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Title: Mass spectrometry and computational study of collision-induced dissociation of 9-methylguanine–1-methylcytosine base-pair radical cation: intra-base-pair proton transfer and hydrogen transfer, non-statistical dissociation, and reaction with a water ligand
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]˙ + ·H 2 O) 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 more » 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]˙ + ·H 2 O. « less
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Physical Chemistry Chemical Physics
Page Range or eLocation-ID:
14875 to 14888
Sponsoring Org:
National Science Foundation
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  1. 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 (Lumore »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.« less
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