyl radicals. Accordingly, the near-infrared emission increases upon raising the peroxynitrite concentration in D(2)O buffer and is suppressed upon addition of O(2) ((1)Delta(g)) quenchers (NaN(3), L-His, H(2)O). Unequivocal evidence of O(2) ((1)Delta(g)) generation was also obtained by chemical trapping of (18)O(2) ((1)Delta(g)) with anthracene-9,10-divinylsulfonate, using HPLC/MS/MS for detection of the corresponding 9,10-endoperoxide derivative. Our studies add insights into the molecular events underlying nitrosative, oxidative, and carbonyl stress in inflammatory processes and aging-associated maladies.