The chemistry of ionized enols in the gas phase

Tureček, František

doi:10.1002/9780470772294.ch3

Cited by 9 publications

(3 citation statements)

References 178 publications

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“…Isomerizations of this type where a nonpolar C–H bond is replaced by a polar X–H bond often result in stabilization of cation radicals, as first reported by Radom and co-workers for distonic ions of the · CH 2 XH + type, where X = O, N . The well-known stabilization of simple enol cation radicals relative to their aldehyde and ketone isomers is also well documented. − With dA3 + · , the noncanonical structure gains further stabilization because of the hydrogen bond to O5. The stabilizing C to N hydrogen transfer can be applied to the other C–H bonds in 2′-deoxyadenosine cation radicals.…”

Section: Resultsmentioning

confidence: 98%

Noncanonical Isomers of Nucleoside Cation Radicals: An Ab Initio Study of the Dark Matter of DNA Ionization

Shu

Tureček

2022

J. Phys. Chem. A

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Cation radicals of DNA nucleosides, 2′-deoxyadenosine, 2′-deoxyguanosine, 2′-deoxycytidine, and 2′-deoxythymidine, can exist in standard canonical forms or as noncanonical isomers in which the charge is introduced by protonation of the nucleobase, whereas the radical predominantly resides in the deoxyribose moiety. Density functional theory as well as correlated ab initio calculations with coupled clusters (CCSD(T)) that were extrapolated to the complete basis set limit showed that noncanonical nucleoside ion isomers were thermodynamically more stable than their canonical forms in both the gas phase and as water-solvated ions. This indicated the possibility of exothermic conversion of canonical to noncanonical forms. The noncanonical isomers were calculated to have very low adiabatic ion–electron recombination energies (REad) for the lowest-energy isomers 2′-deoxy-(N-3H)adenos-1′-yl (4.74 eV), 2′-deoxy-(N-7H)guanos-1′-yl (4.66 eV), 2′-deoxy-(N-3H)cytid-1′-yl (5.12 eV), and 2′-deoxy-5-methylene-(O-2H)uridine (5.24 eV). These were substantially lower than the REad value calculated for the canonical 2′-deoxyadenosine, 2′-deoxy guanosine, 2′-deoxy cytidine, and 2′-deoxy thymidine cation radicals, which were 7.82, 7.46, 8.14, and 8.20 eV, respectively, for the lowest-energy ion conformers of each type. Charge and spin distributions in noncovalent cation-radical dA⊂dT and dG⊂dC nucleoside pairs and dAT, dCT, dTC, and dGC dinucleotides were analyzed to elucidate the electronic structure of the cation radicals. Born–Oppenheimer molecular dynamics trajectory calculations of the dinucleotides and nucleoside pairs indicated rapid exothermic proton transfer from noncanonical T+ · to A in both dAT+ · and dA⊂dT+ ·, leading to charge and radical separation. Noncanonical T+ · in dCT+ · and dTC+ · initiated rapid proton transfer to cytosine, whereas the canonical dCT+ · dinucleotide ion retained the cation radical structure without isomerization. No spontaneous proton transfer was found in dGC+ · and dG⊂dC+ · containing canonical neutral and noncanonical ionized deoxycytidine.

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Section: Resultsmentioning

confidence: 98%

Noncanonical Isomers of Nucleoside Cation Radicals: An Ab Initio Study of the Dark Matter of DNA Ionization

Shu

Tureček

2022

J. Phys. Chem. A

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“…The leveling off of the energies for the 2-oxo form 12 +• and enols 13 +• and 14 +• (0.5 and 1.5 kJ mol –1 , respectively, at the CCSD(T)/CBS level of theory) indicated relative stabilization of the oxo-form upon ionization. This was unusual, because ionized enols are often stabilized relative to oxo forms upon ionization, reversing the order of stability for the neutral molecules. − In contrast to the neutral species, where tautomer 15 has been reported to be virtually isoenergetic with 12 ( cf. also Table S6 data), we found ionized 15 +• to be substantially less stable than 12 +• –14 +• (Table ).…”

Section: Results and Discussionmentioning

confidence: 99%

Cation Radicals of Hachimoji Nucleobases. Canonical Purine and Noncanonical Pyrimidine Forms Generated in the Gas Phase and Characterized by UV–Vis Photodissociation Action Spectroscopy

Shu

Tureček

2020

J. Phys. Chem. A

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Oxidation of nontraditional nucleobases 1-methylcytosine (hachimoji base S) and isoguanine (hachimoji base B) in gas-phase ternary complexes with Cu II (terpyridine) 2+ formed cation radicals that were characterized by tandem mass spectrometry, UV−vis photodissociation action spectroscopy in the 210−700 nm region, and ab initio calculations up to the CCSD(T)/complete basis set level of theory. Oxidation of S was accompanied by exothermic isomerization in the 1-methylcytosine ion (1 +• ), forming 1-methylene-2-hydroxy-4-aminopyrimidine cation radical (9 +• ) as a noncanonical distonic isomer of the nucleobase. Ion 9 +• was characterized by deuterium exchange experiments and provided a matching UV−vis action spectrum with the vibronic absorption spectrum from time-dependent density functional theory calculations. Oxidation of B resulted in the formation of a canonical isoguanine cation radical (12 +• ) as judged from the match of the experimental action spectrum with the calculated vibronic absorption spectrum. The calculated adiabatic ionization energies of canonical S and B, 8.51 and 7.76 eV, respectively, indicated exothermic electron transfer from B to S +• to proceed in an ionized base pair. Contrasting this, the lowest energy tautomer of ionized S (9 +• ) had a low adiabatic recombination energy, RE adiab = 5.70 eV, that would prevent it from oxidizing other nucleobases. Recombination energies of several nucleobase tautomers are reported and discussed.

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“…Apart from some notable exceptions, enols are usually less stable and exist only at very low concentration under equilibrium conditions . However, the situation is different in the corresponding radical cations where gas-phase experiments have shown that enol radical cations are usually more stable than their keto tautomers . This is due to the fact that enol radical cations profit from allylic resonance stabilization that is not available to ketones.…”

Section: Introductionmentioning

confidence: 99%

Electron-Transfer-Induced Tautomerization in Methylindanones: Electronic Control of the Tunneling Rate for Enolization

et al. 2001

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The radical cations generated from 4-methyl- and 4,7-dimethylindanone, as well as their deuterated isotopomers, isolated in Argon matrices, were found to undergo enolization to the corresponding enol radical cations at rates that differ by orders of magnitude. It is shown by quantum chemical calculations that the effect of the remote methyl group in the 4-position is of purely electronic nature in that it stabilizes the unreactive pi-radical relative to the reactive sigma-radical state of the 7-methylindanone radical cation. The observed kinetic behavior of the two compounds can be reproduced satisfactorily on the basis of calculated height and width of the thermal barrier for enolization, using the Bell model for quantum mechanical tunneling. High-level calculations on the methylacrolein radical cation show that barriers for enolization in radical cations are overestimated by B3LYP/6-31G.

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The chemistry of ionized enols in the gas phase

Cited by 9 publications

References 178 publications

Noncanonical Isomers of Nucleoside Cation Radicals: An Ab Initio Study of the Dark Matter of DNA Ionization

Noncanonical Isomers of Nucleoside Cation Radicals: An Ab Initio Study of the Dark Matter of DNA Ionization

Cation Radicals of Hachimoji Nucleobases. Canonical Purine and Noncanonical Pyrimidine Forms Generated in the Gas Phase and Characterized by UV–Vis Photodissociation Action Spectroscopy

Electron-Transfer-Induced Tautomerization in Methylindanones: Electronic Control of the Tunneling Rate for Enolization

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