Phytic acid functionalized Fe3O4 nanoparticles loaded with Ti(IV) ions for phosphopeptide enrichment in mass spectrometric analysis

A novel magnetic nanomaterial for use in metal ion based affinity chromatography is described. It is based on the chelation between the phosphate groups of phytic acid (PA) and Ti(IV) ions. Due to the large number (6) of phosphate groups of PA, it has a large capacity for Ti(IV) ions. PA was first immobilized on magnetite nanoparticles (PA-MNPs) and then loaded with Ti(IV) ions to obtain the sorbent (Ti-PA-MNPs). The fraction of Ti(IV) ions on the surface of PA-MNPs that is exposed to the solution binds the phosphate groups of phosphopeptides. The bound phosphopeptides can then be magnetically separated. The method was applied to the enrichment of the phosphopeptides in a β-casein tryptic digest. A tryptic digest of bovine serum albumin (BSA) was added at a molar ratio (β-casein to BSA) of 1:2000 to study selectivity. The phosphopeptides were quantified by mass spectrometry. The limit of detection can be as low as 8 × 10−10 mol L−1. This sorbent has a high absorption capacity (53.5 μg mg-1) and shows good recoveries (90%). As many as 2145 phosphopeptides were isolated from 500 μg tryptic digest of a rat liver lysate after enrichment by Ti-PA-MNPs. This is superior to that (1568 phosphopeptides) of commercial TiO2 kit.