Chemiluminescence of Cypridina luciferin analogs.: Part 3.: MCLA chemiluminescence with singlet oxygen generated by the retro-Diels-Alder reaction of a naphthalene endoperoxide

The chemiluminescence of the Cypridina luciferin analog, 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazo[1,2-a]pyrazin-3-one (MCLA), with O-2 ((1)Delta(g)) generated by the retro-Diels-Alder reaction of 3-(4'-methyl-1'-naphthyl)-propionic acid endoperoxide was studied in an aqueous solution with pH 7.12 at 37 degrees C. The retro-Diels-Alder reaction occurs with a first-order rate constant laf (4.16 +/- 0.13) x 10(-4)/s to quantitatively yield O-2 ((1)Delta(g)) and 3-(4'-methyl-1'-naphthyl)-propionic acid. MCLA consumed equimolar amounts of O-2 ((1)Delta(g)) with a second-order rate constant (6.96 +/- 0.27) x 10(8)/M/s to emit light in an aqueous solution with pH 7.12 at 37 degrees C, The chemiluminescence spectrum was identified as the fluorescence spectrum of 2-acetylamino-5-(p-methoxyphenyl)pyrazine (OMCLA), a major chemiluminescence reaction. Chemiluminescence spectra and product yields for MCLA reactions with O-2 ((1)Delta(g)), with O-2 ((3)Sigma(g)(-)) and with superoxide anion radicals are identical, suggesting that all of these reactions occur via a common MCLA-2-hydroperoxide intermediate formed by a combination of MCLA radicals and superoxide anion radicals, We have established practical use of NEPO as an O-2 ((1)Delta(g)) source and MCLA as a biological probe for detecting O-2 ((1)Delta(g)).