Electron Capture Dissociation of Oligosaccharides Ionized with Alkali, Alkaline Earth, and Transition Metals

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Abstract
We extend the application of electron capture dissociation (ECD) (which requires at least two charges) to oligosaccharides without basic functionalities by utilizing alkali, alkaline earth, and transition metals (Na+, K+, Ca2+, Ba2+, Mg2+, Mn2+, Co2+, and Zn2+) as charge carriers in electrospray ionization. Both linear and branched oligosaccharides were examined, including maltoheptoase, p-lacto-N-hexaose, and an N-linked glycan from human α1-acid glycoprotein. For comparison, infrared multiphoton dissociation (IRMPD) was also applied to all oligosaccharide species. We show that, for certain metal-adducted oligosaccharides, particularly maltoheptaose, cross-ring cleavage, which can provide saccharide linkage information, is the dominant fragmentation pathway in ECD. By contrast, glycosidic cleavages dominate in IRMPD although cross-ring fragmentation was also observed to varying degrees depending on metal ion type. The branched N-linked glycan did not fragment as easily following ECD compared to the linear oligosaccharides, presumably due to intramolecular noncovalent interactions. However, this limitation was partially overcome with a combined ECD/IRMPD approach (activated ion ECD). For all metal-adducted oligosaccharides, complementary structural information was obtained with ECD as compared to IRMPD. Our results demonstrate that ECD of metal-adducted oligosaccharides is a valuable tool for structural characterization of oligosaccharides.

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