Hydrophilic modification of silica–titania mesoporous materials as restricted-access matrix adsorbents for enrichment of phosphopeptides

A new nano-scale restricted-access matrix (RAM) SiO2 (MCM-41) with relatively high Ti-content (Ti/Si = 0.1), but superior surface area (1129 m2/g), was successfully synthesised for the enrichment of phosphopeptides. The TiO2 was incorporated into the Si-MCM-41 via a hydrothermal process and the external surface was modified with alkyl diol by the successive hydrolysis of γ-(glycidyloxypropyl) trimethoxysilane (GPTMS). Scanning electron microscopy, transmission electron microscopy, N2 adsorption and Fourier transform infrared spectroscopy were used to characterise the alkyl diol-Ti-MCM-41. The appropriate pore diameter (2 nm) coupled to the marshy weeds-like hydrophilic external surface result in an efficient size-exclusion effect for the adsorption of standard cytochrome c with a molecular weight (MW) of ca. 12.4 kDa. At the same time, the strong affinity interaction between the incorporated titanium in the framework and the phosphoryl groups of phosphopeptides demonstrated a selective extract of phosphopeptides from the tryptic digestion. The detection sensitivity for phosphopeptides, determined by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) was as low as 5 fmol for standard tryptic digest of β-casein. Therefore, this alkyl diol-Ti-MCM-41 mesoporous material can be used as a potential adsorbent for applications in MS-based phosphoproteomics.