Paramagnetic calcium melanins

Treatment of catechol, pyrogallol, DOPA, dopamine, norepinephrine, and natural polyhydroxy-1,4-naphthoquinone echinochrome with solution of potassium superoxide (KO2) in the presence of CaCl2 leads to the formation of water-insoluble dark pigments with stable paramagnetic properties (“calcium melanins”). In control experiments in the same procedure without Ca2+, the pigments were not formed. EPR spectra of the calcium melanins had little difference from each other and from known melanins in shape, line width, and the g factor about 2.004. Addition of EDTA water solution to dried paramagnetic pigments leads to their fast dissolving and disappearing of EPR signal. Formation of similar polymers is also observed during autoxidation of o-diphenols in Ca2+-containing alkaline buffer solution, however, this process takes a few days instead of few seconds in the presence of KO2. Thus, calcium (and other divalent cation M2+) can be considered as a key structural element in formation of M2+-catecholate paramagnetic polymer. We assume the existence of two types of paramagnetic centers in melanin-like polymer: M2+-stabilized o-semiquinone radical or bi-radical complex containing o-semiquinone and superoxide anion radicals, stabilized by M2+.