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Photochemistry of the organoselenium compound ebselen: direct photolysis and reaction with active intermediates of conventional reactive species sensitizers and quenchers.

Ebselen (EBS), 2-phenyl-1,2-benzisoselenazol-3(2H)-one, is an organoselenium pharmaceutical with antioxidant and anti-inflammatory properties. Furthermore, EBS is an excellent scavenger of reactive oxygen species. This property complicates conventional protocols for sensitizing and quenching reactive species due to potential generation of active intermediates that quickly react with EBS. In this study, the photochemical reactivity of EBS was investigated in the presence of (1) 1O2 and •OH sensitizers (Rose Bengal (RB), perinaphthanone, H2O2) and (2) reactive species scavenging and quenching agents (sorbic acid, isopropanol, sodium azide, tert-butanol) that are commonly employed to study photodegradation mechanisms and kinetics. The carbon analog of EBS, namely 2-phenyl-3H-isoindol-1-one, was included as a reference compound to confirm the impact of the selenium atom on EBS photochemical reactivity. EBS does not undergo acid dissociation, but pH-dependent kinetics were observed in RB-sensitized solutions, suggesting EBS reaction with active intermediates (3RB2-*, O2⦁-, H2O2) that are not kinetically-relevant for other compounds. In addition, the observed rate constant of EBS increased in the presence of sorbic acid, isopropanol, and sodium azide. These findings suggest that conventional reactive species sensitizers, scavengers, and quenchers need to be carefully applied to highly reactive organoselenium compounds to account for reactions that are typically slow for other organic contaminants.