Stereochemical effects from doubly-charged iron clusters for the structural elucidation of diastereomeric monosaccharides using ESI/IT-MS

Several approaches may be used to contribute to the structural elucidation of carbohydrates. Distinction of diastereomeric monosaccharides is often a tedious task by mass spectrometry, and ultimately the monosaccharide stereochemistry must be determined. Diastereomers of glucose, mannose, galactose and talose stereochemically differ at the C-(2) and C-(4) positions and previous work described an efficient system used to differentiate these four monosaccharides easily. The approach was based on ESI mass spectrometry and scrutinized the behavior of a sugar molecule towards the iron(II) chloride and the resulting effects in the gas phase. The produced cationized monomeric [(MFeCl)-Cl-II](+) and dimeric [(M2FeCl)-Cl-II](+) ions as singly-charged species were studied by sequential MS/MS experiments. When submitted to low resonant excitation conditions these selected ions undergo fragmentation within a kinetic control (i.e., stereochemical effects on the rate constants of competitive unimolecular processes). Enhancement of stereochemical effects was displayed in the CID spectra of the cationized [(M2FeCl)-Cl-II](+) dimeric ions. The present work first reports the influence of the sample preparation on the produced cationized species using the electrospray process. Formation of doubly-charged cluster MnFe2+ ions when the sugar molecule (M) is introduced in excess with the iron(II) chloride was observed. Secondly, behavior of these cluster ions under resonant excitation conditions reveals an increase of stereochemical effects compared with that observed in the case of singly-charged monomeric [(MFeCl)-Cl-II](+) and dimeric [(M2FeCl)-Cl-II](+) ions previously studied [Eur J Mass Spectrom 6 (2000) 421; Eur J Mass Spectrom 7 (2001) 331]. The dissociation pathways of doubly-charged complexes appeared to depend upon the n number of monosaccharides which constitute the formal ligand of the iron(II). MnFe2+ cluster ions are useful to efficiently determine the stereochemistry of the investigated monosaccharides and represent an advantageous alternative for the structural elucidation of their respective stereochemistry.