Cation Radicals of Hachimoji Nucleobases. Canonical Purine and Noncanonical Pyrimidine Forms Generated in the Gas Phase and Characterized by UV-Vis Photodissociation Action Spectroscopy.

Oxidation of nontraditional nucleobases 1-methylcytosine (hachimoji base ) and isoguanine (hachimoji base ) in gas-phase ternary complexes with Cu(terpyridine) formed cation radicals that were characterized by tandem mass spectrometry, UV-vis photodissociation action spectroscopy in the 210-700 nm region, and calculations up to the CCSD(T)/complete basis set level of theory. Oxidation of was accompanied by exothermic isomerization in the 1-methylcytosine ion (), forming 1-methylene-2-hydroxy-4-aminopyrimidine cation radical () as a noncanonical distonic isomer of the nucleobase. Ion was characterized by deuterium exchange experiments and provided a matching UV-vis action spectrum with the vibronic absorption spectrum from time-dependent density functional theory calculations. Oxidation of resulted in the formation of a canonical isoguanine cation radical () as judged from the match of the experimental action spectrum with the calculated vibronic absorption spectrum. The calculated adiabatic ionization energies of canonical and , 8.51 and 7.76 eV, respectively, indicated exothermic electron transfer from to to proceed in an ionized base pair. Contrasting this, the lowest energy tautomer of ionized () had a low adiabatic recombination energy, RE = 5.70 eV, that would prevent it from oxidizing other nucleobases. Recombination energies of several nucleobase tautomers are reported and discussed.