Interaction of free radicals of DOPA-melanin-streptomycin complexes with paramagnetic oxygen O2

Oxygen plays an important role in the metabolism of all tissues. The level of oxygen in the environment of melanin biopolymer in human body may change toxic effects resulted from free radicals formation in melanin–drug complexes.Using electron paramagnetic resonance (EPR) spectroscopy we measured the effect of molecular oxygen on concentration and properties of free radicals in 3,4-dihydroxyphenylalanine–melanin (DOPA–melanin) complexes with streptomycin. Studies have shown that free radical properties of melanin are heavily dependent on the presence of oxygen in the environment of the sample. Streptomycin increases the concentration of free radicals in DOPA–melanin but in the presence of oxygen this concentration is reduced.The EPR studies confirmed the existence of stable o-semiquinone free radicals (∼1018–1019 spin/g) in DOPA-melanin-streptomycin complexes, both in air and in vacuum, and the formation of quasi-chemical bonds between the DOPA-melanin-streptomycin complexes and oxygen molecules, which are responsible for the decrease in the concentration of free radicals. Our study showed that oxygen exerts an influence on spin–spin and spin–lattice interactions in the investigated complexes. Such effects can also be expected in a living organism, because DOPA–melanin is a model eumelanin mainly present in human tissues.