Formation of Protein–Birnessite Complex: XRD, FTIR, and AFM Analysis

Limited information is available on formation chemistry of enzyme–Mn oxide complexes. Adsorption isotherm of protein molecules (tyrosinase) on birnessite (δ-MnO2) at pH 6.0 and room temperature (23°C) was of H type, indicating a very high affinity of the enzyme protein molecules to the birnessite mineral surfaces. After thorough washing of the protein–mineral complex with deionized-distilled water, up to 89% of adsorbed protein molecules remained bound to the mineral surfaces. When a high amount of the protein was immobilized, the X-ray diffractogram shows a significant decrease in the intensity of characteristic d-spacings of birnessite. No shift to higher values of the d-spacings of protein–birnessite complex was observed, indicating that the enzyme molecules were not intercalated in the mineral structure but immobilized at the external surfaces and the edges of the mineral oxide. By comparison to the free enzyme, infrared absorption spectra of the protein–birnessite complexes show a shift by up to 11 cm−1 to lower frequencies in the absorption bands characteristic of amide I and II modes of the polypeptides chains. The mineral surfaces exerted some strain on the protein structure, resulting in an alteration of the protein molecular conformation after binding to the mineral colloid surfaces. In the free state, the globular protein molecules had a spheroid shape with an average cross-sectional diameter of 70±6 nm. The unfolding and flattening of the protein molecules after immobilization is clearly shown in atomic force micrographs. Compared to the tyrosinase–birnessite complex, similar FTIR spectra and atomic force micrographs were observed for the pure protein, bovine serum albumin (BSA), after immobilization on birnessite. The information obtained in this study is of fundamental significance for understanding birnessite as an adsorbent of biopolymers and the catalytic role of the enzyme–birnessite complex.