Probing Copper−Thioether Coordination Chemistry in Rusticyanin and Azurin by 2D ¹H−¹⁹⁹Hg NMR

1H {199Hg} HMQC spectra of the blue copper proteins rusticyanin and azurin exhibit signals from three amino acids that bind the metal center, i.e. one cysteine and two histidine ligands. Spectra of rusticyanin, but not azurin, also exhibit 199Hg coupling to methionine methyl protons suggesting significantly less σ-bonding character or a greater dynamic character of the Hg−S(Met) bond in the latter. 199Hg NMR methods thus reveal subtle aspects of the coordination chemistry in copper proteins.