In this work, multifunctional Ti-immobilized phytic acid-modified magnetic graphene (denoted as MagG@PEI@PA-Ti) nanocomposites were fabricated through a facile route for simultaneous/respective enrichment of N-glyco- and phosphopeptides. Phytic acid (PA), with six phosphate groups, possesses excellent hydrophilicity and metal ion coordination ability, which endowed the MagG@PEI@PA-Ti with combined properties of immobilized metal ion affinity chromatography (IMAC)- and hydrophilic interaction liquid chromatography (HILIC)-based materials. On the basis of the different binding ability of N-glyco- and phosphopeptides on MagG@PEI@PA-Ti, the MagG@PEI@PA-Ti nanocomposites could enrich N-glyco- and phosphopeptides simultaneously or respectively by using different enrichment conditions, achieving controllable selective enrichment of N-glyco- and phosphopeptides. The proposed nanocomposites demonstrated an outstanding performance for selective enrichment of N-glycopeptides (selectivity, 1:1000 molar ratios of IgG/BSA; sensitivity, 0.5 fmol/μL IgG; loading capacity, 300 mg g; recovery, >90%) and phosphopeptides (selectivity, 1:5000 molar ratios of α-casein/BSA; sensitivity, 0.1 fmol/μL α-casein; loading capacity, 100 mg g; recovery, >90%). Taking advantage of these merits, a total of 393 N-glycopeptides derived from 259 glycoproteins and 574 phosphopeptides derived from 341 phosphoproteins were identified from 200 μg of HeLa cell extracts through a single-step enrichment using MagG@PEI@PA-Ti.

In view of the size and hydrophilicity of glycopeptides, materials having suitable channels (size-exclusion) and strong hydrophilic surface (hydrophilic interaction) are preferred to enrich the glycopeptides in biological samples. Metal–organic frameworks (MOFs) are good candidates. However, their smaller microporous channels and low chemical stability have limited the application. Herein, a facile strategy was established to construct hydrophilic mesoporous MOF via synergistic etching and surface functionalization by using phytic acid (PA). Besides, polyvinylpyrrolidone (PVP) was added during MOF synthesis to enhance the water stability of the MOF. Owing to the expanded hydrophilic mesoporous channels, the PA-modified Ce-MOF effectively and selectively captured 422 glycopeptides from 155 glycoproteins in tryptic digests of human serum (2 μL). The present work sheds light on the easy fabrication of hydrophilic mesoporous materials, and this established material holds unique advantages for glycopeptides analysis in biological samples.

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In this work, we fabricated a yolk-shell magnetic composite that contains mesoporous TiO as the inner shell and flowerlike NiO as the outer shell (denoted as FeO@H-TiO@f-NiO) to reduce the limitations of single-component metal oxides in phosphopeptide enrichment. The NiO nanosheets play a synergistic role in phosphopeptide enrichment. And the unique flowerlike structure of NiO with sufficient space can facilitate the reversible insertion/extraction of peptides, which will have less impact on the enrichment process of the inner TiO shell. The yolk-shell structure and two types of porous nanostructures endowed this composite with a high surface area (156.58 m g) and a large pore volume (0.37 cm g). Owing to the high surface area and combined properties of TiO and NiO, the FeO@H-TiO@f-NiO microspheres showed a better performance for phosphopeptide enrichment than the same material without NiO nanosheets (FeO@H-TiO). According to the LC-MS/MS results, 972 unique phosphopeptides were identified from HeLa cell extracts with a high selectivity (91.9%) by FeO@H-TiO@f-NiO relative to 837 phosphopeptides (selectivity: 60.2%) by FeO@H-TiO. The results demonstrated that, compared with single-component metal oxides, composite metal oxides could enhance the selectivity and sensitivity for phosphopeptide enrichment.