Graft modification of cotton with phosphate group and its application to the enrichment of phosphopeptides

Grafting copolymerization, especially "grafting from" approach, has attracted a great attention for the preparation of cellulose-based materials with various functionalities. In this study, a novel phosphate group-containing cotton fiber-based material, CF-NH₂-AZO-p(VPA-x), was successfully synthesized by a "grafting from" radical polymerization approach using vinyl phosphonic acid (VPA) as monomer. The phosphate group content of CF-NH₂-AZO-p(VPA-x) could be easily regulated by adjusting the concentration of VPA monomer. Subsequently, titanium ions immobilized CF-NH₂-AZO-p(VPA-x) (CF-NH₂-AZO-p(VPA-x)-Ti⁴⁺) was prepared and used as an immobilized metal affinity chromatography (IMAC) adsorbent for the selective enrichment of phosphopeptides from various biological samples. The relationship between enrichment efficiency and phosphate group content of CF-NH₂-AZO-p(VPA-x)-Ti⁴⁺ was investigated. The optimized CF-NH₂-AZO-p(VPA-2)-Ti⁴⁺ (x=2) exhibited high selectivity and extraction capacity in phosphopeptides enrichment from standard peptides mixture, non-fat milk digests and protein-rich human serum. In addition, CF-NH₂-AZO-p(VPA-2)-Ti⁴⁺ was further applied for the specific capture of phosphopeptides from tryptic digests of rat brain lysate, and 3241 unique phosphopeptides were identified from 0.5mg of rat brain by combining CF-NH₂-AZO-p(VPA-2)-Ti⁴⁺ pretreatment with liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. We expect the proposed method can promote the development of "grafting from" approach in the preparation of cellulose-based adsorbents and broaden the application of cellulose-based adsorbents for biological analysis.