Charge transfer from 2-aminopurine radical cation and radical anion to nucleobases: a pulse radiolysis study

Abstract

Pulse radiolysis study has been carried out to investigate the properties of the radical cation of 2-aminopurine (2AP) and the probable charge transfer from the radical cation and radical anion of 2AP to natural nucleobases in aqueous medium. The radical cation of 2AP was produced by the reaction of sulfate radical anion (SO₄^•-). The time resolved absorption spectra obtained by the reaction of SO₄^•- with 2AP at neutral pH have two distinct maxima at 380 and 470 nm and is assigned to the formation of a neutral radical of the form 2AP-N²(-H)^• (k₂ = 4.7 × 10⁹ dm³ mol^-1 s^-1 at pH 7). This neutral radical is formed from the deprotonation reaction of a very short-lived radical cation of 2AP. The transient absorption spectra recorded at pH 10.2 have two distinct maxima at 400 and 480 nm and is assigned to the formation of a nitrogen centered radical (2AP-N(9)^•). As the hole transport from 2AP to guanine is a highly probable process, the reaction of SO₄^•- is carried out in the presence of guanosine, adenosine and inosine. The spectrum obtained in the presence of guanosine was significantly different from that in the absence and it showed prominent absorption maxima at 380 and 470 nm, and a weak broad maximum centered around 625 nm which match well with the reported spectrum of a neutral guanine radical (G(-H)^•). The electron transfer reaction from the radical anion of 2AP to thymine (T), cytidine (Cyd) and uridine (Urd) was also investigated at neutral pH. Among the three pyrimidines, only the transient spectrum in the presence of T gave a significant difference from the spectral features of the electron adduct of 2AP, which showed a prominent absorption maximum at 340 nm and this spectrum is similar to the electron adduct spectrum of T. The preferential reduction of thymine by 2AP^•- and the oxidation of guanosine by 2AP^•+ clearly follow the oxidation/reduction potentials of the purines and pyrimidines.