dorsal/arxiv
View SchemaChemical and Structural Disorder in Eumelanins - A Possible Explanation for Broad Band Absorbance
| Authors | M. Linh Tran, B. J. Powell, Paul Meredith |
|---|---|
| Categories | |
| ArXiv ID | q-bio/0506028 |
| URL | https://arxiv.org/abs/q-bio/0506028 |
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.
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"abstract": "We report the results of an experimental and theoretical study of the\nelectronic and structural properties of a key eumelanin precursor -\n5,6,-dihydroxyindole-2-carboxylic acid (DHICA) and its dimeric forms. We have\nused optical spectroscopy to follow the oxidative polymerization of DHICA to\neumelanin, and observe red shifting and broadening of the absorption spectrum\nas the reaction proceeds. First principles density functional theory\ncalculations indicate that DHICA oligomers (possible reaction products of\noxidative polymerization) have red shifted HOMO-LUMO gaps with respect to the\nmonomer. Furthermore, different bonding configurations (leading to oligomers\nwith different structures) produce a range of gaps. These experimental and\ntheoretical results lend support to the chemical disorder model where the broad\nband monotonic absorption characteristic of all melanins is a consequence of\nthe superposition of a large number of inhomogeneously broadened Gaussian\ntransitions associated with each of the components of a melanin ensemble. These\nresults suggest that the traditional model of eumelanin as an amorphous organic\nsemiconductor is not required to explain its optical properties, and should be\nthoroughly re-examined. These results have significant implications for our\nunderstanding of the physics, chemistry and biological function of these\nimportant biological macromolecules. Indeed, one may speculate that the robust\nfunctionality of melanins in vitro is a direct consequence of its\nheterogeneity, i.e. chemical disorder is a \"low cost\" natural resource in these\nsystems.",
"arxiv_id": "q-bio/0506028",
"authors": [
"M. Linh Tran",
"B. J. Powell",
"Paul Meredith"
],
"categories": [
"q-bio.BM",
"physics.bio-ph",
"physics.chem-ph"
],
"title": "Chemical and Structural Disorder in Eumelanins - A Possible Explanation for Broad Band Absorbance",
"url": "https://arxiv.org/abs/q-bio/0506028"
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