Ionization‐Induced Proton Transfer in Model DNA Base Pairs: A Theoretical Study of the Radical Ions of the 7‐Azaindole Dimer

Abstract

Proton-transfer reactions of the radical anion and cation of the 7-Azaindole (7AI) dimer were investigated by means of density functional theory (DFT). The calculated results for the dimer anion and cation were very similar. Three equilibrium structures, which correspond to the non-proton-transferred (normal), the single-proton-transferred (SPT) and the double-proton-transferred (tautomeric) forms, were found. The transition states for proton-transfer reactions were also located. The calculations showed that the first proton-transfer reaction (normal-->SPT) is exothermic and almost barrier-free; therefore, it should occur spontaneously in the period of a vibration. In contrast, the second proton-transfer reaction (SPT-->tautomer) was found to be far less-probable in terms of reaction energy and barrier. Hence, it was concluded that both (7Al)2*- and (7Al)2*+ exist in the SPT form. The conclusion was further confirmed by the calculated electron vertical detachment energy (VDE) of the SPT form of (7Al)2*-, 1.33 eV, which is very close to the experimental measurement of 1.35 eV. The calculated VDEs of the normal and tautomeric (7Al)2*- forms were too small compared to the experimental value. The proton transfer process was found to be multidimensional in nature involving not only proton motion but also intermolecular rocking motion. In addition, IR spectra were calculated and reported. The spectra of the three structures showed very different features and, therefore, can be considered as fingerprints for future experimental identifications. The implications of these results to biology and spectroscopy are also briefly discussed.

Excited‐State Proton Transfer via Hydrogen‐Bonded Dimers and Complexes in Condensed Phase

“…They concluded that the evidence supported the stepwise mechanism, which is in agreement with their previous proposal [30] and is consistent with the recent theoretical study by Serrano-Andr es and Merch an [32]. It is worth noting here that another theoretical approach reported by Chen and Chao [40] was not consistent with the stepwise model claimed by Serrano-Andr es and Merch an.…”

“…Experiments with subpicosecond resolution further resolved a time constant of 1.4 ps for the rate of 7AI dimer ESDPT in non-polar solvents [26]. In view of the ESDPT dynamics of the 7AI dimer, ever since a non-concerted, two-step reaction model was proposed by Zewail and coworkers [27], there has been a longstanding and stimulating debate regarding the cooperative motion of two protons in a stepwise [27][28][29][30][31][32][33][34] versus concerted [35][36][37][38][39][40][41][42][43][44][45] double-proton transfer pathway (Figure 24.1).…”

Ionization of cytosine monomer and dimer studied by VUV photoionization and electronic structure calculations

“…Therefore, one can expect, that the ionization-induced proton transfer in these CCdimers is a barrierless or nearly barrierless process (Fig. 3).The existence of efficient channels of ionization-induced proton transfer in hydrogen-bonded base pairs and related systems was previously reported by both theory [65][66][67][68] and experiment. 34, 69 The dissociation of the proton-transferred dimer can give rise to the CH + signal.…”

Double-Proton-Transfer Reaction in Guanine-Cytosine Base Pair Embedded in B-Form DNA

“…[12][13][14][15][16] 碱基对内 质子转移反应受到多种环境因素的影响, 水溶剂、 碱基序列、 堆积效应和各种离子等都会影响碱基对 内质子转移. 因此科学家开始关注这些环境因素的 作用, [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] 目前的研究结果表明, 水分子有助于碱基 对阴离子发生单质子转移反应, 17,18 不同碱基排序对 碱基对阴离子的单质子转移分别起到促进和阻碍 作用, 19 堆积效应在质子转移过程中改变氢键键长 并保持碱基对的平面性, 20 一些金属离子也有助于 单质子转移反应的发生. 32 Table 2 Charge…”