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

¹ School of Chemistry, University of Bristol
² Theory of Condensed Matter Physics Group, School of Physical Sciences, University of Queensland
³ Soft Condensed Matter Physics Group, School of Physical Sciences, The University of Queensland

^* To whom correspondence should be addressed. E-mail: meredith{at}physics.uq.edu.au.

Submitted on June 19, 2005
Revised on August 15, 2005
Accepted on 26 September 2005

	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.

Key Words: Chemical Disorder, DHICA, Eumelanin, Organic Semicondutcor, Quantum Chemistry, Secondary Structure

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