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Melanins are polymorphous and multifunctional biopolymers with a relatively high concentration of freeradicals. EPR spectroscopy was used to study o-semiquinone free radicals in model eumelanins synthesizedfrom 3,4-dihydroxyphenylalanine (DOPA) and tyrosine in the presence of tyrosinase, and melanins isolatedfrom A-375 and G-361 human melanoma malignum cells exposed to two compounds: 5,7-dimethoxycoumarin(DMC) and valproic acid (VPA). Changes were determined in the concentrations of free radicals in theindividual melanins from tumour cells treated with DMC and VPA. A strong decrease in the concentrationsof free radicals characterizes melanins isolated from tumour cells treated together with DMC and VPA.Slow spin-lattice relaxation processes were noted in the melanins tested with homogeneous broadened EPRspectra. The EPR technique may be useful not only for the elucidation of free radicals in melanins fromA-375 and G-361 cells treated with VPA and DMC but it could also be applied to establish the relationshipbetween melanin type and the malignancy of melanoma malignum

Electron paramagnetic resonance (EPR) spectroscopy was applied to measure the influence of two aminoglycoside antibiotics: gentamicin and kanamycin on free radical properties of DOPAmelanin. DOPA-melanin was formed by oxidative polymerization of 3,4-dihydroxyphenylalanine. Different concentrations of gentamicin and kanamycin (from 1 9 10 -4 to 1 9 10 -2 M) were used. o-Semiquinone free radicals with a g factor of 2.0043 were found in all studied melanin samples. Their concentrations in the DOPA-melanin-drug complexes were higher than in DOPA-melanin, and increased with the increase of gentamicin and kanamycin concentration. A single EPR line of the analyzed samples (ABp~, 0.48-0.52 mT) indicates that aminoglycoside antibiotics do not create a new type of free radicals in DOPA-melanin. Microwave saturation behavior of the experimental lines indicates the homogeneous broadening of resonance absorption curves for DOPA-melanin and its complexes with aminoglycosides. The EPR lines saturate at low microwave powers. Slow spin-lattice relaxation processes were characteristic for all studied melanin samples.

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