Photochemical Transformation of Emodin in Alcohols: Access to Physcion and (–)-Biemodin and their Bioactivity Profiles

Emodin, a natural anthraquinone possessing anticancer, anti-inflammatory, antioxidant, and antiviral activities, has recently garnered interest as a photosensitizer for photodynamic therapy (PDT). In this study, we investigated its solvent-dependent photochemical reactivity under UV–visible irradiation (320–800 nm) to access structurally diverse derivatives. Photoirradiation in methanol afforded physcion (1), a C-6 methoxylated analogue formed via phenoxyl radical-mediated O-methylation, whereas prolonged irradiation in ethanol yielded (–)-biemodin (2), a symmetric C-7–linked dimer. Structures were elucidated by 1H-NMR, HR-ESI-MS, and electronic circular dichroism (ECD) spectroscopy analysis. Biological activities were assessed in three assays including cytotoxicity, DPPH radical scavenging, and indoleamine 2,3-dioxygenase (IDO) inhibition. Notably, only emodin itself showed moderate cytotoxicity at 50 µM; neither physcion (1) nor (–)-biemodin (2) displayed significant activity in any assay. These findings illustrate that photochemical modification of emodin enhances structural diversity but does not necessarily improve biological efficacy. Moreover, the pronounced photoreactivity of emodin highlights the importance of light-protected storage to prevent unintended degradation or structural alteration.