PHOTOLYSIS OF MELANIN PRECURSORS: FORMATION OF SEMIQUINONE RADICALS AND THEIR COMPLEXATION WITH DIAMAGNETIC METAL IONS

Abstract— Semiquinone free radicals have been generated by the UV photolysis of aqueous solutions of melanin precursors (catechols and catecholamines) and characterized by electron spin resonance (ESR) spectroscopy. Hyperfine parameters of the semiquinones are sensitive to the nature of the substituents on the aromatic ring, reflecting (i) the ionization state of the substituent; (ii) restricted rotation of methylene protons; and (iii) the presence of a chiral carbon center. In the presence of complexing metal ions (Zn²⁺, Cd²⁺) the semiquinones form chelate complexes whose ESR spectra differ from those of the uncomplexed radicals. Satellite peaks demonstrating hyperfine coupling to metal isotopes (⁶⁷Zn, ¹¹¹Cd and ¹¹³Cd) present in natural abundance were detected in several instances. Concentrations of metal complexes are much higher than those of the uncomplexed radicals, suggesting that diamagnetic metal ions may be useful in identifying the presence of semiquinone radicals of this kind in various systems.