Cu(II) complexes in bacterial growth medium: electron spin resonance study

In this study we report a spectroscopic investigation on the structure and stability of Cu(II)-complexes that are formed in a minimum growth medium (M M), normally used for Bacillus subtilis cultures. As other transition metal s, Cu(II) compounds are toxic to this bacterium and the toxicity depends on the Cu(II) concentration. MM contained NH; ions and asparagine (asn) as th e source of inorganic and organic nitrogen. Both ESR and electronic spectra demonstrated the very important role played by the amino acid asparagine i n the coordinative behaviour of Cu(II). In particular, three different comp lexes were evidenced: Cu(H2O)(6)(2+);Cu(asn)(+) and Cu(asn)(2). The relativ e amount of these three species strongly depended on pH, on Cu:asn ratio an d on the presence of the phosphate ions. They were identified and evaluated quantitatively by extensive simulation of the electron spin resonance (ESR ) spectra recorded in different experimental conditions. The bis-complex wa s found to be more stable in MM than in an asparagine-containing water solu tion with the same Cu:asn ratio. A comparison of the spectroscopic results with microbiological investigations is also made. (C) 2000 Elsevier Science B.V. All rights reserved.