Electronic spectra of 2-aminopurine and 2,6-diaminopurine: phototautomerism and fluorescence reabsorption

Ultraviolet absorption, fluorescence and excitation spectra of 2-aminopurine (2AP) and 2,6-diaminopurine (2,6DAP) have been studied using aqueous solutions at different pH values. The molecules are found to fluoresce most efficiently near the neutral pH. It is also found that the fluorescence caused by the solutions of these compounds is partly reabsorbed. Molecular orbital calculations on 2AP, its tautomers, anion and cation, and excited state life-time measurements corresponding to the emissions of 2AP and 2,6DAP near 370 and 342 nm, respectively, have been carried out, and have been found to be useful in the interpretation of the experimental observations.