Robust Dual-Biomimetic Titanium Dioxide-Cellulose Monolith for Enrichment of Phosphopeptides

Metal oxide affinity chromatography (MOAC) is considered to be one of the most effective methods for phosphopeptide enrichment. However, most of the materials used in the method are powder; frequent centrifugation is necessitated during the enrichment process, and potential risks of loss of peptides and materials and clogging of the column employed for liquid chromatography-mass spectrometry (LC-MS) arise. Moreover, the reusability of these materials to achieve sustainability was hardly investigated. To overcome these limitations, herein, inorganic titanium dioxide (TiO₂) was coated onto the skeletal surface of the organic cellulose monolith (CM) material with a coral-like structure via a sol-gel method. This produced an organic-inorganic hybrid TiO₂-CM material, which contained a combination of organic and inorganic substances, making it mimic the mollusk shell in terms of composition. The prepared TiO₂-CM material as monolith exhibited excellent mechanical strength and did not break during the enrichment process; thus, the tedious implementation of multiple centrifugation cycles was prevented, thereby streamlining the experimental procedure and avoiding the loss of peptides and materials. Moreover, a large amount of TiO₂ was introduced onto the surface of the CM material, and thus, the resultant TiO₂-CM material exhibited a large surface area. As a result, the fabricated TiO₂-CM material was successfully applied to the enrichment of phosphopeptides obtained from the tryptic digests of a BSA/β-casein (mass ratio, 500/1) mixture. The results were superior to those achieved for commercial TiO₂ beads, confirming that TiO₂-CM has excellent selectivity for phosphopeptides and reusability. Furthermore, 9287 unique phosphopeptides derived from the 2661 phosphoproteins were successfully identified from two milligrams of tryptic digests of Hela cell exosomes obtained through five independent replications after enriching using the TiO₂-CM material. The results indicated that the material has good application prospects in the analysis of protein phosphorylation. Furthermore, TiO₂-CM consists of green and cheap cellulose as the skeleton, and its synthesis process is environment-friendly, simple, and inexpensive.