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Fluorescence Emission Properties of the Cation of 4-Aminopyrazole(3,4-d) pyrimidine, an Adenine Analogue: Evidence for Phototautomerism

A study has been made of the emission spectra at room temperature, in aqueous and alcoholic media, of 4-aminopyrazolo(3,4-d)pyrimidine (APP) and some of its methylated derivatives. The cationic forms APPH+, N2-methyl-APPH+ and N7-methyl-APPH+ exhibit intense fluorescence under these conditions, the first two exhibiting excitation spectra which differ from the absorption spectra, pointing to the existence of a tautomeric equilibrium in the ground state. From the shape of the excitation spectra, and comparisons with methylated analogues in fixed tautomeric forms, it follows that the emission of APPH+ originates exclusively from the species N(2)-H,N(7)-H+, the other forms being non-fluorescent. The proportion of the emitting species, calculated from the excitation wavelength dependence of the quantum yield, is in good agreement with data for the ground state. The emission spectrum of APPH+ in aqueous medium consists of two bands with λmax 360 nm and 430 nm, which exhibit identical excitation spectra, but are quenched to different extents by H3O+. The 430 nm emission band is absent in alcoholic media. A similar behaviour is exhibited by N7-methyl-APPH+, whereas the neutral form of this analogue exhibits only the 430 nm band. These results indicate that the long wavelength emission band of APPH+ originates from the rare tautomeric species N(7)-H formed in the excited state by photodissociation of the N(2)-H proton from the species N(2)-H,N(7)-H+. This is further confirmed by results obtained with the aid of the basicity method, as well as by salt efects in non-aqueous media. Consideration is given to the possibility of such processes occurring in other analogues of nucleic acid derivatives.