Density functional calculations have been applied to study reactions of the HSSS. radical with guanine at the C2, C4, C5, and C8 sites of guanine that yields [GCn-SSSH].n = 2, 4, 5, and 8 adducts. Also, formation of 8-thioguanine from [GC8-SSSH]. adduct has been studied. In all the adducts [GCn-SSSH]., HSSS. radical is placed on the top of the plane of the guanine ring. The results show that there is a possibility of intraresidue S-H…N hydrogen bonding in each [GCn-SSSH]. adduct. Besides, the C8 site of guanine is the most reactive site to add to HSSS. radical with appreciably high rate constant, and the adduct formed at this site would be significantly stable. According to NBO analysis, delocalization energies for all the hydrogen bondings, i.e., E(2) for nN⟶σ*SH interactions, are obtained to be in the range of 1.31–3.46 kcalmo1−1. The biggest E(2) and the largest amount of charge transfer is found for [GC8-SSSH]. adduct, i.e., for the S-H…N7 (Gua) interaction, in which RN…H = 2.588 Å, to be 1.78 kcalmo1−1 and 0.0024e, respectively, well correlated with the prediction of most of its stability among the [GCn-SSSH]. adducts. The ZPE-corrected enthalpy changes for the reactions are obtained to be 0.2–1.4 kcalmo1−1. The reaction enthalpy changes and the barrier energies related to the different sites of guanine are found to be in the order of C8 < C2 < C5 < C4. Starting with the adduct [GC8-SSSH]., 8-thioguanine could be achieved in two steps with two transition states (TS1 and TS2) and an intermediate complex (IM); the overall reaction is exothermic. The first reaction step, the rate-determining step, involves going from the adduct [GC8-SSSH]. to IM via TS1, in which HSS moiety gets dissociated from the HSSS moiety attached to the C8 site of guanine. Graphical abstract Graphical abstract

中文翻译：

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HSSS 的反应。带有鸟嘌呤的自由基和 8-硫鸟嘌呤的形成：一项计算研究

密度泛函计算已应用于研究 HSSS 的反应。在鸟嘌呤的 C2、C4、C5 和 C8 位点与鸟嘌呤自由基，产生 [GCn-SSSH].n = 2、4、5 和 8 加合物。此外，从 [GC8-SSSH] 形成 8-硫鸟嘌呤。加合物进行了研究。在所有加合物中 [GCn-SSSH]., HSSS. 自由基位于鸟嘌呤环平面的顶部。结果表明，每个[GCn-SSSH]中都有可能存在残基内SH…N氢键。加合物。此外，鸟嘌呤的 C8 位点是添加到 HSSS 中最具反应性的位点。自由基具有明显高的速率常数，并且在该位点形成的加合物将非常稳定。根据 NBO 分析，所有氢键的离域能，即 nN⟶σ*SH 相互作用的 E(2)，都在 1.31–3.46 kcalmo1-1 的范围内。[GC8-SSSH] 的 E(2) 和电荷转移量最大。加合物，即对于 SH…N7 (Gua) 相互作用，其中 RN…H = 2.588 Å，分别为 1.78 kcalmo1−1 和 0.0024e，与 [GCn- SSH]。加合物。ZPE 校正的反应焓变为 0.2–1.4 kcalmo1-1。发现与鸟嘌呤不同位点相关的反应焓变化和势垒能的顺序为 C8 < C2 < C5 < C4。从加合物 [GC8-SSSH] 开始，8-硫鸟嘌呤可以通过两个过渡态（TS1 和 TS2）和一个中间复合物（IM）分两步实现；整个反应是放热的。第一个反应步骤，即速率确定步骤，涉及从加合物 [GC8-SSSH] 开始。通过 TS1 到 IM，其中 HSS 部分与连接到鸟嘌呤 C8 位点的 HSSS 部分分离。图形摘要图形摘要