Dielectric response and ac conductivity of synthetic dopa-melanin polymer

The dielectric spectra of dopa-melanin have been examined over the wide frequency range 100 Hz-1000 MHz at room temperature. Synthetic dopa-melanin was obtained by oxidative polymerization of L-3,4-dihydroxyphenylalanine. The purpose of the present study was to characterize polarization effects, the electrical conduction mechanism and structural arrangement of the units taking part in these processes as a basis for future biological applications. Real and imaginary components of the dielectric permittivity show an anomalous low-frequency dispersion. Values of the power-law exponents were interpreted in terms of the cluster model of potentially mobile charges and the structural arrangement of molecular units in melanin. The fractal circuit model (self-similar branched porous electrode system) was found to be equivalent to the cluster model in the description of the dielectric response in melanin. It has been shown that relations between cluster model parameters p and n, and fractal dimensionalities of melanin, are fulfilled very well. The presented results support the earlier findings based on d.c. conductivity measurements, that charge hopping is the main conduction mechanism, which contributes to the dielectric polarization in the law-frequency region. Dielectric spectra at frequencies above 10MHz show small beta-relaxation features due to molecular polar segments and water molecules bound to melanin. (C) 1998 Kluwer Academic Publishers.