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Experimental Evidence for Noncanonical Thymine Cation Radicals in the Gas Phase

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journal contribution
posted on 2017-12-08, 00:00 authored by Andy Dang, Huong T. H. Nguyen, Heather Ruiz, Elettra Piacentino, Victor Ryzhov, František Tureček
Thymine cation radicals were generated in the gas phase by collision-induced intramolecular electron transfer in [Cu­(2,2′:6,2″-terpyridine)­(thymine)]2+• complexes and characterized by ion–molecule reactions, UV–vis photodissociation action spectroscopy, and ab initio and density functional theory calculations. The experimental results indicated the formation of a tautomer mixture consisting chiefly (77%) of noncanonical tautomers with a C-7-H2 group. The canonical 2,4-dioxo-N-1,N-3-H isomer was formed as a minor component at ca. 23%. Ab initio CCSD­(T) calculations indicated that the canonical [thymine]+• ion was not the lowest-energy isomer. This contrasts with neutral thymine, for which the canonical isomer is the lowest-energy structure. Exothermic unimolecular isomerization by a methyl hydrogen migration in the canonical [thymine]+• ion required a low energy barrier, forming a C-7-H2,O-4-H isomer. Noncanonical thymine tautomers with a C-7-H2 group were also identified by calculations as low-energy isomers of 2′-deoxythymidine phosphate cation radicals. The relative energies of thymidine ion isomers were sensitive to the computational method used and were affected by solvation. The noncanonical [thymine]+• ions have extremely low adiabatic recombination energies (REadiab < 5.9 eV), making them potential ionization hole traps in ionized nucleic acids.

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