(439j) Understanding the Self-Assembly of Allomelanin Dimers and the Impact of Poly (ethylene glycol) on the Assembly Using Atomistic Simulations

Melanin, a natural pigment known for imparting color to human skin/hair, is used in bio-inspired synthetic materials for a variety of applications. Allomelanin is a type of melanin found in fungi that helps fungi withstand hostile radiation environments, making it an attractive material for engineering protective coatings. Synthetic mimics of allomelanin can be formed by assembly of dimers/oligomers of the synthetic precursor of allomelanin, 1,8-dihydroxy naphthalene (1,8-DHN). Past experimental studies have focused on the synthesis and assembly into melanin-mimicking nanoparticles, but the molecular driving forces underlying assembly is much less known. In this talk we will present our recent work [1] linking the structure within these assembled melanin nanoparticles to the DHN dimer structure and DHN dimer mixture composition. We study DHN dimers (2-2’, 2-4’ and 4-4’, and their mixture) in aqueous solutions using explicit-solvent atomistic molecular dynamics (MD) simulations. We find that hydrogen-bonding interactions drive the assembly of like-dimers whereas stacking interactions between unlike-dimers brings the like-dimer domains together. The position of hydroxyl groups on the dimer drive the shape of the assembly with the aggregate assembled from 2-2’ dimers assuming a spherical morphology as opposed to 4-4’ dimers that adopt an anisotropic shape. The surface of the aggregate formed by 2-2’ dimers is primarily hydrophobic while the surface of aggregates formed by 2-4’ dimers, 4-4’ dimers and their mixtures are amphiphilic. We also find that, although addition of linear polyethylene glycol (PEG) chains to the assembled particle does not alter the aggregate structure formed by a single dimer type, PEG chains do prefer to interact with 4-4’ dimers more than 2-2’ and 2-4’. Contrastingly, in aggregates formed by mixtures of dimers PEG chains interact preferentially with 2-4’ dimers than 4-4’ and 2-2’ dimers, thus demonstrating the use of dimer mixture composition as an effective means to tailor the interactions between DHN-dimer and PEG chains.

[1] Kapoor, U.; Jayaraman, A. Self-Assembly of Allomelanin Dimers and the Impact of Poly (ethylene glycol) on the Assembly: A Molecular Dynamics Simulation Study, J. Phys. Chem. B, 2020, 124, 2702-2714