To understand the anomalous collision-induced dissociation (CID) behavior of the proton-bound Hoogsteen base pair of cytosine (C) and guanine (G), C:H⁺∙∙∙G, we investigated CID of a homologue series of proton-bound heterodimers of C, 1-methylcytosine, and 5-methylcytosine with G as a common base partner. The CID experiments were performed in an energy-resolved way (ER-CID) under both multiple and near-single collision conditions. The relative stabilities of the protonated complexes examined by ER-CID suggested that the proton-bound complexes produced by electrospray ionization in this study are proton-bound Hoogsteen base pairs. On the other hand, in contrast to the other base pairs, CID of C:H⁺∙∙∙G exhibited more abundant productions of C:H⁺, the fragment protonated on the moiety with a smaller proton affinity, than that of G:H⁺. This appeared to contradict general prediction based on the kinetic method. However, further theoretical exploration of potential energy surfaces found that there can be facile proton transfers in the proton-bound Hoogsteen base pairs during the CID process, which makes the process accessible to an additional product state of O-protonated C for C:H⁺ fragments. The presence of an additional dissociation channel, which in other words corresponds to twofold degeneracy in the transition state leading to C:H⁺ fragments, effectively doubles the apparent reaction rate for production of C:H⁺. In this way, the process gives rise to the anomaly, the observed pronounced formation of C:H⁺ in the CID of the proton-bound Hoogsteen base pair, C:H⁺∙∙∙G.

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为了理解反常碰撞诱导解离（CID）行为（C）的质子结合Hoogsteen碱基对胞嘧啶和鸟嘌呤（G）的，C：H ⁺ ∙∙∙G，我们调查的同源物系列质子的CID与C，1-甲基胞嘧啶和5-甲基胞嘧啶结合的异二聚体，其中G为共同的碱基伴侣。CID实验是在多个碰撞条件和接近单个碰撞条件下以能量分辨方式（ER-CID）进行的。ER-CID检测的质子化复合物的相对稳定性表明，本研究中电喷雾电离产生的质子结合的复合物是质子结合的Hoogsteen碱基对。另一方面，与其他碱基对相比，C：H ⁺ ∙∙∙G的CID显示出更多的C：H ^+产生，该片段在部分上质子化的质子亲和力小于G：H ⁺。这似乎与基于动力学方法的一般预测相矛盾。但是，对势能面的进一步理论探索发现，在CID过程中，质子结合的Hoogsteen碱基对中可能存在容易的质子转移，这使得该过程可用于C：H ⁺的O质子化C的其他产物状态碎片。另一个解离通道的存在，换言之对应于导致C：H ⁺片段的过渡态的双重简并，有效地使产生C：H ⁺的表观反应速率加倍。这样，该过程引起异常，即在质子结合的Hoogsteen碱基对CID中C：H ⁺ ∙∙∙G的CID中明显观察到C：H ^+的形成。

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