Oxygen Dependent Purine Lesions in Double-Stranded Oligodeoxynucleotides: Kinetic and Computational Studies Highlight the Mechanism for 5',8-Cyplopurine Formation

The reaction of HOo radical with DNA is intensively studied both mechanistically and analytically for lesions formation.Several aspects related to the reaction paths of purine moieties with the formation of 5?,8-cyclopurines (cPu), 8-oxopurines(8-oxo-Pu) and their relationship are not well understood. In this study, we investigated the reaction of HOo radical with a 21-merdouble-stranded oligodeoxynucleotide (ds-ODNs) in ?-irradiated aqueous solutions under various oxygen concentrations and accuratelyquantified the six purine lesions (i.e., four cPu and two 8-oxo-Pu) by LC-MS/MS analysis using isotopomeric internal standards.In the absence of oxygen 8-oxo-Pu lesions are only ~4 times more than cPu lesions. By increasing oxygen concentration, the8-oxo-Pu and the cPu gradually increase and decrease, respectively, reaching a gap of ~130 times at 2.01×10-4 M of O2. Kinetictreatment of the data allows to estimate the C5? radical competition between cyclization and oxygen trapping in ds-ODNs, and lastlythe rate constants of the four cyclization steps. Tailored computational studies by means of dispersion-corrected DFT calculationswere performed on the CGC and TAT in their double-strand models for each cPu diastereoisomer along with the completereaction pathways of the cyclization steps. Our findings reveal unheralded reaction mechanisms that resolve the long-standing issueswith C5? radical cyclization in purine moieties of DNA sequences.