Acid–base equilibria in aqueous solutions of 2-aminopurine radical cations generated by two-photon photoionization

Abstract

The acid–base equilibria of the nucleic acid analogue 2-aminopurine (2AP) and 2-aminopurine riboside (2-APr) radicals generated by two-photon photoionization of the 2AP residues with intense 308 nm XeCl excimer laser pulses (fwhm = 12 ns, ca. 70 mJ cm⁻² pulse⁻¹) have been investigated using transient absorption spectroscopy techniques. It is of interest to investigate the properties of these radicals as a function of pH because of their abilities to undergo proton-coupled electron transfer reactions with other nucleic acid bases. Pronounced differences in the 2AP and 2APr radical absorption bands are observed in the 330–550 nm spectral region within the pH range of 1 to 12. These differences are attributed to the deprotonation of the 2AP and 2APr radical cations, 2AP^·+(pK_a1^r = 2.8 ± 0.2), and 2APr^·+(pK_a1^r = 2.7 ± 0.2), with the concomitant formation of the neutral radicals, 2AP(−H)^· and 2APr(−H)^·. At higher pH values, the 2AP(−H)^· radical (pK_a2^r = 9.5 ± 0.2) loses a second proton at the N⁹ position to form the radical anion 2AP(−2H)^·−; this deprotonation reaction does not occur in the case of the 2APr(−H)^· radical because of the ribofuranosyl substitution at the N⁹ position.