Interfacial Hyperactivation of Candida rugosa Lipase onto Ca₂Fe₂O₅ Nanoparticles: pH and Ionic Strength Fine-Tuning...

The current study provides a comprehensive look of the adsorption process of Candida rugosa lipase (CRL) on Ca₂Fe₂O₅ iron oxide nanoparticles (NPs). Protein-support interactions were identified across a broad range of pH and ionic strengths (mM) through a response surface methodology, surface charge determination, and spectroscopic and in silico analyses. The maximum quantity of immobilized protein was achieved at an ionic strength of 50 mM and pH 4. However, this condition did not allow for the greatest hydrolytic activity to be obtained. Indeed, it was recorded at acidic pH, but at 150 mM, where evaluation of the recovered activity revealed hyperactivation of the enzyme. These findings were supported by adsorption isotherms performed under different conditions. Based on zeta potential measurements, electrostatic interactions contributed differently to protein-support binding under the conditions tested, showing a strong correlation with experimentally determined immobilization parameters. Raman spectra revealed an increase in hydrophobicity around tryptophan residues, whereas the enzyme immobilization significantly reduced the phenylalanine signal in CRL. This suggests that this residue was involved in the interaction with Ca₂Fe₂O₂ and molecular docking analysis confirmed these findings. Fluorescence spectroscopy showed distinct behaviors in the CRL emission patterns with the addition of Ca₂Fe₂O₅ at pH 4 and 7. The calculated thermodynamic parameters indicated that the contact would be mediated by hydrophobic interactions at both pHs, as well as by ionic ones at pH 4. In this approach, this work adds to our understanding of the design of biocatalysts immobilized in iron oxide NPs.