Gas-Phase Intercluster Thiyl-Radical Induced C–H Bond Homolysis Selectively Forms Sugar C2-Radical Cations of Methyl D-Glucopyranoside: Isotopic Labeling Studies and Cleavage Reactions

Abstract

A suite of isotopologues of methyl D-glucopyranosides is used in conjunction with multistage mass spectrometry experiments to determine the radical site and cleavage reactions of sugar radical cations formed via a recently developed 'bio-inspired' method. In the first stage of CID (MS²), collision-induced dissociation (CID) of a protonated noncovalent complex between the sugar and S-nitrosocysteamine, [H₃NCH₂CH₂SNO + M]⁺, unleashes a thiyl radical via bond homolysis to give the noncovalent radical cation, [H₃NCH₂CH₂S^• + M]⁺. CID (MS³) of this radical cation complex results in dissociation of the noncovalent complex to generate the sugar radical cation. Replacement of all exchangeable OH and NH protons with deuterons reveals that the sugar radical cation is formed in a process involving abstraction of a hydrogen atom from a C–H bond of the sugar coupled with proton transfer to the sugar, to form [M – H^• + D⁺]. Investigation of this process using individual C-D labeled sugars reveals that the main site of H/D abstraction is the C2 position, since only the C2-deuterium labeled sugar yields a dominant [M – D^• + H⁺] product ion. The fragmentation reactions of the distonic sugar radical cation, [M – H^•+ H⁺], were studied by another stage of CID (MS⁴). ¹³C-labeling studies revealed that a series of three related fragment ions each contain the C1–C3 atoms; these arise from cross-ring cleavage reactions of the sugar.

Graphical Abstract

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