Low‐Energy‐Barrier Proton Transfer Induced by Electron Attachment to the Guanine⋅⋅⋅Cytosine Base Pair

Abstract

The photoelectron spectrum of the anion of the guanine...cytosine base pair (GC)(*-) is recorded for the first time. The observed variation in the spectral peak-height ratios with the source conditions suggests the presence of two or more anionic isomers. Two maxima of the broad bands in the photoelectron spectrum were measured at about 1.9 and about 2.6 eV. These values are very well reproduced by the vertical detachment energies of the B3LYP/6-31++G(d,p) calculated low-energy anionic structures, which are 1) the Watson-Crick base-pair anion with proton transferred from N1 of guanine to N3 of cytosine, 2) its analogue in which the proton is transferred from N9 of guanine to N7 of guanine, and 3) the global minimum geometry, which is formed from the latter anion by rotation of guanine about the axis approximately defined by C2 of guanine and C4 of cytosine. Furthermore, a minor difference in the stabilities of the two lowest energy anions explains the experimentally observed source (temperature) dependence of the PES spectrum. A rational procedure, based on the chemistry involved in the formation of anionic dimers, which enables the low-energy anions populated in the photoelectron spectrum to be identified is proposed. In contrast to the alternative combinatorial approach, which in the studied case would lead to carrying out quantum chemical calculations for 2000-2500 structures, the procedure described here reduces the computational problem to only 15 geometries.

Valence Anions of DNA-Related Systems in the Gas Phase: Computational and Anion Photoelectron Spectroscopy Studies

“…Therefore, in order to interpret PES spectra using a computational approach, one has to scrutinize the conformational space accessible to a studied system in order to identify the most stable configurations. If the system of interest is a molecule having several tautomeric forms [22,[41][42][43][44][45][46][47][48] or a conformationally flexible complex [9], consisting of two or more monomers, the configurational space that has to be probed within calculations may be enormous [49], which makes the task of identifying the most stable form(s) non-trivial. This is also an argument that support the usage of cost-effective DFT methods rather than accurate but very expensive coupled-cluster approach for such type of studies.…”

“…The former is responsible for the feature at~2.6 eV and the latter for the~2 eV one. Despite significant difference in VDEs predicted for both complexes, their adiabatic affinities are almost identical (~0.7 eV) [49].…”

“…When it comes to non-methylated guanine···cytosine complexes, the relative thermodynamic stabilities obtained at the DFT level suggest that 3 anionic structures are likely to show up in the photoelectron experiment [49]. Their geometries are presented in Fig.…”

“…While the MAMT •À and AT •À complexes differ in their propensity to undergo intermolecular proton transfer, both the MGMC •À and GC •À valence anions exist as proton-transferred structures in the gas phase [49,87]. The photoelectron spectrum of MGMC…”

“…The Watson-Crick AT [80][81][82][83] and GC [49,50,82,[84][85][86][87] base pairs were also shown to form stable valence anions in the gas phase, in which the excess charge is located entirely at the pyrimidine moiety. The hydrogen bonding between complementary bases increases the AEA values of pyrimidine nucleobase.…”