Conformational changes of proteins adsorbed onto ZnO nanoparticle surfaces investigated by concentration-dependent infrared spectroscopy

We conducted an infrared spectroscopic study of the concentration-dependent conformational changes of bovine serum albumin (BSA) and transferrin (Tf) on ZnO nanoparticles (NPs) functionalized with self-assembled monolayers. The ZnO NPs used as a platform for protein conjugation were characterized by their UV absorption spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Self-assembled thin films of 16-phosphonohexadecanoic acid (16-PHDA) for anchoring proteins onto ZnO surfaces were examined by diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy. The presence of the strong ν(CO) band at ∼1700 cm−1, along with the significant changes of the P–OH, P–O, and PO bands at 930–1215 cm−1 in the DRIFT spectra, indicate that at least some of the 16-PHDA appeared to adsorb onto the ZnO surfaces via its phosphonic acid group. Protein-ZnO nanoparticle (NP) conjugates were assembled by a 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC) coupling reaction. The protein conjugations of bovine serum albumin (BSA) and transferrin (Tf) were verified by identifying their amide bands in the infrared spectra. By referring to the peak analysis of the amide I bands, different surface-induced conformational changes of the two proteins could be observed upon their adsorption on ZnO.