Water-Soluble Melanin-Protein-Fe/Cu Conjugates Derived from Norepinephrine as Reliable Models for Neuromelanin of Human Brain Locus Coeruleus

Water-soluble melanin-protein-Fe/Cu conjugates derived from norepinephrine and fibrillar beta-lactoglobulin are reliable models for neuromelanin (NM) of human brain locus coeruleus. Both iron and copper promote catecholamine oxidation and exhibit strong tendency to remain coupled in oligonuclear aggregates. The Fe-Cu clusters are EPR silent and affect the H-1 NMR spectra of the conjugates through a specific sequence of signals. Derivatives containing only Fe or Cu exhibit different NMR patterns. The EPR spectra show weak signals of paramagnetic Fe-III in conjugates containing Fe or mixed Fe-Cu sites due to small amounts of mononuclear centers. The latter derivatives exhibit EPR signals for isolated Cu-II centers. These features parallel the EPR behavior of NM from locus coeruleus. The spectral data indicate that Fe-III is bound to the melanic fraction, whereas Cu-II is bound on the protein fibrils, suggesting that the Fe-Cu clusters occur at the interface between the two components of the synthetic NMs.