Ground and Excited States of Gas-Phase DNA Nucleobase Cation-Radicals. A UV–vis Photodisociation Action Spectroscopy and Computational Study of Adenine and 9-Methyladenine

Abstract

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.

Singlet Oxygen Oxidation of the Radical Cations of 8‐Oxo‐2′‐deoxyguanosine and Its 9‐Methyl Analogue: Dynamics, Potential Energy Surface, and Products Mediated by C5‐O2‐Addition

“…The O′Hair [28a,b,32] and Bohme [28c] laboratories extended this approach to the formation of guanosine radical cations via collision‐induced dissociation (CID) of Cu(II)‐nucleoside(G) complexes wherein the redox separation of [Cu II G 3 ] .2+ leads to the formation of complementary product ions G .+ and [Cu I G 2 ] + . This method was successfully used to generate the radical cations of guanine, 9‐methylguanine, 2′‐deoxyguanosine and guanosine, [28h,33] as well as those of Hachimoji nucleobases [28e–g] . In the present work, 9MOG .+ was generated by CID of the [Cu II (9MOG) 3‐n (2′‐deoxyguanosine) n ] .2+ (n=0–2) complexes which were formed by electrospray ionization (ESI) of a methanol/water solution containing equimolar concentration of 9MOG, 2′‐deoxyguanosine (used as a co‐ligand) and Cu(NO 3 ) 2 .…”

The Elusive Noncanonical Isomers of Ionized 9-Methyladenine and 2′-Deoxyadenosine

“…1,17,18 Two low-energy noncanonical isomers of 9-methyladenine cation radical (1 +• ) have recently been identified by ab initio calculations as 9-methylene-(1H)adenine (2 +• ) and 9-methylene-(3H)adenine (3 +• ) (Scheme 1). 33 However, synthetic pathways to generating ions 2 +• and 3 +• have not been established, especially in light of the energy analysis of the potential energy surface that has indicated large energy barriers separating the isomers and precluding unimolecular isomerization. 33 We reasoned that an adenine derivative carrying an N-9 CH 2 −X group can be protonated, yielding a stable gas-phase ion in which a labile C−X bond, such as C−I, could be homolytically cleaved upon collisional activation to form a ringprotonated 9-CH 2 radical, such as 2 +• (Scheme 2) or its N-3 (3 +• ) or N-7-protonated tautomers (4 +• ).…”

Threshold photoelectron spectroscopy of 9-methyladenine: theory and experiment

“…Our work confirms thus the assumptions of Turec ˇek and co-workers. 21 We would like to note that we locate the lowest quartet state (1 4 A (Q 1 )) in the vicinity of D 5 . Although the single photoionization of neutral 9-methyladenine does not populate the quartet cationic state, the latter may couple to the close lying doublets (e.g.…”

“…The geometrically optimized parameters of 9MA + are listed in Tables S2 and S3 (ESI †), which are close to those determined by Turec ˇek and co-workers. 21 Obviously, the Franck-Condon factors (FCFs) for the 9MA X + hn -9MA + X + + e À photoionization transition should be close to zero whereas non-zero FCFs are expected for 9MA 0 X + hn -9MA + X + + e À because of the transcis change at the N9-C14 (methyl) bond in the former as illustrated in Fig. 2.…”

“…For both cations, they proposed the following state ordering: D 0 (p À1 ) (first band), D 1 (n À1 ) (second band) and then D 2 (p À1 ) & D 3 (n À1 ) (third band) where the energies of the bands were lower for 9MA compared to 9HA. In 2020, Turec ˇek and co-workers 21 carried out UV-vis photodissociation action spectroscopy and collision induced dissociation (CID) mass spectrometry on gas phase 9MA trapped cations. Through density functional theory and coupled clusters computations, they identified a unique form of the 9MA cation (denoted in what follows as 9MA + ).…”