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 Jul 8;22(26):14875-14888. doi: 10.1039/d0cp01788d.
A combined experimental and theoretical study is presented on the collision-induced dissociation (CID) of 9-methylguanine-1-methylcytosine base-pair radical cation (abbreviated as [9MG·1MC]˙+) and its monohydrate ([9MG·1MC]˙+·H2O) with Xe and Ar gases. Product ion mass spectra were measured as a function of collision energy using guided-ion beam tandem mass spectrometry, from which cross sections and threshold energies for various dissociation pathways were determined. Electronic structure calculations were performed at the DFT, RI-MP2 and DLPNO-CCSD(T) levels of theory to identify product structures and map out reaction potential energy surfaces. [9MG·1MC]˙+ has two structures: a conventional structure 9MG˙+·1MC (population 87%) consisting of hydrogen-bonded 9-methylguanine radical cation and neutral 1-methylcytosine, and a proton-transferred structure [9MG - H]˙·[1MC + H]+ (less stable, population 13%) formed by intra-base-pair proton transfer from the N1 of 9MG˙+ to the N3 of 1MC within 9MG˙+·1MC. The two structures have similar dissociation energies but can be distinguished in that 9MG˙+·1MC dissociates into 9MG˙+ and 1MC whereas [9MG - H]˙·[1MC + H]+ dissociates into neutral [9MG - H]˙ radical and protonated [1MC + H]+. An intriguing finding is that, in both Xe- and Ar-induced CID of [9MG·1MC]˙+, product ions were overwhelmingly dominated by [1MC + H]+, which is contrary to product distributions predicted using a statistical reaction model. Monohydration of [9MG·1MC]˙+ reversed the populations of the conventional structure (43%) vs. the proton-transferred structure (57%) and induced new reactions upon collisional activation, of which intra-base-pair hydrogen transfer produced [9MG + H]+ and the reaction of the water ligand with a methyl group in [9MG·1MC]˙+ led to methanol elimination from [9MG·1MC]˙+·H2O.
本文对 9-甲基鸟嘌呤-1-甲基胞嘧啶碱基对自由基阳离子（简称[9MG·1MC]˙+）及其一水合物（[9MG·1MC]˙+·H2O）与 Xe 和 Ar 气体的碰撞诱导解离（CID）进行了实验和理论研究。使用引导离子束串联质谱法测量了作为碰撞能函数的产物离子质谱，从中确定了各种解离途径的截面和阈值能。在 DFT、RI-MP2 和 DLPNO-CCSD(T)理论水平上进行了电子结构计算，以确定产物结构并绘制反应势能面。[9MG·1MC]˙+有两种结构：一种是由氢键结合的 9-甲基鸟嘌呤自由基阳离子和中性 1-甲基胞嘧啶组成的常规结构 9MG˙+·1MC（占 87%），另一种是由 9MG˙+·1MC 内碱基对质子转移形成的质子转移结构[9MG - H]˙·[1MC + H]+（不太稳定，占 13%）。两种结构具有相似的解离能，但可以区分，因为 9MG˙+·1MC 解离为 9MG˙+和 1MC，而[9MG - H]˙·[1MC + H]+解离为中性[9MG - H]˙自由基和质子化[1MC + H]+。一个有趣的发现是，在 Xe 和 Ar 诱导的[9MG·1MC]˙+CID 中，产物离子 overwhelmingly 以[1MC + H]+为主，这与使用统计反应模型预测的产物分布相反。[9MG·1MC]˙+的一水化作用逆转了常规结构（43%）与质子转移结构（57%）的比例，并在碰撞激活时诱导了新的反应，其中碱基对内的氢转移产生了[9MG + H]+，水配体与[9MG·1MC]˙+中的甲基反应导致甲醇从[9MG·1MC]˙+·H2O 中消除。
