Guanine Tautomerism in Ionic Complexes with Ag⁺ Investigated by IRMPD Spectroscopy and Mass Spectrometry

In this paper, we present the IRMPD spectra of three ionic complexes between guanine (G) and silver (Ag⁺): [GAg-H₂O]⁺, [GAgG]⁺ produced in the electrospray ionization source of the mass spectrometer, and [GAg]⁺ produced by collision induced dissociation of the [GAgG]⁺ complex. On the basis of the comparison of theoretically calculated IR spectra, we show that there are two isomers of each complex containing two different keto-amino (KA) tautomers of G (GKA(1,9) and GKA(1,7)). The observed isomers are the most stable structures in aqueous solution, and their experimentally estimated relative populations are in better agreement with the calculated relative populations in solution than in the gas phase, both at 298 K. We concluded that these observations suggest that GKA(1,9) and GKA(1,7) coexist in solution according to previous theoretical reports (Colominas, C.; et al. J. Am. Chem. Soc. 1996, 118, 6811). We were unable to find any evidence of the presence of the GEA(9), GKA(3,7), GKA(3,9), or GKA(7,9), whose relative stabilities in solution are strongly dependent on the theoretical method used to account for the solvent effect (Hanus, M.; et al. J. Am. Chem. Soc. 2003, 125, 7678).