Stereochemical Differentiation of Four Monosaccharides Using Transition Metal Complexes by Electrospray Ionization/Ion-Trap Mass Spectrometry

Several approaches may contribute to the structural elucidation of carbohydrates. The distinction of diastereoisomers by mass spectrometry is often a tedious task and, ultimately, the stereochemistry of monosaccharides must be determined. α/β anomers of glucose, mannose, galactose and talose stereochemically differ at the C₂ and C₄ positions and this work describes an efficient system used to differentiate four isomeric monosaccharides easily. The approach is based on mass spectrometry and it uses the gas-phase interaction between a sugar molecule and the iron(II) transition metal ion for differentiation of the species. Electrospray ionization allows the generation of cationized [M + FeCl]⁺ monosaccharides. Such ions have been prepared in high abundance in the external source of an electrospray ionization/ion-trap mass spectrometer instrument. Resonant excitation of these selected ions gives rise to dissociative collisions. Very likely, the strong interaction of iron^II with nucleophilic sites (i.e. hydroxyl groups) seems to favor regioselective and stereoselective iron^II ion attachment to each epimer. Consequently, when submitted to resonant excitation conditions, the cluster [M + FeCl]⁺ ions undergo fragmentations that can either be stereospecific in certain cases or, more usually, kinetically orientated (i.e. stereochemical effects on the rate constants of competitive unimolecular processes). Thus, by scrutinizing the variation in the relative abundances of diagnostic fragment ions, it is possible to distinguish stereoisomeric sugars. The origin of some product ions was proved by MS² and MS ⁿ experiments through ¹³C and ²H-labeling that may enlighten the mechanisms.