UV-light-driven photooxidation of harmaline catalyzed by riboflavin: Product characterization and mechanisms

β-Carboline alkaloid harmaline (HA) is a candidate drug molecule that has been proven to have broad and significant biological activity. Herein, the effects of HA on the riboflavin (RF)-sensitized photooxidation under aerobic conditions were studied for the first time. The photooxidation reaction of HA catalyzed by RF is triggered by UV light at 365 nm and shows a time-dependent stepwise reaction process. Seven transformed products, including five undescribed compounds, oxoharmalines A-E (1-4 and 7), and two known compounds, N-(2-(6-Methoxy-2-oxoindolin-3-yl)ethyl)acetamide (5) and harmine (6), were isolated and identified from the reaction system, following as the gradual oxidation mechanisms. The rare polymerization and dehydrogenation processes in radical-mediated photocatalytic reactions were involved in the process. The transformed products 2-7 exhibited significant neuroprotective activity in a model of H₂O₂-introduced injury in SH-SY5Y cells, which suggested that the products of the interaction between HA and vitamins may be beneficial to health.