Influence of coordination number and ligand size on the dissociation mechanisms of transition metal-monosaccharide complexes

Abstract

Several features of metal-carbohydrate complexes, in the form of deprotonated metal/ N-glycoside ions, were varied in order to determine which types of complexes would best enable mass spectrometric differentiation of the stereochemical features of the coordinated monosaccharides. Metal complexes were generated by using transition metals such as nickel, copper, and zinc. Additionally, the size and coordination number of ligands in the complex, as well as the number of these ligands coordinated to the metal, were varied. By using a quadrupole ion trap mass spectrometer, multistage mass spectrometric experiments were performed on the electrospray-generated metal N-glycoside complexes. Several tricoordinate Ni N-glycoside systems were capable of differentiating the stereochemistry about the C-2 center in the monosaccharide ring, whereas the tricoordinate Cu/ en system allowed for the differentiation of both the C-2 and C-4 stereocenters. No stereochemical differentiation was possible from four- or five-coordinate species with the exception of the four-coordinate Zn/ dien complexes. Changes in the metal center or size of the N-glycoside ligand generated the greatest changes in the product ion spectra of the tricoordinate complexes. Such alterations in four- or five-coordinate complexes often did not result in greatly differing product ion spectra. Whereas the product ion spectra of most metal/ N-glycoside complexes could be easily categorized according to structural features of the precursor ion, exceptional species such as the four coordinate [Zn( dien/monosaccharide)–H] + precursor ion, also capable of differentiating the stereochemical features about C-2 and C-4, can give unexpected stereochemical information.