Modeling deoxyribonucleic acid and ribonucleic acid damage in the gas phase

This short review outlines the tandem mass spectrometric methods for the generation and analysis of transient nucleobase radicals relevant to deoxyribonucleic acid and ribonucleic acid damage. Radical hydrogen atom adducts to uracil, adenine, cytosine and N-methylcytosine were generated by femtosecond electron transfer to the corresponding gas-phase cations in fast beams at 8 keV kinetic energy. Radical unimolecular dissociations were monitored by product analysis following collisional ionization to cations or anions using neutralization-reionization mass spectrometry. The radical energetics and dissociation kinetics were further analyzed by mapping the potential energy surfaces by high-level ab initio calculations in combination with Rice-Remsberger-Kassel-Marcus calculations of unimolecular rate constants. This first- principles-based approach allows one to model radical dissociations occurring from doublet ground electronic states of radical intermediates, assign reaction mechanisms and derive quantitative branching ratios for dissociation channels that are in agreement with experiments. Theoretical analysis also provides distinction between radical dissociations occurring on the ground and excited electronic state potential energy surfaces. Specific characterization of excited state dissociations of nucleobase and other polyatomic radicals remains a challenging topic for both experimentalists and computational chemists.