Adsorption of trypsin onto pH sensitive poly-N-vinylpyrrolidone-co-poly-4-vinylpyridine and its magnetic based hydrogel: Sorption isotherms and thermodynamic parameters

Amphiphilic pH-sensitive hydrogels containing magnetite Fe3O4 nanoparticles were obtained by radical copolymerization of N-vinylpyrrolidone and 4-vinylpyridine and cross-linking of the latter with N,N-methylenebisacrylamide. The synthesized neat and magnetite hydrogels were analyzed by physicochemical characterization as FT-IR, TQA, SEM, X-Ray and swelling ratio. The incorporation of Fe3O4 nanoparticles into the hydrogel structure has been shown to improve crystallinity and thermal stability. The sorption of trypsin by PVPr-so-P4VP and PVPr-so-P4VP/Fe3O4 hydrogels in aqueous solution was studied as well as the effect of the hydrogel dose, average pH, temperature, ionic strength, contact time and initial enzyme concentration on the process. The best adsorption equilibrium state for PVPr-so-P4VP-amphiphile hydrogel was demonstrated at 75 mg, pH = 7 (initial trypsin concentration: 100 mg×L−1, contact time: 80 min) at 25–45 °C. However, for PVPr-so-P4VP/Fe3O4-based gel, it happens within 60 min at a dosage of 100 mg and an initial solution of trypsin of 150 mg×L−1, and the protein capacity was 53.5 ± 2 and 68.54 ± 3 mg×gr−1, respectively. The results of trypsin adsorption were applied to Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models, I and II order kinetics. The maximum monolayer protein capacity was determined to be 71–72 mg×gr−1 at 45 °C for the hydrogel PVPr-so-P4VP/Fe3O4. It was determined that trypsin sorption by hydrogel obtained from the cross-linking of the copolymer PVPr-so-P4VP was more suitable for the isothermal models Freundlich and Langmuir, partially D-R (R2 =0.98). Trypsin adsorption by crosslinking of the hydrogel PVPr-so-P4VP/Fe3O4 is only suitable for Langmuir isotherm models (R2 =0.9961). The KL of the two hydrogels PVPr-co-P4VP and PVPr-co-P4VP/Fe3O4 was greater than one and was 1.218 and 5.880, respectively. The thermodynamic parameters such as free energy, enthalpy, and entropy were quantified. It has been observed that the temperature increase up to 45 °C proves that the process is endothermic, and after 45 °C the trypsin molecule begins to become more difficult to adsorb. The value of ∆G° is negative for hydrogels, but ∆S° is positive (83.39 and 116.41 J×mol−1K−1, respectively). Based on the results, PVPr-co-P4VP and PVPr-co-P4VP/Fe3O4 hydrogels can be used as adsorbents for chromatographic separation of proteins and drugs, and as matrices for administration and long-term effects for enzymes.