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A first-principles density-functional calculation of the electronic and vibrational structure of the key melanin monomers

University of Queensland, Brisbane, Queensland, Australia
The Journal of Chemical Physics (Impact Factor: 3.12). 06/2004; 120(18):8608-15. DOI: 10.1063/1.1690758
Source: PubMed

ABSTRACT We report first-principles density-functional calculations for hydroquinone (HQ), indolequinone (IQ), and semiquinone (SQ). These molecules are believed to be the basic building blocks of the eumelanins, a class of biomacromolecules with important biological functions (including photoprotection) and with the potential for certain bioengineering applications. We have used the difference of self-consistent fields method to study the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, Delta(HL). We show that Delta(HL) is similar in IQ and SQ, but approximately twice as large in HQ. This may have important implications for our understanding of the observed broadband optical absorption of the eumelanins. The possibility of using this difference in Delta(HL) to molecularly engineer the electronic properties of eumelanins is discussed. We calculate the infrared and Raman spectra of the three redox forms from first principles. Each of the molecules have significantly different infrared and Raman signatures, and so these spectra could be used in situ to nondestructively identify the monomeric content of macromolecules. It is hoped that this may be a helpful analytical tool in determining the structure of eumelanin macromolecules and hence in helping to determine the structure-property-function relationships that control the behavior of the eumelanins.

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Available from: M. R. Pederson, Jun 06, 2014
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    • "Based on the work of Powell et al. (2004), we were able to assign several of these peaks to hydroquinone, indolequinone and semiquinone , the key monomers of eumelanin. In the calculated spectrum of Powell et al. (2004) the peaks are very narrow, whereas our spectra, as well as those of Capozzi et al. (2005), show broad bands. This may be caused by the high fluorescence background, and by the degradation of the melanin molecules as a result of the Eocene age of our sample. "
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    • "This value corresponds to the minimum energy required to cause a transition between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of the system. As such, it corresponds to the HOMO-LUMO gap and is consistent with our first principles density functional theory calculations of the gap of indolequinone (Powell et al. 2004). We believe our measurements of b to be the first reported for melanin in the solid-state. "
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    ABSTRACT: Photopyroelectric (PPE) spectroscopy, in the 350-1,075 nm wavelength range, was used to study the optical properties of electropolymerized melanin films on indium tin oxide (ITO) coated glass. The PPE intensity signal as a function of the wavelength lambda, V (n)(lambda) and its phase F (n)(lambda) were independently measured. Using the PPE signal intensity and the thermal and optical properties of the pyroelectric detector, we were able to calculate the optical absorption coefficient beta of melanin in the solid-state. We believe this to be the first such measurement of its kind on this material. Additionally, we found an optical gap in these melanin films at 1.70 eV.
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    ABSTRACT: Acknowledgments This thesis (and I) would not be here if it weren’t for the support of many people. Firstly, of course, my two most-excellent supervisors|Paul Meredith and Ben Powell. I extend my sincerest thanks to both of them for their unfailing faith in me and this project. Their time and support, and their expertise was greatly appreci- ated. Also thanks to Andrew Watt for his help with all things physics and Evan Moore for his help with all things chemistry. Thank-you too to Ross McKenzie for his thoughts on various aspects of the theory. And to Shuzhi Cai for his assistance with various things around the lab but especially for taking the SEM images of my samples. On a more specic note, thanks to: Surya Subianto at the QUT Chemistry Department for his kind donation of some samples; to Chris Vale and Andrew White for the loan of various experimental equipment; and to Paul Bern and Kevin ??? at Oxford for running the TGA on my samples. And nally , behind the scenes, thanks to my family for getting me this far. I
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