Getting COSY with Methanobactin : NMR chromopeptide characterization

Abstract

Methanobactin (mb) is a chalkophore produced by Methylosinus trichosporium OB3b. This non-ribosomal chromopeptide is a part of the copper acquisition system of these methane oxidizing bacteria. We have obtained NMR spectra of mb after titration with Cu(II). This is only possible because mb reduces Cu(II) to Cu(I) upon binding and thus becomes diamagnetic and slightly less soluble. We have not yet identified the specific reductant which is also capable of reducing Hg(II), Ag(I) and Au(III). Cu(I) in solution is typically unstable; however, it remains stable in solution when bound to mb. Mb is thought to bind metal ions using nitrogen and sulfur ligands from its 4-thiocarbonyl-5-hydroxy imidazole (THI), 4-hydroxy-5-thiocarbonyl imidazole (HTI) and possibly tyrosine, methionine and other residues. Our experiments show that there are considerable changes in their environments denoted by significant changes in the proton NMR spectra of mb when it is bound to copper versus copper-free mb. Residue assignments have been made using COSY and TOCSY measurements. Protons on the nitrogens of HTI and THI have been identified using 15N NMR.