We report the generation of deoxyriboadenosine dinucleotide cation radicals by gas-phase electron transfer to dinucleotide dications and their noncovalent complexes with crown ether ligands. Stable dinucleotide cation radicals of a novel hydrogen-rich type were generated and characterized by tandem mass spectrometry and UV-vis photodissociation (UVPD) action spectroscopy. Electron structure theory analysis indicated that upon electron attachment the dinucleotide dications underwent a conformational collapse followed by intramolecular proton migrations between the nucleobases to give species whose calculated UV-vis absorption spectra matched the UVPD action spectra. Hydrogen-rich cation radicals generated from chimeric riboadenosine 5'-diesters gave UVPD action spectra that pointed to novel zwitterionic structures consisting of aromatic π-electron anion radicals intercalated between stacked positively charged adenine rings. Analogies with DNA ionization are discussed.

Thymine cation radicals were generated in the gas phase by collision-induced intramolecular electron transfer in [Cu(2,2':6,2″-terpyridine)(thymine)] 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-H 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-H,O-4-H isomer. Noncanonical thymine tautomers with a C-7-H 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 (RE < 5.9 eV), making them potential ionization hole traps in ionized nucleic acids.

The radical cation of cytosine (Cyt ) is generated by dissociative oxidation from a ternary Cu complex in the gas phase. The radical cation is characterized by infrared multiple photon dissociation (IRMPD) spectroscopy in the fingerprint region, UV/Vis photodissociation (UVPD) spectroscopy, ion-molecule reactions, and theoretical calculations (density functional theory and ab initio). The experimental IRMPD spectrum features diagnostic bands for two enol-amino and two keto-amino tautomers of Cyt that are calculated to be among the lowest energy isomers, in agreement with a previous study. Although the UVPD action spectrum can also be matched to a combination of the four lowest energy tautomers, the presence of a nonclassical distonic radical cation cannot be ruled out. Its formation is, however, unlikely due to the high energy of this isomer and the respective ternary Cu complex. Gas-phase ion-molecule reactions showed that Cyt undergoes hydrogen-atom abstraction from 1-propanethiol, radical recombination reactions with nitric oxide, and electron transfer from dimethyl disulfide.

Cation radicals of adenine (A•+) and 9-methyladenine (MA•+) were generated in the gas phase by collision-induced intramolecular electron transfer in copper–terpyridine–nucleobase ternary complexes and characterized by collision-induced dissociation (CID) mass spectra and UV–vis photodissociation action spectroscopy in the 210–700 nm wavelength region. The action spectra of both A•+ and MA•+ displayed characteristic absorption bands in the near-UV and visible regions. Another tautomer of A•+ was generated as a minor product by multistep CID of protonated 9-(2-bromoethyl)­adenine. Structure analysis by density functional theory and coupled-clusters ab initio calculations pointed to the canonical 9-H-tautomer Ad1 •+ as the global energy minimum of adenine cation radicals. The canonical tautomer MA1 •+ was also calculated to be a low-energy structure among methyladenine cation radicals. However, two new noncanonical tautomers were found to be energetically comparable to MA1 •+. Vibronic absorption spectra were calculated for several tautomers of A•+ and MA•+ and benchmarked on equation-of-motion coupled-clusters excited-state calculations. Analysis of the vibronic absorption spectra of A•+ tautomers pointed to the canonical tautomer Ad1 •+ as providing the best match with the action spectrum. Likewise, the canonical tautomer MA1 •+ was the unequivocal best match for the MA•+ ion generated in the gas phase. According to potential-energy mapping, MA1 •+ was separated from energetically favorable noncanonical cation radicals by a high-energy barrier that was calculated to be above the dissociation threshold for loss of a methyl hydrogen atom, thus preventing isomerization. Structures and energetics of all four DNA nucleobase cation radicals are compared and discussed.

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 •+ .

No abstract

Adenosine radicals tagged with a fixed‐charge group were generated in the gas phase and structurally characterized by tandem mass spectrometry, deuterium labeling, and UV/Vis action spectroscopy. Experimental results in combination with Born–Oppenheimer molecular dynamics, ab initio, and excited‐state calculations led to unambiguous assignment of adenosine radicals as N‐7 hydrogen atom adducts. The charge‐tagged radicals were found to be electronically equivalent to natural DNA nucleoside radicals.

Hydrogen-rich DNA dinucleotide cation radicals (dGG + 2H)+•, (dCG + 2H)+•, and (dGC + 2H)+• represent transient species comprising protonated and hydrogen atom adducted nucleobase rings that serve as models for proton and radical migrations in ionized DNA. These DNA cation radicals were generated in the gas phase by electron-transfer dissociation of dinucleotide dication–crown–ether complexes and characterized by UV–vis photodissociation action spectra, ab initio calculations of structures and relative energies, and time-dependent density functional theory calculations of UV–vis absorption spectra. Theoretical calculations indicate that (dGG + 2H)+• cation radicals formed by electron transfer underwent an exothermic conformational collapse that was accompanied by guanine ring stacking and facile internucleobase hydrogen atom transfer, forming 3′-guanine C-8–H radicals. In contrast, exothermic hydrogen transfer from the 5′-cytosine radical onto the guanine ring in (dCG + 2H)+• was kinetically hampered, resulting in the formation of a mixture of 5′-cytosine and 3′-guanine radicals. Conformational folding and nucleobase stacking were energetically unfavorable in (dGC + 2H)+• that retained its structure of a 3′-cytosine radical, as formed by one-electron reduction of the dication. Hydrogen-rich guanine (G + H)• and cytosine (C + H)• radicals were calculated to have vastly different basicities in water, as illustrated by the respective pK a values of 20.0 and 4.6, which is pertinent to their different abilities to undergo proton-transfer reactions in solution.