UV-Vis Action Spectroscopy of Guanine, 9-Methylguanine, and Guanosine Cation Radicals in the Gas Phase.

Abstract

Cation radicals of guanine (G•+), 9-methylguanine (MG•+), and guanosine (rG•+) were generated by dissociative oxidation of gas-phase copper complexes and characterized by UV-vis photodissociation action spectra and ab initio calculations. Comparison of the action spectra of G•+ with the calculated vibronic absorption spectra of several cation radical tautomers showed the best match for the canonical 6-oxo-N-9-H structure (G1•+). The formation of G1•+ was favored by the stability of its precursor CuII ion complexes in solution and the gas phase. G1•+ was the marginally lowest-energy guanine tautomer according to CCSD(T) calculations extrapolated to the complete basis set limit (CBS). A canonical 6-oxo structure (MG1•+) was also assigned to the 9-methylguanine cation radical on the basis of a match between the action spectrum and the calculated vibronic absorption spectra. MG1•+ was calculated by CCSD(T)/CBS to be marginally less stable than the 6-OH enol tautormer, but its formation was favored by the superior stability of its precursor CuII ion complexes in solution and the gas phase. Action spectroscopy allowed us to assign the canonical 6-oxo structure (rG1•+) to the gas-phase guanosine cation radicals that were formed as the lowest-energy tautomers. The absorption bands in the action spectra were assigned on the basis of time-dependent density functional theory calculations that were benchmarked on equation-of-motion coupled cluster calculations of G•+.