Free radicals in ultraviolet irradiated melanins and melanin complexes with Cd(II) and Cu(II) – EPR examination

• Free radical concentration in eumelanins increased after UV irradiation.
• Free radical concentration decreased in irradiated pheomelanin.
• Cd(II) and Cu(II) change free radical concentration in the studied melanins.
• EPR lines of all the studied samples are homogeneously broadened.
• EPR is useful to study the effect of UV irradiation on free radicals in melanins.

Effect of UV irradiation on free radicals in different types of melanins and melanin complexes with diamagnetic Cd(II) and paramagnetic Cu(II) was examined by the use of electron paramagnetic resonance (EPR) spectroscopy. The aim of this studies was to compare o-semiquinone free radicals formation in two model eumelanins synthesized from 3,4-dihydroxyphenylalanine (DOPA) and tyrosine in the presence of tyrosinase, and in synthetic pheomelanin, under exposition on ultraviolet, because of the important role of free radicals and melanins in human organism. UV may change free radical concentrations in melanin. Changes in EPR spectra of DOPA-melanin-Cd(II) and DOPA-melanin-Cu(II) complexes after UV irradiation were determined. Diamagnetic Cd(II) strongly increased free radical concentrations in DOPA-melanin. UV irradiation during 30 and 60 min slightly increased and decreased free radical concentrations in DOPA-melanin-Cd(II) complexes, respectively. Paramagnetic Cu(II) quenched free radical lines of DOPA-melanin, and only the Cu(II) signals were detected for both UV-irradiated and nonirradiated samples. Free radical concentration in both eumelanins increased after UV irradiation, but it decreased in irradiated pheomelanin. EPR spectra of free radicals in the studied samples were homogeneously broadened. Slow spin-lattice relaxation processes exist in all the examined melanins and DOPA-melanin-Cd(II) complexes. Fast spin-lattice relaxation processes characterized Cu(II) in DOPA-melanin-Cu(II) complexes.

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