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

The acid-base equilibria of the nucleic acid analogue 2-aminopurine (2AP) a nd 2-aminopurine riboside (2-APr) radicals generated by two-photon photoion ization 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 tran sient absorption spectroscopy techniques. It is of interest to investigate the properties of these radicals as a function of pH because of their abili ties to undergo proton-coupled electron transfer reactions with other nucle ic acid bases. Pronounced differences in the 2AP and 2APr radical absorptio n 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 2A P 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 rad icals, 2AP(-H)(.) and 2APr(-H)(.) radical (pK(a2)(r) = 9.5 +/- 0.2) loses a second proton at the N-9 position to form the radical anion 2AP(-2H)(.-); this deprotonation reaction does not occur in the case of the 2APr(-H)(.) r adical because of the ribofuranosyl substitution at the N-9 position.