Halogen Counter-Ion Effect on the Dissociation of Monosaccharide—iron Complexes Generated by Electrospray Ionization Combined with an Ion Trap Mass Spectrometer

A primary focus of research in our laboratory has been to determine the stereochemistry of four isomeric monosaccharides using the electrospray ionization/desorption mode combined with an ion trap mass spectrometer. α/β-Anomers of glucose, mannose, galactose and talose differ stereochemically at the C₂ and C₄ positions; previous work described an efficient system able to easily differentiate these four isomeric hexoses when combined with the iron(II)ion. This approach was based upon interaction between the sugar analyte and iron(II)yielding cationized [MFe^IICl]⁺ monosaccharides which were then studied by sequential tandem mass spectrometry. The daughter ions of these clusters were produced using the resonant excitation mode and are characterized by variable rate constants related to the hydroxyl stereochemistry. When submitted to resonant excitation conditions, the [MFe^IICl]⁺ cluster ions undergo processes that can be either stereospecific in certain cases or, more usually, kinetically controlled by stereochemical effects. It is, therefore, possible to distinguish stereoisomeric sugars by examining the variation in relative abundance of diagnostic fragment ions. The present work reports data on the influence of counter-ions associated with iron(II)(viz. FeCl₂, FeBr₂, FeI₂ and FeF₂) on desorbed species obtained by electrospray desorption/ionization. The cationized monomeric [MFe^IIX]⁺ and dimeric [M₂Fe^IIX]⁺ ions were studied by collision-induced dissociation (CID)to demonstrate a significant halogen effect from the resonant excitation experiments and on the extent of the differentiation of the diastereomers. The data presented demonstrate the influence of thermochemical properties (i.e. acidity and basicity), mainly related to the halide size, on the dissociation of cationic [MFe^IIX]⁺ and [M₂Fe^IIX]⁺ sugar–metal complexes in CID experiments. Only weak effects due to the halide were observed during desorption itself, when comparison of the latter cluster with the former shows a relatively enhanced desorption yield of the monomer with an iodide counter-ion. However, the more significant counter-ion effects concern the cationized dimeric species, which present spectacular product-ion abundance changes in CID spectra. In all cases, it has been possible to distinguish the diastereomers.