Gas phase structure and fragmentation of [Cytosine-Guanine]

The silver-mediated cytosine-guanine [CGAg]$$^{+}$$ pair was characterized by mass-resolved IRMPD spectroscopy and electronic structure calculations at the DFT level. Three fragments were observed: [GAg]$$^{+}$$, [CAg]$$^{+}$$ and [CH]$$^{+}$$. From the mass-resolved IRMPD spectra recorded on the mass of each fragment, two isomers of the [CGAg]$$^{+}$$ complex were identified. The most populated isomer is a Hoogsteen structure [HooAg]$$^{+}$$ between canonical cytosine (CKA(1)) and canonical guanine (GKA(9)), while the second isomer is an altered-Hoogsteen-like pair [*HooAg]$$^{+}$$ in which guanine is in a non-canonical form (GKA(7)). The determined yields of each fragment are strongly dependent of the isomer of the precursor ion because of the fragmentation energy of the different channels. Finally, the [CH]$$^{+}$$ is suggested to be produced by a metal-assisted proton transfer from guanine to cytosine preceded by an isomerization of the [HooAg]$$^{+}$$ isomer