Properties of 2,6-diamino-8-azapurine, a highly fluorescent purine analog and its N-alkyl derivatives: Tautomerism and excited-state proton transfer reactions

Fluorescence emission properties of 2,6-diamino-8-azapurine (2-amino-8-azaadenine) in aqueous medium were examined. The title compound exhibits strong fluorescence centered at 365 nm, with quantum yield 0.40 and bi-exponential decay with lifetimes of ∼7.5 and ca. 0.2 ns. The spectral properties of 2,6-diamino-8-azaapurine are very similar to those of its 9-alkoxyphosphonate, suggesting that the protomer responsible for the emission of the former must be identified as N(9)H form. In acidic media (pH ∼ 2.5), fluorescence of the title compound is shifted to 410 nm, and is very similar to that of the neutral form of the N8-methyl derivative, the latter emitting at 410 nm with yield 0.85 and lifetime 12.7 ns. The N8-methyl derivative in the moderately acidic media undergoes rapid excited-state deprotonation, with pK* calculated to be as low as −2, compared to pKa 4.8 in the ground state, and resulting in the presence of the 410 nm fluorescence band well below the ground-state pKa. We therefore propose that the 410 nm band observed in the fluorescence of 2,6-diamino-8-azapurine cation is the consequence of (a) shift in tautomerism upon protonation from N(9)H toward N(8)H form and (b) rapid excited-state deprotonation of the latter. This conclusion is supported by the study of solvent, isotope and buffer concentration effects.