Links between peptides and Mn oxide: nano-sized manganese oxide embedded in a peptide matrix

Abstract

Herein we report on a poly-peptide/Mn oxide nanocomposite as a model for the water-oxidizing catalyst in Photosystem II. The material is obtained by the reaction of poly-L-tyrosine, Mn(OAc)₂·4H₂O and KMnO₄. HRTEM images of the nanocomposite indicate the presence of nanoparticles dispersed in a poly-L-tyrosine matrix with no crystalline phases. A broad peak at ∼400–550 nm in the UV-visible spectrum is related to Mn oxide formation. X-ray absorption studies suggest that the Mn atoms are in the oxidation state of +IV and a coordination number of around three was obtained for Mn in the structure. The water-oxidizing activity of the material under electrochemical conditions was also investigated. Electrochemical experiments proved the presence of Mn(IV) oxide. Determination of oxygen coupled to electrochemistry revealed the role of poly-L-tyrosine in decreasing the overpotential for water oxidation. In the next step, the interaction of the nanocomposite and imidazole as a model for the tyrosine 161 and histidine 190 residues in Photosystem II was studied with fluorescence spectroscopy using the multivariate curve resolution-alternating least squares method.