Facile synthesis of Ti4+-immobilized magnetic covalent organic frameworks for enhanced phosphopeptide enrichment

In this work, core-shell structured Ti4+-immobilized magnetic covalent organic frameworks (denoted as Fe3O4@TAPTDHTA-Ti(4+)composites) were prepared for enhanced phosphopeptide enrichment by one-pot synthesis of COFs shell with inherent bifunctional groups on Fe3O4 NPs and further Ti4+ immobilization. The widely distributed bifunctional groups could provide abundant chelating sites for Ti4+ immobilizing. Combining with the high specific surface area and mesoporous structure, the Fe3O4@TAPTDHTA-Ti4+ composites exhibited excellent enrichment efficiency for phosphopeptides, such as low detection limit (0.05 fmol mu L-1), high selectivity (1:5000 of molar ratio of beta-casein/bovine serum albumin (BSA) tryptic digests), high adsorption capacity (62.9 mu g mg(-1)) and strong size-exclusive effect (1:250:250 of molar ratio of beta-casein tryptic digest/beta-casein/ BSA). In addition, this method was general for immobilizing other metal ions (Zr4+ and Fe3+). Notably, the Fe3O4@TAPTDHTA-Fe3+ composites exhibited controllable affinity towards mono-phosphopeptides and multi-phosphopeptides. Furthermore, the Fe3O4@TAPTDHTA-Ti4+ composites were successfully applied to selectively capture phosphopeptides from complex biological samples including the tryptic digest of nonfat milk, human serum and human saliva. More significantly, 3333 phosphopeptides derived from 1409 phosphoproteins with 3492 phosphorylation sites were clearly identified from the tryptic digest of HeLa cell lysate. In addition to providing a potential excellent enrichment probe for comprehensive phosphoproteomic analysis, this study also offers a new perspective for the functionalization of COFs.