Sulfonated calix[4]arene functionalized SiO₂@TiO₂ for recognition of lysine methylation

Lysine methylations are common protein post-translational modifications (PTMs), that play significant roles in regulating gene activities. Studies of their functions and connections with diseases have important values. However, due to the small variations from their native structures and very low component proportions, it is very difficult to extract methylated peptides from biological mixtures. In this research, a new material that utilizes sulfonated calix[4]arene (SC4A) as the recognition unit and silica coated with TiO₂ as carrier, denoted as SiO₂@TiO₂@SC4A, was synthesized. The equilibrium binding experiments demonstrated that SiO₂@TiO₂@SC4A can identify lysine and arginine methylation and peptides with these methylated residues. The maximum isotherm binding capacities are 70.0, 55.9, 31.4 and 24.8 μmol g^-1 for Lys(Me₃), Lys(Me)₂, Lys(Me) and Lys, respectively. It demonstrated that the higher the degree of methylation, the stronger the interactions. In addition, the analyses of high performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) demonstrated that peptides with methylated lysine or arginine can be selectively extracted from spiked histone trypsin digestion. The recoveries for the spiked GGAK(Me)R, GGAKR(Me)₂ and GGAK(Me)₃R are 83%, 78%, and 84% respectively. The experiments from the nuclear extracts of HeLa cells also illustrated that SiO₂@TiO₂@SC4A holds a potential in the enrichment and identification of lysine methylations.