Electron paramagnetic resonance and ultraviolet-visible spectroscopic evidence for copper porphyrin in actinomycete melanins

Abstract

The coordination chemistry of iron (III) (Fe³⁺) and copper (II) (Cu²⁺) in melanins synthesized by seven actinomycetes isolated from Brazilian latosol soils under savanna (cerrado) vegetation was investigated using electron paramagnetic resonance (EPR) and ultraviolet-visible (UV-Vis) spectroscopy. The EPR spectra indicated the presence of Cu²⁺ ions bound to oxygenated and nitrogenous functional groups, and rhombic coordinated Fe³⁺ ions. In some of the actinomycete melanins the EPR hyperfine splitting of Cu⁺² ion complexes was well resolved, and indicated four magnetically equivalent nitrogen atoms in a plane. This result suggested the presence of Cu⁺² porphyrin complexes, which was confirmed by Soret bands in the 400-nm region of the UV-Vis spectra. The concentration of Mn in actinomycete melanins, determined by inductively coupled plasma atomic emission spectroscopy, was lower than those of Cu and Fe.