NMR, Mass Spectrometry and Chemical Evidence Reveal a Different Chemical Structure for Methanobactin That Contains Oxazolone Rings

Methanobactin (mb) is a small copper-binding peptide produced by methanotrophic bacteria and is intimately involved in both their copper metabolism and their role in the global carbon cycle. The structure for methanobactin comprises seven amino acids plus two chromophoric residues that appear unique to methanobactin. In a previously published structure, both chromophoric residues contain a thiocarbonyl attached to a hydroxyimidazolate ring. In addition, one is attached to a pyrrolidine ring, while the other is attached to an isopropyl ester. A published X-ray determined structure for methanobactin shows these two chromophoric groups forming an N2S2 binding site for a single Cu(I) ion with a distorted tetrahedral geometry. In this report we show that NMR, mass spectrometry, and chemical data reveal a chemical structure that is significantly different than the previously published one. Specifically, the 1H and 13C NMR assignments are inconsistent with an N-terminal isopropyl ester and point instead to a 3-m...