Mechanistic Aspects of Photodegradation of Deoxynucleosides Induced by Triplet State of Effluent Organic Matter

Excited triplet states of wastewater effluent organic matter ((3)EfOM*) are known as important photo-oxidants in the degradation of extracellular antibiotic resistance genes (eArGs) in sunlit waters. In this work, we further found that (3)EfOM* showed highly selective reactivity toward 2 '-deoxyguanosine (dG) sites within eArGs in irradiated EfOM solutions at pH 7.0, while it showed no photosensitizing capacity toward 2 '-deoxyadenosine, 2 '-deoxythymidine, and 2 '-deoxycytidine (the basic structures of eArGs). The (3)EfOM* contributed to the photooxidation of dG primarily via one-electron transfer mechanism, with second-order reaction rate constants of (1.58-1.74) x 10(8) M-1 s(-1), forming the oxidation intermediates of dG (dG(-H)(center dot)). The formed dG(-H)(center dot) could play a significant role in hole hopping and damage throughout eArGs. Using the four deoxynucleosides as probes, the upper limit for the reduction potential of (3)EfOM* is estimated to be between 1.47 and 1.94 V-NHE. Compared to EfOM, the role of the triplet state of terrestrially natural organic matter ((NOM)-N-3*) in dG photooxidation was minor (similar to 15%) mainly due to the rapid reverse reactions of dG(-H)(center dot) by the antioxidant moieties of NOM. This study advances our understanding of the difference in the photosensitizing capacity and electron donating capacity between NOM and EfOM and the photodegradation mechanism of eArGs induced by (3)EfOM*.