Setting the Bases of the Photogenotoxicity of p-Aminobenzoic Acid.

Despite its wide use in the past, the UVB filter p-aminobenzoic acid (PABA) is currently considered unsafe in the cosmetic industry. Among other reasons, there is the claimed formation of photoadducts with the nucleobases in DNA. We provide theoretical evidence showing the spontaneous intercalation of PABA in a (dAdT)6·(dAdT)6 strand. The π-π stacking interactions between PABA and the nucleobases result in an effective coupling of the native excited states of both molecules, altering the optical properties of DNA. Transition density matrix analysis shows that although the absorption spectrum of the DNA-PABA complex is dominated by excitons, the dark charge-separated states, which have been associated with photodamaging radical reactions in the literature, are overall more numerous in the entire absorption energy range. We show that PABA → DNA charge transfer states are more abundant, more energetically accessible, and potentially longer-lived than those across nucleobases, hence explaining the radicallic origin of the photoadducts between the filter and DNA.