Melanins are the large organic compounds, which belong to natural pigments [1]. Melanin biopolymers bind major chemical compounds, especially metal ions, polycyclic aromatic hydrocarbons and drugs [1]. The binding of drugs by melanin biopolymers depends on the presence of free radicals and metal ions in their structure [1-3].
In this work effect of Cd(II) on free radicals in the model eumelanin as DOPA-melanin was examined. Electron paramagnetic resonance (EPR) spectroscopy was used as the experimental method. EPR spectra of DOPA-melanin, and DOPA-melanin-Cd(II) complexes were measured by an X-band (9.3 GHz) EPR spectrometer of Radiopan Firm (Poznań, Poland) and the Rapid Scan Unit of Jagmar Firm (Kraków, Poland). Synthetic DOPA-melanin was formed by the autooxidative polymerization of 3,4-dihydroxyphenylalanine (DOPA). The metal ions/DOPA molar ratios in the reaction mixtures were 2:1, 1:1 or 1:2. Free radical concentration in DOPA-melanin was high. Formation of melanin complexes with Cd(II) increased free radical concentration in DOPA-melanin. For the samples DOPA-melanin-Cd(II) (2:1) and DOPA-melanin-Cd(II) (1:1) free radical concentration increased with the increasing of Cd(II) in melanin. Free radical concentration in DOPA-melanin-Cd(II) (1:2) complexes were lower than for DOPA-melanin-Cd(II) (1:1) complexes. This effect was the result of free radical recombination during formation of these samples. g-Values characteristic for o-semiquinone free radicals were obtained. Broad EPR lines were measured. Linewidths increased after Cd(II) binding to DOPA-melanin. Changes of amplitudes and linewidths with microwave power shown the homogeneous broadening of EPR lines of DOPA-melanin complexes with Cd(II), independently on the metal ion concentration. Slow spin-lattice relaxation processes existed in all the tested samples, their EPR lines saturated at low microwave powers. Cd(II) caused fastening of spin-lattice relaxation processes in DOPA-melanin. The results are very important for evaluation of drugs binding to eumelanin in the presence of Cd(II).

References:

1. Tran M.L., Powell B.J., Meredith P. (2006). Chemical and structural disorder in eumelanins: a possible explanation for broadband absorbance. Biophysical Journal, Vol. 90, pp. 743-752.
2. Buszman E., Pilawa B., Zdybel M., Wrześniok D., Grzegorczyk A., Wilczok T. (2005). EPR examination of Zn²⁺ and Cu²⁺ effect on free radicals in DOPA-melanin-netilmicin complexes. Chemical Physics Letters, Vol. 403, pp. 22-28.
3. Matuszczyk M., Buszman E., Pilawa B., Witoszyńska T., Wilczok T. (2004). Cd²⁺ effect on free radicals in Cladosporium cladosporioides-melanin tested by EPR spectroscopy. Chemical Physics Letters, Vol. 394, pp. 366-371.

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