Unfolding and inactivation of proteins by counterions in protein-nanoparticles interaction

• CLC-IONPs can selectively bind charged protein and denature it using counterions.
• Protein can be deactivated by denaturing secondary folding on RCPC interaction.
• Living cells can be killed by deactivating its constituent proteins.
• We propose cell apoptosis using CLC-IONPs via the RCPC model.
• Selective cell targeting by CLC-IONPs needs to be investigated.

In this work, the structure and activity of proteins; such as, hen egg lysozyme (HEWL) and calf intestine alkaline phosphatase (CIAP); have been investigated after incubation with surface coated iron oxide nanoparticles (IONPs) in water. IONPs were coated with counterions bound charge-ligands and were named as the charge-ligand counterions iron oxide nanoparticles (CLC-IONPs). The coating was done with tri-lithium citrate (TLC) and tri-potassium citrate (TKC) to have negative surface charge of CLC-IONPs and Li+ and K+, respectively, as counterions. To have positive surface charge, IONPs were coated with cetylpyridinium chloride (CPC) and cetylpyridinium iodide (CPI) having Cl− and I−, respectively, as counterions. The secondary structure of proteins was measured using far ultraviolet circular dichroism (CD) spectroscopy which showed that both proteins were irreversibly unfolded after incubation with CLC-IONPs. The unfolded proteins were seen to be functionally inactive, as confirmed through their activity assays, i.e., HEWL with Escherichia coli (E. coli) and CIAP with para-nitrophenyl phosphate (pNPP). Additionally, we have observed that monomeric hemoglobin (Hb) from radio-resistant insect Chironomus ramosus (ChHb) was also partially unfolded upon interaction with CLC-IONPs. This work clearly shows the role of counterions in protein inactivation via protein-nanoparticles interaction and, therefore, CLC-IONPs could be used for therapeutic purpose.

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