Cation Radicals of Hachimoji Nucleobases P and Z.  Generation in the Gas Phase and Characterization by UV-Vis Photodissociation Action Spectroscopy and Theory

We reported the generation of gas-phase cation radicals of unusual nucleobases 5-aza-7-deazaguanine (P) and 6-amino-5-nitro-(1H)pyrid-2-one (Z) that have been used as building blocks of base-expanded (hachimoji) DNA. The cation radicals were generated by collision-induced intramolecular electron transfer and dissociation of ternary copper-terpyridine complexes. The cation radicals were characterized by deuterium labeling and tandem mass spectrometry including MS³ collision-induced dissociation, UV-Vis photodissociation, and action spectroscopy.  Vibronic absorption UV-Vis spectra were calculated by time-dependent density functional theory (TD-DFT) and compared with the action spectra to unequivocally assign the most closely matching structures for the gas-phase cation radicals. Ab initio calculations up to the coupled clusters-complete basis set (CCSD(T)/CBS) level of theory were used to rank by energy the P and Z neutral molecules and cation-radical isomers, and provided transition-state and dissociation energies. The 5-aza-7-deazaguanine cation radicals were determined to have the canonical N-1-H, 6-oxo structure (P1^+●) that was the global energy minimum within this group of isomers. The Z cation radicals were found to have the 1H-pyrid-2-one structure (Z1^+●). The formation of P1^+● and Z1^+● was shown to be controlled by the solution thermodynamics of the Cu-terpyridine complexes and the kinetics of their dissociations.  We also report and compare CCSD(T)/CBS-calculated adiabatic recombination energies of cation radicals for the entire hachimoji set of eight nucleobases, P^+● (7.92 eV), Z^+● (8.51 eV), S^+● (8.51 eV), B^+● (7.76 eV), T^+● (8.98 eV), C^+● (8.62 eV), A^+● (8.32 eV),  and G^+● (7.97 eV), to assess the thermodynamics of base-to-base electron transfer  following random ionization.