Structural characterization of intact polysaccharides by MALDI-in-source decay-Fourier transform ion cyclotron resonance (FTICR) MS

Polysaccharides occur throughout all kingdoms of life including plants, microorganisms and animals. They differ profoundly in structure and function ranging from heparin that serves as an anti-coagulant to pectins that are used to modify food properties. Mass spectrometry (MS) is a powerful technique for the characterization of polysaccharides and is often applied in conjunction with chemical or enzymatic degradation. Here, we show that wide mass range ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI) in-source decay (ISD) Fourier transform ion cyclotron resonance (FTICR) MS is suitable as a generic and robust method for the analysis of polysaccharides at the level of large fragments. We successfully analyzed polysaccharides that differ widely regarding composition and origin. The ISD fragment ions conferred information on monosaccharide composition as well as modifications including sulfation, amidation and methoxylation. Additionally, we could determine the pectins' degree of methoxylation from these ISD fragment ions. Performing MS² of the ISD fragments confirmed the proposed identity and provided additional structural characteristics. We conclude that MALDI-ISD-MS is a fast and robust method for the characterization of polysaccharides complementing other analytical techniques. Specifically, the method may be evaluated for a rapid determination of differences between polysaccharide variants and batches, for establishing structure-function relationships.