EPR Evidence on the Structure of the Copper(II)-Bacitracin A Complex

Bacitracin is an antibiotic that is effective only in the presence of divalent metal cations. In this paper, the possible sites for the binding of divalent transition metal ions to bacitracin A are investigated by electron paramagnetic resonance. Manganese(II) shows only the characteristics of the aqueous ion. High-spin cobalt, suggesting distorted octahedral coordination, is indicated from the cobalt(II) EPR spectra. The copper(II) complex yields a rich spectrum and detailed structural information. The electronic Zeeman, hyperfine, and nuclear quadrupole coupling parameters show that copper is most probably square planar, coordinated to the glutamate γ-carboxyl, the histidine imidazole, the aspartate β-carboxyl, and the thiazoline nitrogen. This is the first reported case of a significant influence of quadrupole coupling on the EPR spectra of solutions of metal compounds of such complexity or of biological significance.