Light-induced full aromatization and hydroxylation of 7-methoxy-1-methyl-3,4-dihydro-2H-pyrido[3,4-b]indole alkaloid: Oxygen partial pressure as a key modulator of the photoproducts distribution

Full-aromatic and partially hydrogenated β-carboline (βC) derivatives constitute a group of alkaloids widely distributed in a great variety of living systems. In plants and bacteria, tetrahydro-βCs are the primary product of the Pictet-Spengler enzymatically catalyzed condensation. Tetrahydro-βC skeleton is further modified giving rise to the formation of a vast set of derivatives including dihydro- and full-aromatic βCs. However, in most of the cases, the later processes still remain unclear and other sources, such as photo-triggered reactions, deserve to be explored. In this context, the photophysic and photochemistry of 7-methoxy-1-methyl-3,4-dihydro-2H-pyrido[3,4-b]indole or harmaline (Hlina) in aqueous solution is reported herein. UV–visible absorption and fluorescence emission spectroscopy coupled with multivariate data analysis (PARAFAC), HPLC and HRESI-MS techniques were used for both quantitative and qualitative analysis. The formation singlet oxygen and hydrogen peroxide reactive oxygen species (ROS) was quantified and their role together with the influence of pH and oxygen partial pressure on the photochemical degradation of HlinaH+ was assessed. We report herein the first study on photochemical full-aromatization of a dihydro-βC derivative. These results can shed some light on the βCs biosynthesis and role in living systems. Upon photoexcitation harmaline alkaloid shows a distinctive photochemical behavior modulated by pH and oxygen partial pressure. [Display omitted] •Harmaline, a partially hydrogenated βC alkaloid, shows a quite distinctive photophysic and photochemical behaviour.•Photosensitized formation of reactive oxygen species by photoexcited harmaline is rather small or null.•Oxygen partial pressure and pH strongly modulate the photochemical properties of harmaline.•Under anaerobic conditions, photoexcited harmaline gives rise to the formation of the full-aromatic βC, harmine.•Light might promote additional pathways for the in vivo synthesis and/or photochemical modification of endogenous βCs.