Chemical and Structural Disorder in Eumelanins: A Possible Explanation for Broadband Absorbance

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 the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital red-shifted 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 broadband monotonic absorption characteristic of all melanins is a consequence of the superposition of a large number of nonhomogeneously 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 reexamined. 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.

Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies

“…The chemical disorder model posits that eumelanin is made of many chemically distinct species such that its broadband absorption spectrum results from the convolution over the spectra of these species. [29] However, it has been reported that the spectrum is affected by interactions among eumelanin protomolecules. [30] Recently, Buehler and co-workers, considering excitonic couplings between eumelanin oligomers (tetramers, pentamers, and octamers), found that the computational spectra with geometric (packing) disorder of a single species of DHI oligomers ( Fig.…”

Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin

“…Besides its pigmentation, eumelanin is also an efficient photoprotective pigment. The broadband absorption spectrum of eumelanin [1][2][3][4] is a result of biological evolution for protecting living organisms from intense sunlight 5,6 . Any gaps in the spectrum would leave living organisms vulnerable to these specific wavelengths of sunlight.…”

“…The chemical disorder model is a widely accepted model to explain the broadband absorption spectrum of eumelanin 3,11,12 . Accordingly, eumelanin consists of many chemically distinct species and its broadband absorption spectrum is a result of averaging over the spectra of these species.…”

The Red and the Black

“…The literature values for the absorption cross sections of DHI and DHICA vary depending on the solvent or pH; [49][50][51][52][53][54] , respectively. These results suggest that the absorption of the melanosome will vary with the relative content of DHI and DHICA.…”