Stabilization and linkage analysis of metal-ligated sialic acid containing oligosaccharides.

The dissociation of metal-ligated sialyllactose and sialyl-N-acetyllactosamine was investigated. Metal-ligand derivatization of the carbohydrate samples with the diethylenetriamine ligand and one of four transition metals [Co(II), Ni(II), Cu(II), Zn(II)] suppressed sialic acid loss in the collision-induced dissociation process. Suppression of sialic acid loss allows sialic acid linkage information to be gained through tandem mass spectrometry. Sialic acid stabilization is postulated to occur due to the doubly charged metal ion which allows for deprotonation of the sialic acid moiety. Furthermore, a connection between the metal center and the amount of sialic acid loss was found. These results were rationalized using the Irving-Williams series and a competition between different sites of deprotonation. Analysis of the product ion spectra showed a clear differentiation of sialic acid linkage. Linkage determination is proposed to be effective due to the available conformations allowed by the different linkages. A more flexible linkage will allow more coordination of the sialic acid residue with the metal center, whereas a less flexible linkage will make this interaction unlikely.