Chemical and Structural Disorder in Eumelanins - A Possible Explanation for Broad Band Absorbance

TL;DR

This study combines experimental spectroscopy and theoretical calculations to explore how chemical and structural disorder in eumelanins leads to their broad absorption spectrum, challenging traditional models of their optical properties.

Contribution

It provides new insights into eumelanin's optical behavior by linking disorder-induced heterogeneity to broad absorption, suggesting a need to revise existing models.

Findings

Ongoing oxidative polymerization causes red-shifted broadening of absorption spectrum.

Oligomers exhibit reduced HOMO-LUMO gaps with diverse bonding configurations.

Chemical disorder explains the broad, monotonic absorption characteristic of melanins.

Abstract

We report the results of an experimental and theoretical study of the electronic and structural properties of a key eumelanin precursor - 5,6,-dihydroxyindole-2-carboxylic acid (DHICA) and its dimeric forms. We have used optical spectroscopy to follow the oxidative polymerization of DHICA to eumelanin, and observe red shifting and broadening of the absorption spectrum as the reaction proceeds. First principles density functional theory calculations indicate that DHICA oligomers (possible reaction products of oxidative polymerization) have red shifted HOMO-LUMO gaps with respect to the monomer. Furthermore, different bonding configurations (leading to oligomers with different structures) produce a range of gaps. These experimental and theoretical results lend support to the chemical disorder model where the broad band monotonic absorption characteristic of all melanins is a consequence of the superposition of a large number of inhomogeneously broadened Gaussian transitions associated with each of the components of a melanin ensemble. These results suggest that the traditional model of eumelanin as an amorphous organic semiconductor is not required to explain its optical properties, and should be thoroughly re-examined. These results have significant implications for our understanding of the physics, chemistry and biological function of these important biological macromolecules. Indeed, one may speculate that the robust functionality of melanins in vitro is a direct consequence of its heterogeneity, i.e. chemical disorder is a "low cost" natural resource in these systems.