Electronic spectra of adenine: interaction with dissolved oxygen in solution, oscillation and intensification of n-π* transition

The electronic absorption and fluorescence spectra of adenine were studied in aqueous solution at two different pH values, and in the presence and absence of oxygen dissolved in the solution as a function of time. Because of interaction with the dissolved oxygen in the solution, adenine shows a slow oscillation similar to that observed previously for guanine; eventually, the spectrum is significantly modified and a strong absorption band appears near 305 nm which has been interpreted as a n-π* transition in adenine. It has been concluded that, like guanine, adenine has asymmetric double-well potential surfaces in its ground and excited states, but the double-well nature of the potential surfaces is not as prominent as it is in guanine. It has been shown that the rare (protonated) tautomer of adenine, the presence of which at low pH has been suggested previously, fluoresces near 370 nm, while the normal species, under interaction with oxygen, fluoresces near 345 nm from the singlet n-π* excited state.