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Abstract DNA–protein crosslinks (DPCs) remain as a poorly understood DNA lesion. Herein, crosslinking between guanosine and lysine was explored using a model system comprising 9-methylguanine (9MG) and CH3NH2. Crosslinking was induced by one-electron oxidized 9MG•+ radical cations and doubly oxidized [9MG – HN2]+ cations, and analyzed as a function of reaction energy using an electrospray ionization tandem mass spectrometer. Experiment was augmented by dynamics simulations and kinetics modeling. Alongside the formation of X-NH2CH3[9MG]•+ (X = C2, C8) via direct addition, 8-CH2NH2[9MG + HN7]+ was discovered as a new crosslink between 9MG•+ and CH3NH2. This crosslink results from methyl–hydrogen abstraction of CH3NH2 by the N7 of 9MG•+, followed by adding •CH2NH2 to [9MG + HN7]+. Notably, crosslinking is dramatically enhanced between [9MG – HN2]+ and CH3NH2, yielding major products X-+NH2CH3[9MG – HN2] (X = N2, N3, C5, and C8, along with their proton tautomers), which form from the direct CH3NH2 addition to [9MG – HN2]+, and minor products X-CH2NH2[9MG – HN2 + HO6]+ (X = N2, N3, C5, N7, and C8), which arise from the combination of methyl–hydrogen abstraction products. This work dissected and distinguished the roles of one- versus two-electron oxidized guanosine in DPC formation, offering novel insights into oxidative DNA damage. 

Abstract 8‐Oxoguanosine is the most common oxidatively generated base damage and pairs with complementary cytidine within duplex DNA. The 8‐oxoguanosine−cytidine lesion, if not recognized and removed, not only leads to G‐to‐T transversion mutations but renders the base pair being more vulnerable to the ionizing radiation and singlet oxygen (¹O₂) damage. Herein, reaction dynamics of a prototype Watson−Crick base pair [9MOG ⋅ 1MC]⋅⁺, consisting of 9‐methyl‐8‐oxoguanine radical cation (9MOG⋅⁺) and 1‐methylcystosine (1MC), was examined using mass spectrometry coupled with electrospray ionization. We first detected base‐pair dissociation in collisions with the Xe gas, which provided insight into intra‐base pair proton transfer of 9MOG⋅⁺ ⋅ 1MC[9MOG − H_N1]⋅ ⋅ [1MC+H_N3′]⁺and subsequent non‐statistical base‐pair separation. We then measured the reaction of [9MOG ⋅ 1MC]⋅⁺with¹O₂, revealing the two most probable pathways, C5‐O₂addition and H_N7‐abstraction at 9MOG. Reactions were entangled with the two forms of 9MOG radicals and base‐pair structures as well as multi‐configurations between open‐shell radicals and¹O₂(that has a mixed singlet/triplet character). These were disentangled by utilizing approximately spin‐projected density functional theory, coupled‐cluster theory and multi‐referential electronic structure modeling. The work delineated base‐pair structural context effects and determined relative reactivity toward¹O₂as [9MOG − H]⋅>9MOG⋅⁺>[9MOG − H_N1]⋅ ⋅ [1MC+H_N3′]⁺≥9MOG⋅⁺ ⋅ 1MC. 

Abstract 8‐Oxo‐2′‐deoxyguanosine (OG) is the most common DNA lesion. Notably, OG becomes more susceptible to oxidative damage than the undamaged nucleoside, forming mutagenic products in vivo. Herein the reactions of singlet O₂with the radical cations of 8‐oxo‐2′‐deoxyguanosine (OG^.+) and 9‐methyl‐8‐oxoguanine (9MOG^.+) were investigated using ion‐molecule scattering mass spectrometry, from which barrierless, exothermic O₂‐addition products were detected for both reaction systems. Corroborated by static reaction potential energy surface constructed using multi‐reference CASPT2 theory and molecular dynamics simulated in the presence of the reactants′ kinetic and internal energies, the C5‐terminal O₂‐addition was pinpointed as the most probable reaction pathway. By elucidating the reaction mechanism, kinetics and dynamics, and reaction products and energetics, this work constitutes the first report unraveling the synergetic damage of OG by ionizing radiation and singlet O₂.