Magnetite nanoparticles coated with chitosan and polyethylenimine as anion exchanger for sorptive enrichment of phosphopeptides

An anion exchange solid-phase sorbent is described. Chitosan coated magnetite nanoparticles were modified with polyethylenimine which is positively charged at pH 3 and therefore can be used for the magnet-supported enrichment of phosphopeptides which are negatively charged at this pH value. A 2-step strategy was used to synthesize the sorbent. The materials were characterized by transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetry and magnetic moment analysis. The anion exchanger was applied to extract phosphopeptides from a beta-casein digest. Characteristic analytical figures include (a) a loading buffer of pH 3, (b) and elution buffer of pH 11, (c) a loading time of 5 min, (d) good selectivity (the beta-casein to BSA ratio is 1:1000), and (e) excellent sensitivity (1 fmol). The optimized method was applied to egg yolk digest, non-fat milk digest, and diluted human serum.