MODELING NUCLEOBASE RADICALS IN THE MASS-SPECTROMETER

Neutralization-reionization mass spectrometry, complemented by ab init io and density-functional theory calculations, provides a powerful too l for the investigation of polyatomic radicals relevant to transient i ntermediates in the process of radiation or chemically induced DNA dam age. Precursor ions for analogues of DNA radicals are prepared by gas- phase protonation of nucleobases or their heterocyclic models. The pro ton affinity of the gas-phase acid is tuned to the topical proton affi nity in the substrate molecule to achieve selective protonation. Femto second electron transfer imprints the structure of the precursor ion o n to that of the radical. Deuterium labeling, collisional activation a nd variable-time measurements on a microsecond time-scale are used to study unimolecular dissociations of transient heterocyclic radicals an d distinguish them from ion dissociations. Ab initio and density-funct ional theory calculations provide energies which are not available fro m neutralization-reionization experiments. Topical proton affinities, Franck-Condon energies pertinent to vertical electron transfer, radica l stabilities and isomerization barriers are used to elucidate the str uctures and dissociations of isomeric heterocyclic radicals. (C) 1998 John Wiley & Sons, Ltd.