Copper complexation of methanobactin isolated from Methylosinus trichosporium OB3b: pH-dependent speciation and modeling

Methanobactins are copper-binding ligands produced by aerobic methanotrophic microorganisms. A quantitative understanding of their potential role in methanotrophic copper acquisition requires the investigation of their copper complexes under relevant pH conditions. In this study, a chemical speciation model describing the pH-dependence of copper binding and the formation of the different complexes by methanobactin (mb) is released by Methylosinus trichosporium OB3b was developed. Potentiometric and spectrophotometric titrations of the free ligand indicated the presence of four protonation sites consistent with the molecular structure of methanobactin. Metal titrations revealed a distinct pH-dependence of copper binding to methanobactin between pH 5 and 8. Based on evidence from size-exclusion chromatography coupled to inductively coupled plasma mass spectrometry (ICP-MS), the copper binding was quantitatively described with three different types of copper-methanobactin complexes which can additionally undergo protonation reactions. The high affinity observed upon initial copper additions resulted from the predominant occurrence of copper-methanobactin dimer complexes, mb(2)H(4)Cu and mb(2)H(3)Cu with log K values of 58 and 52, respectively. With increasing copper to methanobactin ratios, methanobactin bound copper as monomers, mbHCu (log K=25) and mbCu (log K=18), whereas at elevated copper activities methanobactin was able to bind two copper ions (mbHCu(2) and mbCu(2)). Model calculations based on the fitted complexation constants suggest that in natural systems, copper-methanobactin complexes are mostly present as monomers.