Theoretical Investigation of the Photochemistry and Phototoxicity of Aloe-Emodin as a Natural Sensitizing Dye in Photodynamic Therapy

The photosensitizing properties of aloe-emodin, linked to its anticancer and antimicrobial effects in photodynamic therapy, were investigated under physiological conditions using computational chemistry tools. Neutral and monoanionic species of aloe-emodin were found to coexist in a ratio of approximately 1:3. Dissociation leads to a rearrangement of molecular orbitals, resulting in a redshift of the absorption spectrum. Excited-state dynamics analysis revealed an average triplet state quantum yield of 0.76, with triplet lifetimes of 78.3 ns for the neutral species and 2.08 ns for the anionic species. Both species exhibit comparable propensities for type I photoreactivity, although the neutral species more readily oxidizes biomolecules during type III photoreactivity. Additionally, the neutral form effectively intercalates DNA, preferentially at the AT–TA site, altering the absorption profile and inducing eventually cytotoxic structural rearrangements to the nucleotides.