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

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.