Mechanistic Study of the Peroxyoxalate System in Completely Aqueous Carbonate Buffer

The peroxyoxalate reaction is one of the most efficient chemiluminescence transformations, with emission quantum yields of up to 50%; additionally, it is widely utilized in analytical and bioanalytical assays. Although the real reason for its extremely high efficiency is still not yet understood, the mechanism of this transformation has been well elucidated in anhydrous medium. Contrarily, only few mechanistic studies have been performed in aqueous media, which would be of great importance for its application in biological systems. We report here our experimental results of the peroxyoxalate reaction in completely aqueous carbonate buffer, using fluorescein as chemiluminescence activator. The kinetics are very fast in the used basic conditions (pH > 9); despite this, reproducible kinetic results were obtained. The reaction proceeds by specific base catalysis, with rate-limiting attack of hydrogen peroxide anion to the oxalic ester, in competition with ester hydrolysis by hydroxide ion. Emission quantum yields increase with the hydrogen peroxide concentration up to an optimal concentration of 10 mmol L^-1 . The infinite singlet quantum yield of (5.8 ± 0.2) × 10^-7 is much lower than in anhydrous medium; however, it is similar to quantum yields measured before in partially aqueous media.