The spectroscopic properties of melanin

Riesz, Jennifer Jean (2007). The spectroscopic properties of melanin PhD Thesis, School of Physical Sciences, University of Queensland.

       
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Author Riesz, Jennifer Jean
Thesis Title The spectroscopic properties of melanin
School, Centre or Institute School of Physical Sciences
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Dr Paul Meredith
Abstract/Summary Melanin exhibits a broadband absorbance spectrum that appears to be unique amongst func- tional biomolecules. Explaining the origin of this spectrum is one of the primary challenges facing this field. Three main theories have been proposed: 1) The optical density is due to scattering, 2) Melanin is an amorphous semiconductor which then naturally exhibits broad- band absorbance, or 3) Melanin has a large amount of chemical heterogeneity (or disorder) which leads to a broadband absorbance spectrum through superposition of the absorbance spectra of many species (the chemical disorder model). We discuss these models and the experimental evidence for each. We report direct experimental evidence that the optical density is not a consequence of scattering processes, and is due to true absorbance. Distin- guishing between the two remaining explanations leads us to a discussion of the molecular structure of melanin, an issue which remains unresolved. We include a literature review, summarising the important structural findings for melanins, and conclude that the evidence suggests that melanin consists of a heterogeneous collection of small oligomers, which points towards the chemical disorder explanation for broadband absorbance. We caution, however, that the amount of chemical disorder obviously prevalent in this system is often neglected in discussions of the structure of melanin. We show that the oscillator strengths of melanin and its fundamental components are naturally explained by the chemical disorder model, as are their emission and excitation spectra and their radiative quantum yields. We show that the emission properties of melanin are not consistently reported in the literature, which is in part due to the strong re-absorption that occurs in this system, and we emphasise the im- portance of correcting for this effect in any measurement of the fluorescence of melanin. The emission properties of synthetic pheomelanin are reported, and are found to be different to those for eumelanin, suggesting different energy dissipation processes for these pigments. We also report a careful study of the spectroscopic properties of DHICA (5,6-dihydroxyindole-2- carboxylic acid), a key melanin monomer, and propose, based on these results, that proton transfer processes contribute to energy dissipation in this molecule. The radiative yield of melanin is measured to be extremely small (< 0.1%), which is consistent with melanin’s biological role as a photoprotectant, and suggests that non-radiative energy dissipation pro- cesses are very important for melanin. As a result of this finding we measure the vibronic structure of eumelanin related molecules using inelastic neutron scattering and accurately assign the vibrational modes using density functional theory calculations. This provides an important first step towards understanding the non-radiative processes in melanin energy dissipation and its biological functionality.

 
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