Tautomer contribution's to the near UV spectrum of guanine: towards a refined picture for the spectroscopy of purine molecules

By using depopulation laser techniques, like IR-UV population labeling coupled to mass-selected R2PI detection, we confirm that four tautomers are responsible for the near UV spectroscopy (310-280 nm) of guanine: two enol and two keto forms, each pair having a 7NH and a 9NH form. Besides the UV spectroscopy of each tautomer, additional information on the excited state nature and dynamics is obtained from fluorescence stiidies. In particular, the quenching of fluorescence beyond 285 nm, the existence of a background absorption, as well as the existence of a strongly red-shifted component in the fluorescence emission provides evidence for a strong electronic mixing in the excited state together with an efficient non-radiative process. The details of these features are found to be tautomer-dependent. Comparison of the present results with literature data on other purine molecules, like adenine or 9-substituted guanines, enables us to propose a new insight on the spectroscopy and dynamics of the purine molecules. First, a large variability of the tautomer distribution in the gas phase is found within the purine family, in particular a molecular change, as simple as a 9-methylation on guanine, can reduce the tautomer distribution to a single species (enol form). Since the absorption spectrum is tautomer-dependent as well as substituent-dependent, it turns-out that the tautomer population is one of the major parameters that control the overall shape of the UV spectrum. Second, the excited state model, often evoked in the literature, which involves electronic coupling between excited states of different natures, namely pipi* and npi* states, might account for the present fluorescence measurements on guanine, providing an extensive excited state electronic mixing is assumed for these systems.