Mechanistic Study of the Peroxyoxalate System in Completely Aqueous Carbonate Buffer

Abstract

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.

Tuning the Intramolecular Chemiexcitation of Neutral Dioxetanones by Interaction with Ionic Species

“…The light-emission from both CL and BL arises from a two-step process [12][13][14]. There is the initial oxygenation of the reactant, with subsequent generation and almost immediate thermolysis of an energy-rich peroxide intermediate (Scheme 1).…”

Water Effect on Peroxyoxalate Kinetics and Mechanism for Oxalic Esters with Distinct Reactivities

“…The solvent effect might be a general medium polarity effect or a specific molecular water effect on the stabilization of radical ion pairs formed during the CIEEL sequence. The observed increase is surprising in a first instance, as it is known that the emission quantum yields of chemiluminescence transformations are generally much lower in aqueous media, as compared to anhydrous conditions (2,3,6,7,22,24–27,29). Therefore, this intriguing observation made in this work will be further investigated using photophysical approaches to rationalize the chemiexcitation quantum yields in the peroxyoxalate reaction of TCPO and DNPO in DME:H 2 O mixtures, in the conditions well established in the present work.…”

“…Using ACTs with distinct photophysical and electronic properties, it was also possible to confirm the involvement of the CIEEL mechanism (21) in the chemiexcitation step of this transformation (7). However, few mechanistic studies have been carried out involving the peroxyoxalate system in aqueous systems, even though it is an essential analytical chemiluminescent method (22–25).…”

“…One of the recent studies established experimental conditions in an aqueous phosphate‐buffered medium with reproducible reaction kinetics; depending on the medium's pH, it was possible to verify the occurrence of general acid or general base catalysis by phosphate ions (25). Mechanistic studies in aqueous borate and carbonate buffer media demonstrated that the reaction proceeds by specific basic catalysis (SBC) (22,26). Kinetic studies, designed to distinguish between nucleophilic, specific base, and general base catalysis, in the reaction of TCPO with H 2 O 2 using 2,6‐lutidine (2,6‐dimethylpyridine) as catalyst, indicated the role of 2,6‐lutidine as general basic catalyst in both organic and aqueous medium (24).…”

“…Kinetic studies, designed to distinguish between nucleophilic, specific base, and general base catalysis, in the reaction of TCPO with H 2 O 2 using 2,6‐lutidine (2,6‐dimethylpyridine) as catalyst, indicated the role of 2,6‐lutidine as general basic catalyst in both organic and aqueous medium (24). All these studies, performed in aqueous organic mixtures or entirely aqueous medium, revealed that the chemiluminescence emission efficiency of the peroxyoxalate reaction decreases drastically in aqueous media, as compared to anhydrous organic medium (6,7,22,26–29).…”