Guanidyl-Functionalized Magnetic Bimetallic MOF Nanocomposites Developed for Selective Enrichment of Phosphopeptides

In this work, guanidine group-modified magnetic hafnium and titanium bimetallic metal-organic framework (MOF) nanocomposites denoted as Fe3O4@Hf/Ti-MOF-Gua were developed by a solvothermal method and post-synthetic route. Through combining metal affinity chromatography with hydrophilic interaction, the as-prepared nanomaterial was used for identifying phosphopeptides from tryptic digests of alpha-/beta-casein by matrixassisted laser desorption ionization-time of flight mass spectrometry and showed high sensitivity (20 fmol), high selectivity for phosphopeptides from alpha-/beta-casein tryptic digests/phosphoprotein a-casein/BSA (1:1:2000:2000), and great reusability of three circles for capturing phosphopeptides. Because of excellent enrichment performance with a high recovery of 85.4%, this affinity probe was subsequently applied to real samples, and 27 and 15 phosphopeptides were identified from nonfat milk and human saliva, respectively. The above remarkable advantages benefitted from the strong affinity of abundant Hf4+/Ti4+ on the large surface of the MOF shell with the improved hydrophilicity from a large number of guanidyl groups. Consequently, the novel Fe3O4@Hf/Ti-MOF-Gua nanocomposites not only efficiently captured phosphopeptides but also removed macromolecular proteins, indicating their great potential for the application in identification and further analysis of low-abundance phosphopeptides from complex biological samples.