Investigation of copper–saccharide complexation reactions using potentiometry and electrospray mass spectrometry

Abstract

The associations between cupric ions and oligosaccharides of cellulose, chitin and chitosan were investigated using potentiometry and electrospray mass spectrometry. The goal of these studies was to understand the degree to which electrospray mass spectra reflect the solution chemistry of copper–oligosaccharide complexes. To this end, electrospray mass spectrometric data were compared with potentiometric studies. The potentiometric data revealed no copper binding for mono- and tetrasaccharides of cellulose and chitin, while the mass spectrometric data suggested the presence of distinct copper complexes, these attributed to charge attachment during desorption from the highly charged electrospray droplets. Difference in the distribution of tetrasaccharide–copper complexes for cellulose and chitin was attributed to possible differences in droplet solution conformations of these saccharides. For mono-, di- and tetrasaccharides of chitosan the potentiometric data showed strong copper complexation. For the tetrasaccharide the major species expected at pH 7 is a neutral copper complex. This neutral copper–tetrasaccharide complex was observed in the protonated form at relatively low abundances under high-voltage conditions in the electrospray interface. In this complex, Cu(II) is co-ordinated to the C(1)-alkoxide, the four amine nitrogens of chitosan tetrasaccharide and an anion. The ability of electrospray mass spectrometry to probe the immediate environment of the complex provides a clearer picture of the complex as it exists in solution, the associated counter-ions and possible differences in conformation. However, electrospray mass spectrometric data best reflect chemistries in highly charged droplets rather than systems at equilibrium, highlighting the significance of supporting techniques to fully understand a chemical system. © 1997 John Wiley & Sons, Ltd.