Proton Transfer Accounting for Anomalous Collision-Induced Dissociation of Proton-Bound Hoogsteen Base Pair of Cytosine and Guanine.

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. Graphical Abstract ᅟ.