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WIREs Comput Mol Sci
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Computational aspects towards understanding the photoprocesses in eumelanin

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Abstract Eumelanin is the active ingredient in skin pigment found in human beings. It is also present in animals, plants, and some bacteria and fungi. However, there are still open questions about its biological activity. The detailed molecular mechanism of its photoprotective property is topic of intense research. While it has been implicated in photo‐damage in certain conditions, the extent of this effect is largely unknown. Furthermore, its involvement in a wide range of metal chelation and radical scavenging properties remain unclear. This rather sparse knowledge about such as important biological system is mainly due to the extreme level of heterogeneity in eumelanin structure, its low solubility, and lack of unique structural features. Due to these difficulties theoretical studies and modeling appear to be a crucial cornerstone towards the understanding of eumelanin system. This review aims to report the current status of the understanding in molecular mechanism of photoprocesses in eumelanin and the theoretical approaches that are used for that. Furthermore, the outstanding open questions and difficulties in understanding the eumelanin system will be dealt with. This article is categorized under: Software > Quantum Chemistry Structure and Mechanism > Computational Biochemistry and Biophysics Structure and Mechanism > Molecular Structures
The dimeric structures of dihydroxyindole carboxylic acid (DHICA). There are only two sites open for polymerization in DHICA, and therefore, only three unique structures of DHICA dimers as shown in References 37, 38
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Qualitatively featureless spectra of eumelanin. It is monotonically decreasing in the visible region
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The monomers of eumelanin. (a) Dihydroxyindole (DHI), (b) mono‐keto indole (MKI), (c) di‐keto indole (DKI), and (d) dihydroxyindole carboxylic acid (DHICA)
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Schematics of formation of different forms of melanin, namely eumelanin and pheomelanin. Eumelanin is formed from dopaquinone in the absence of cysteine. Pheomelanin, the reddish pigment, on the other hand is formed in the presence of cysteine
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Frontier molecular orbitals for the lowest energy dimers of mono‐keto indole (MKI). The FMOs of the MKI monomer is also shown for comparison. The HOMO‐LUMO gaps of the two dimers are similar
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Conical intersection of mono‐keto indole is barrier‐less and along the CO elongation mode
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Schematics of the effect on environment (H bonding with water molecule) on the photo‐deactivation channels of dihydroxyindole (DHI). The black lines correspond to the energies at the Franck Condon region as is used as the origin. Blue and red lines denote the NH and OH elongated CIs, respectively. The purple lines denote the non‐planar CIs due to six membered ring puckering. The bold line denotes the gas phase DHI, dashed and dotted lines refer to the D‐O1 and D‐N1 structures, respectively
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The photoprocesses of the eumelanin monomers, dihydroxyindole, mono‐keto indole, and di‐keto indole show that there are modes that can cause interchange between the monomers, thus retaining the main structure of eumelanin intact
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Schematic diagram of the potential energy surface of di‐keto indole photo‐deactivation. There are two low energy conical intersections. The planar CO elongated CI is energetically unfavorable while the non‐planar CI is barrier‐less and energetically favorable
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Photo‐deactivation processes in dihydroxyindole are shown. The right side shows the OH and NH elongation modes. The left side shows the out of plane ring puckering mode, which is barrier‐less. The geometries of the conical intersections are also shown
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A dual excited state proton transfer photo‐cycle is responsible for excitation and de‐excitation of dihydroxyindole carboxylic acid without photo‐damage
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The complete active space self‐consistent field orbitals involved in low lying excitations in dihydroxyindole, mono‐keto indole, and di‐keto indole. The schematics shows that the lowest unoccupied π* orbital is lowered in the oxidized forms. The red arrows denote the lowest optically active excitation
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The porphyrin‐like tetrameric structure of eumelanin oligomer proposed as a model structure
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Structure and Mechanism > Molecular Structures
Structure and Mechanism > Computational Biochemistry and Biophysics
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