Elucidation of the buildup mechanism of eumelanin particles and specific isolation of intermediates

• Applicants: Professor Dr. Klaus Huber; Professor Dr. Oliver Strube
• Subject Area: Preparatory and Physical Chemistry of Polymers ; Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: from 2019 to 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429844944

• Final Report Year: 2025

Final Report Abstract

The tyrosinase catalysed formation of eumelanin pigments proceeds according to a supramolecular buildup including four steps. At the onset is the tyrosinase-catalysed polymerisation of L-dopa to oligomers based on indolic derivatives as chemical constituents. The resulting oligomers form protoparticles, which further grow to type-A particles. In a final step, type-A particles agglomerate to spherically shaped type-B particles with a size of roughly 400 nm. Der gesamte Aufbau wird durch eine Alterung begleitet und/oder abgeschlossen. This buildup is accompanied or completed by a maturation. Important goals of the project include the targeted preparation of the intermediates as well as the revelation of the building mechanism of the entire buildup by means of UV-vis-spectroscopy and time-resolved light scattering. The detailed screening of reaction conditions eventually led to the targeted synthesis of type-A particles at pH > 6.2. Their successful use as starting material for the formation of type-B particles on demand clearly demonstrated the successful stabilization of the corresponding intermediate (type-A). The mechanism of the formation of type-B from type-A particles could be resolved. To begin with, successful initiation of formation of type-B particles required a critical concentration of type-A particles. Only if this minimum concentration is surpassed, formation of type-B particles commences. Depending on the actual pH of the medium, the mechanism of growth either obeys a monomer-addition process (similar to an initiation followed by a chain growth in polymer chemistry) or a step growth process, where each intermediate can associate with any other intermediate leading to the corresponding agglomerate. Dis-integration of type-B particles back to type-A particles occurs only during a short period of time after the start of the reaction. Obviously maturation increasingly interferes with the reversibility of agglomeration of type-A particles. The results established in the present project greatly assist development of new materials with e.g. interesting optical or magnetic properties.

Publications

• Building mechanism and controlled synthesis of eumelanin particles. 17th Zsigmondy Colloquium 2022 of the German Colloid Society
  Kollmann, F.; Büngeler, A.; Strube, O.I. & Huber, K.
  
• Gezielte Synthese definierter Eumelanin-Pigmentpartikel. 2022, Tagungsbeitrag: Jahrestagung GDCh Fachgruppe Lackchemie, Würzburg, Deutschland
  Hense, D.; Büngeler, A. & Strube, O. I.
  
• Structural Analysis of early Eumelanin intermediates and their growth mechanisms. 2022, ESRF, Grenoble, Experiment number SC-5172
  Kollmann, F.; Büngeler, A. & Huber, K.
  
• Targeted Synthesis of the Type-A Particle Substructure from Enzymatically Produced Eumelanin. Biomacromolecules, 23(3), 1020-1029.
  Büngeler, Anne; Kollmann, Fabian; Huber, Klaus & Strube, Oliver I.
  
• Targeted Synthesis of Eumelanin Pigment Particles. 2023, Tagungsbeitrag: Coatings Science International (CoSI), Noordwijk, Niederlande
  Hense, D.; Büngeler, A.; Huber, K. & Strube, O. I.
  
• Analysis of the Growth Mechanism of Eumelanin Particles by Time-Resolved Static and Dynamic Light Scattering. Biomacromolecules, 26(5), 3104-3112.
  Kollmann, Fabian; Büngeler, Anne; Splett, Miriam; Strube, Oliver I. & Huber, Klaus