The oxidation of guanine by photoionized 2‑aminopurine

Bull, G. and Thompson, Katherine (2021) The oxidation of guanine by photoionized 2‑aminopurine. Journal of Photochemistry and Photobiology 6 (100025), ISSN 2666-4690.

Abstract

2-Aminopurine (2AP) is a fluorescent nucleobase analogue of the DNA nucleobase adenine and can be inserted in place of adenine into nucleic acids with minimal structural perturbation. The inserted 2-aminopurine can be selectively excited, and the fluorescence observed used as a sensitive probe for local structure, in addition selective ionization of 2‑aminopurine within a nucleic acid is possible and once formed the ionized 2‑aminopurine will oxidize nearby guanine bases, providing a route to study charge transfer within nucleic acids. Time resolved infrared spectroscopy is a powerful tool to study oxidation processes on picosecond, and longer timescales. There is a lack of availability of high quality IR spectra in the literature of the individual, short lived, photoproducts, required to fully elucidate the mechanism of the interaction of photoionized 2-aminopurine with guanine. Density functional theory (DFT) methods (B3LYP/6–31G+(d) and EDF1/6–31G+(d)) have been used to calculate the energies and infrared spectra of 2‑aminopurine, guanine and potential photoproducts that may result from photoionization reactions between 2-aminopurine and guanine. Direct comparison can therefore be made between areas of bleaching of bands in the IR spectra of 2‑aminopurine or guanine and the appearance of new transient bands of potential photoproducts. These potential photoproducts could include deprotonated neutral radical species, deprotonated anion species, radical cations and radical anions. It was found in the current work that the key intermediates in the oxidation reaction between photoionized 2-aminopurine and guanine were: 2AP•(−H,N2) (formed from deprotonation from the 2AP●+ amino group); G•(−H) (formed from deprotonation from the N1 nitrogen, or possibly the amino group, of the initially formed G●+), and 2AP−(−H,N2) (formed from the reduction of 2AP•(−H,N2) by guanine).

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