Disassembly Of Allomelanin Nanoparticles In Solution: A Molecular Dynamics Simulation Study

Abstract

Allomelanin, a naturally occurring nitrogen-free melanin, can be synthetically produced from the polymerization of 1,8-dihydroxynapthalene (DHN) precursor molecule to form spherical nanoparticles. Recent experiments have shown that the structure of synthetic allomelanin nanoparticles can be altered by subjecting the nanoparticle to different organic solvents. To determine the molecular forces impacting the structure of these nanoparticles in different organic solvents, we used atomistic molecular dynamics (MD) simulations to study both the assembly of three isomers of DHN dimers in explicitly represented water molecules, and the disassembly of these aggregates in explicitly represented organic solvent (methanol, acetic acid, and ethyl acetate) molecules. We analyzed the structure of the aggregate as it disassembled and the dynamics of disassembly. We also identified the intermolecular forces that contribute to the structure and disassembly. Aggregates of DHN44 were found to be more anisotropic than DHN22 or DHN24, likely contributing to DHN44 aggregates’ faster rate of disassembly. All aggregates exhibited the fastest rates of disassembly in methanol, followed by ethyl acetate, and then acetic acid. We found that the disruption of dimer-dimer hydrogen bonds and formation of dimer-solvent hydrogen bonds is the largest contributing factor to the disassembly of the dimer aggregate; the solvent size, polarity, and ability to form hydrogen bonds with aggregated dimers affect the solvent diffusion and formation of dimer-solvent hydrogen bonds.