Hydrophilic spacer-arm containing magnetic nanoparticles for immobilization of proteinase K: Employment for speciation of proteins for mass spectrometry-based analysis.

Proteinase K (ProK) is used for the degradation of proteins in cell lysates to isolate nucleic acids, and for the speciation of proteins for mass spectrometry analysis. In this work, a novel and sensitive immobilization process was developed for examination of protein mixtures by combining MALDI-ToF-MS and nLC-TIMS-ToF-MS/MS systems. To achieve these goals, magnetic nanoparticles (MPs) were prepared via thermal coprecipitation reaction under alkaline condition. The MPs were grafted with a silica layer (i.e., 3-(2,3-epoxypropoxy) propyltrimethoxysilane; EPTES) containing reactive epoxy groups. Then, the silica-grafted magnetic particles were coated with a long chain hydrophilic poly(ethylene glycol) diamine polymer (PEGDAP). The prepared materials were characterized by the Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM) and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. The VSM data show that the MPs@EPTES@PEGDAP has paramagnetic performance with a saturation magnetization of approximately 32.3 emu g-1. Proteinase K (EC 3.4.21.64) was covalently immobilized on the MPs@EPTES by reaction of its epoxy groups with amine groups of the enzyme. On the other hand, the ProK was immobilized on the MPs@EPTES@PEGDAP after activation with glutaraldehyde and the immobilization reaction was realized by the coupling reaction between aldehyde groups of the support and amine groups of the enzyme. The amounts of immobilized ProK on the MPs@EPTES and MPs@EPTES@PEGDAP were found to be 27.4 and 19.6 mg g-1and the retained activities were determined to be 29 and 87%, respectively. For the first time, some important features such as thermal and storage stabilities, reusability and potential use in protein speciation for mass spectrometry-based techniques were also evaluated. For examples, after six weeks of storage at 4 °C, the immobilized ProK on the MPs@EPTES@PEGDAP-ProK still maintained 59% of its initial activity. However, at the end of the six-week storage period, its free counterpart had lost all of its initial activity. The immobilized ProK was also utilized for degradation and identification of model proteins (i.e., α-2-HS glycoprotein, ß-casein, bovine serum albumin and immunoglobulin). After enzymatic treatment, the digested peptides were analyzed and mapped by using nLC-TIMS-ToF-MS/MS systems.