Uric Acid: A Less-than-Perfect Probe for Singlet Oxygen.

Uric acid and/or its monoanion has long been used as chemical-trapping agents to demonstrate the presence of singlet oxygen, O2 (a1 Δg ), in aqueous systems. "Oxidative bleaching" of uric acid, generally monitored through changes in the uric acid absorption spectrum, is often used in support of claims for the intermediacy of O2 (a1 Δg ). The bleaching of uric acid has also been used to quantify photosensitized O2 (a1 Δg ) yields in selected systems. Unfortunately, experiments performed to these ends often neglect processes and phenomena that can influence the results obtained. For the present study, we experimentally examined the behavior of uric acid under a variety of conditions relevant to the photoinitiated creation and subsequent removal of O2 (a1 Δg ). Although the oxidative destruction of uric acid can indeed be a useful tool in some cases, we conclude that caution must be exercised such as not to incorrectly interpret the data obtained.