On the photochemistry of purine nucleobases

We present R2PI spectra of a series of purine nucleobases, nucleosides, and related derivatives, including adenine, guanine, guanosine, 2-aminopurine, 2,6-diaminopurine, 9-ethyladenine, and 9-methylguanine. We compare the results with models involving two interacting excited states, which have been proposed to explain the short excited-state lifetime of most nucleobases. The amino group in the 2 position appears to have a strong effect on the excited-state potentials, as does any substitution in the 9 position. This explains the very different fluorescence characteristics of 2-aminopurine, but it raises new questions concerning the short lifetime of guanine in solution.