Analysis of the Growth Mechanism of Eumelanin Particles by Time-Resolved Static and Dynamic Light Scattering

Tyrosinase-catalyzed formation of eumelanin proceeds along four successive phases. The final phase is an aggregation of smaller type-A particles to larger type-B particles with a final radius of 200 nm. It is this fourth phase that is analyzed by means of time-resolved static and dynamic light scattering and UV-vis spectroscopy, providing the following significant insight into the formation mechanism of eumelanin. Growth of type-B particles requires a critical concentration of substrate l-Dopa. During an initial period of the reaction, disintegration of type-B particles is achieved with an appropriate increase of pH, demonstrating partial reversibility of the aggregation. Correlation of the size and mass of the growing type-B particles at variable pH levels and l-Dopa concentrations reveals that depending on the pH, the mechanism shifts from nucleation, followed by monomer addition, with type-A particles acting as the monomers, to a step growth process, where any two aggregates combine to form a corresponding larger aggregate.