The present work reports a combined experimental and theoretical study of the 9-methylguanine (9MG) nucleobase and its monohydrate in the gas phase. Laser spectroscopy provides optical evidence for the presence of only one tautomer of 9MG, among the three forms expected from an energetic basis. Comparison with the calculated vibrational frequencies enables us to assign with confidence this form to the anti rotamer of the 9MG enol tautomer. The same approach allows us to conclude that the monomer conformation is retained in the monohydrate, with the water molecule bridging the OH proton donor and the N7 acceptor sites of 9MG. A complete comparison between all IR absorption data known for the enol tautomers of guanine and related compounds in the gas phase and the present calculations enables us to propose a consistent assignment for all of these enol tautomers in terms of rotamers. Finally, the fact that one does not observe either the most stable (and biologically relevant) keto form or the other enol rotamer (very close in energy) raises the question of the efficiency of UV spectroscopy to detect properly these missing species.
