Synthesis and structure characterization of soluble neuromelanin analogues provides important clues on its biosynthesis

The elucidation of neuromelanin (NM) structure and biosynthesis is of capital importance to understand its putative role in the pathogenesis of Parkinson’s disease. A useful integration to the studies aiming at understanding the origin and properties of this essentially insoluble natural substance is the preparation of synthetic derivatives resembling NM. With this aim, water soluble conjugates between dopamine derived melanin and bovine serum albumin (BSA) were synthesized. Melanin-BSA adducts were prepared both with eumelanic oligomers, obtained by oxidative polymerization of dopamine, and pheomelanic oligomers, obtained in the same conditions from dopamine and cysteine. Iron ions were added in the synthesis to understand the interaction between the pigment and this metal ion, as in neurons of several human brain regions NM contains significant amounts of iron. The structure of the conjugates was analyzed by 1H-NMR spectroscopy and controlled proteolysis/MS experiments. The binding of iron(III) ions was evaluated by ICP analysis and EPR spectroscopy. The EPR signal of bound iron(III) indicates high spin octahedral sites and, as for NM, the signal is coupled to a radical signal associated to the melanic component of the conjugates. However, the intensity of the EPR iron signal accounts for a reduced fraction of total iron, indicating that most of the iron is strongly coupled in the matrix. The amount of paramagnetic, mononuclear iron(III) is larger in the conjugates of pheomelanin-BSA type, suggesting that iron clustering is reduced in the sulfur-containing pigment. The melanin-BSA conjugates appear to be good models for the natural pigment.