  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.
