Proton NMR studies of Co(II) complexes of the peptide antibiotic bacitracin and analogues: insight into structure-activity relationship.

Bacitracin is a widely used metal-dependent peptide antibiotic produced by Bacillus subtilis and Bacillus licheniformis with a potent bactericidal activity directed primarily against Gram-positive organisms. This antibiotic requires a divalent metal ion such as Zn(II) for its biological activity, and has been reported to bind several other transition metal ions, including Co(II), Ni(II), and Cu(II). Despite the wide use of bacitracin, a structure-activity relationship for this drug has not been established, and the structure of its metal complexes has not been fully determined. We report here one- and two-dimensional nuclear magnetic resonance (NMR) studies of the structure of the metal complexes of several bacitracin analogues by the use of paramagnetic Co(II) as a probe. The Co(II) complex of this antibiotic exhibits many well-resolved isotropically shifted (1)H NMR signals in a large spectral window ( approximately 200 ppm) due to protons near the metal, resulting from both contact and dipolar shift mechanisms. The assignment of the isotropically shifted (1)H NMR features concludes that bacitracin A(1), the most potent component of the bacitracin mixture, binds to Co(II) via the His-10 imidazole ring N(epsilon), the thiazoline nitrogen, and the monodentate Glu-4 carboxylate to form a labile complex in aqueous solutions. The free amine of Ile-1 does not bind Co(II). Several different analogues of bacitracin have also been isolated or prepared, and the studies of their Co(II) binding properties further indicate that the antimicrobial activity of these derivatives correlates directly to their metal binding mode. For example, the isotropically shifted (1)H NMR spectral features of the high-potent bacitracin analogues, including bacitracins A(1), B(1), and B(2), are virtually identical. However, Glu-4 and/or the thiazoline ring does not bind Co(II) in the bacitracin analogues with low antibiotic activities, including bacitracins A(2) and F.