Chemiluminescence of Cypridina luciferin analogues. Part 1. Effect of pH on rates of spontaneous autoxidation of CLA in aqueous buffer solutions

Abstract

From the pH–rate profile of autoxidation of the Cypridina luciferin analogue {2-methyl-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one, CLA}, CLA was found not to react with ground-state molecular oxygen at a measurable rate, while the conjugate base of CLA was found to react with molecular oxygen with a second-order rate constant of 0.434 ± 0.001 dm³ mol^–1 s^–1 at 25 °C. No chain reaction process contributed in the autoxidation of CLA. Superoxide dismutase (SOD) was observed to reduce the rate of the autoxidation to a small extent in the reaction with low-pH buffers, while SOD did not affect the reaction in buffers of pH higher than 7. The intimate radical pair (CLA˙O₂˙^–) is proposed to be the primary product of the reaction.