Specific enrichment of phosphopeptides by using magnetic nanocomposites of type FeO@graphene oxide and FeO@C coated with self-assembled oligopeptides.

Iron(III-immobilized magnetic nano-composites (MNCs) were first fabricated using one-step aqueous self-assembly of oligopeptides (Glu-Pro-Ala-Lys-Ala-Lys-Ala-Lys; EPAK-VI) for the highly selective capture of phosphopeptides from complex biological samples. Under physiological conditions, EPAK-VI can readily self-organize into a robust and complete coating layer mainly composed of β-sheets and β-turns on the surface of FeO@GO and FeO@C MNCs. Tailored by the cyclic structure of proline, the Glu-Pro motifs of EPAK-VI are vertically erected on the surface and thus serve as an effective linker to chelate Fe through carboxyl (COO-) group in the glutamic acid (E) residues. The ionic hydrogen bonds between the ε-amino groups and the surface negative charges coupled with intermolecular hydrogen bonds render the EPAK-VI coating on the MNCs insusceptible to repeated extreme washing conditions. The Fe-EPAK-VI coated MNCs exhibit high enrichment efficiency for β-casein tryptic digest (0.05 fmol μL), excellent selectivity from mixed digests (β-casein/bovine serum albumin, mass ratio 1:500), and high recovery rate (over 80%). Graphical abstractSchematic representation of the fabrication of Fe-immobilized MNCs for phosphopeptide enrichment.