Size-Dependent Local Ordering in Melanin Aggregates and Its Implication on Optical Properties

Area of Science:

• Biophysics
• Materials Science
• Computational Chemistry

Background:

• Eumelanin, a biopolymer, exhibits broad-band absorption crucial for photoprotection.
• Understanding eumelanin's optical properties requires detailed knowledge of its nanoscale structure.
• Existing models often focus on chemical disorder, potentially overlooking structural contributions.

Purpose of the Study:

• To model atomic-scale structures of eumelanin nanoaggregates of varying sizes.
• To investigate the relationship between aggregate size, π-stacking geometry, and spectral properties.
• To elucidate the role of structural disorder in eumelanin's optical absorption.

Main Methods:

• Atomistic molecular dynamics (MD) simulations were employed to generate eumelanin nanoaggregate models.
• Simulated annealing procedures were integrated with MD to explore conformational space.
• Semiempirical spectral calculations were performed on simulated nanoaggregate structures.

Main Results:

• Formation of secondary structures via π-stacking was observed in eumelanin nanoaggregates.
• A wide distribution of stack geometries (size, displacement, torsion angles) was found.
• Aggregate size significantly impacts the distribution of stacking angles, controlled by surface and bulk energies.
• Spectral properties strongly correlate with specific stack structures, leading to spectral broadening.

Conclusions:

• The geometric disorder of eumelanin nanoaggregates plays a critical role in its optical properties.
• The findings support a geometric disorder model that complements the chemical disorder model for eumelanin absorption.
• Aggregate size is a key factor in determining eumelanin's nanoscale structure and optical response.