We report the generation and spectroscopic study of hydrogen-rich DNA tetranucleotide cation radicals (GATC+2H)(+center dot) and (AGTC+2H)(+center dot). The radicals were generated in the gas phase by one-electron reduction of the respective dications (GATC +2H)(2+) and (AGTC+2H)(2+) and characterized by collision-induced dissociation and photodissociation tandem mass spectrometry and UV-vis photodissociation action spectroscopy. Among several absorption bands observed for (GATC+2H)(+center dot), the bands at 340 and 450 nm were assigned to radical chromophores. Time-dependent density functional theory calculations including vibronic transitions in the visible region of the spectrum were used to provide theoretical absorption spectra of several low-energy tetranucleotide tautomers having cytosine-, adenine-, and thy- mine-based radical chromophores that did not match the experimental spectrum. Instead, the calculations indicated the formation of a new isomer with the 7,8-H-dihydroguanine cation radical moiety. The isomerization involved hydrogen migration from the cytosine N-3-H radical to the C-8 position in N-7-protonated guanine that was calculated to be 87 kJ mol(-1) exothermic and had a low-energy transition state. Although the hydrogen migration was facilitated by the spatial proximity of the guanine and cytosine bases in the low-energy (GATC+2H)(+center dot) intermediate formed by electron transfer, the reaction was calculated to have a large negative activation entropy. Rice-Ramsperger-Kassel-Marcus (RRKM) and transition state theory kinetic analysis indicated that the isomerization occurred rapidly in hot cation radicals produced by electron transfer with the population-weighed rate constant of k = 8.9 x 10(3) s(-1). The isomerization was calculated to be too slow to proceed on the experimental time scale in thermal cation radicals at 310 K.