Department of Chemistry and Biochemistry, Queens College of the City University of New York, 65-30 Kissena Blvd., Queens, NY 11367, USA.
Phys Chem Chem Phys. 2020 Nov 21;22(43):24986-25000. doi: 10.1039/d0cp04243a. Epub 2020 Oct 28.
A guided-ion beam tandem mass spectrometric study was performed on collision-induced dissociation (CID) of a protonated 9-methylguanine-1-methylcytosine Watson-Crick base pair (designated as WC-[9MG·1MC + H]), from which dissociation pathways and dissociation energies were determined. Electronic structure calculations at the DFT, RI-MP2 and DLPNO-CCSD(T) levels of theory were used to identify product structures and delineate reaction mechanisms. Intra-base-pair proton transfer (PT) of WC-[9MG·1MC + H] results in conventional base-pair conformations that consist of hydrogen-bonded [9MG + H] and 1MC and proton-transferred conformations that are formed by PT from the N1 of [9MG + H] to the N3' of 1MC. Two types of conformers were distinguished by CID in which the conventional conformers produced [9MG + H] product ions whereas the proton-transferred conformers produced [1MC + H]. The conventional conformers have a higher population (99.8%) and lower dissociation energy than the proton-transferred counterparts. However, in contrast to what was expected from the statistical dissociation of the equilibrium base-pair conformational ensemble, the CID product ions of WC-[9MG·1MC + H] were dominated by [1MC + H] rather than [9MG + H]. This finding, alongside the non-statistical CID reported for deprotonated guanine-cytosine (Lu et al.; PCCP, 2016, 18, 32222) and guanine-cytosine radical cation (Sun et al.; PCCP, 2020, 22, 14875), reinforces that non-statistical dissociation is a distinctive feature of singly-charged Watson-Crick guanine-cytosine base pairs. It implies that intra-base-pair PT facilitates the formation of proton-transferred conformers in these systems and the ensuing conformers have loose transition states for dissociation. The monohydrate of WC-[9MG·1MC + H] preserves non-statistical CID kinetics and introduces collision-induced methanol elimination via the reaction of the water ligand with a methyl group.
采用导向离子束串联质谱研究了质子化 9-甲基鸟嘌呤-1-甲基胞嘧啶 Watson-Crick 碱基对（标记为 WC-[9MG·1MC+H]）的碰撞诱导解离（CID），确定了解离途径和解离能。在 DFT、RI-MP2 和 DLPNO-CCSD(T)理论水平上进行电子结构计算，以识别产物结构并描绘反应机制。WC-[9MG·1MC+H]的碱基对内质子转移（PT）导致形成氢键的[9MG+H]和 1MC 以及由[9MG+H]的 N1 到 1MC 的 N3'的质子转移的常规碱基对构象和质子转移构象。通过 CID 区分了两种类型的构象体，其中常规构象体产生[9MG+H]产物离子，而质子转移构象体产生[1MC+H]。常规构象体的含量（99.8%）较高，解离能较低。然而，与从平衡碱基对构象体混合物的统计解离预期相反，WC-[9MG·1MC+H]的 CID 产物离子主要是[1MC+H]而不是[9MG+H]。这一发现与去质子化鸟嘌呤-胞嘧啶（Lu 等人；PCCP，2016，18，32222）和鸟嘌呤-胞嘧啶自由基阳离子（Sun 等人；PCCP，2020，22，14875）报道的非统计 CID 一起，强调了非统计解离是单电荷 Watson-Crick 鸟嘌呤-胞嘧啶碱基对的一个显著特征。这意味着碱基对内的 PT 促进了这些系统中质子转移构象体的形成，而随后的构象体具有松散的解离过渡态。WC-[9MG·1MC+H]的一水合物保留了非统计 CID 动力学，并通过水配体与甲基的反应引入了碰撞诱导的甲醇消除。
