Unraveling the Degradation Mechanism of Purine Nucleotides Photosensitized by Pterins: The Role of Charge-Transfer Steps

Photosensitized reactions contribute to the development of skin cancer and are used in many applications. Photosensitizers can act through different mechanisms. It is currently accepted that if the photosensitizer generates singlet molecular oxygen (O-1(2)) upon irradiation, the target molecule can undergo oxidation by this reactive oxygen species and the reaction needs dissolved O-2 to proceed, therefore the reaction is classified as O-1(2)-mediated oxidation (type II mechanism). However, this assumption is not always correct, and as an example, a study on the degradation of 2'-deoxyguanosine 5'-monophosphate photosensitized by pterin is presented. A general mechanism is proposed to explain how the degradation of biological targets, such as nucleotides, photosensitized by pterins, naturally occurring O-1(2) photosensitizers, takes place through an electron-transfer-initiated process (type I mechanism), whereas the contribution of the O-1(2)-mediated oxidation is almost negligible.