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Effect of Dimerization on Vibrational Spectra of Eumelanin Precursors

Authors

  • Stephen P. Nighswander-Rempel,

    Corresponding author
    1. Centre for Biophotonics and Laser Science, School of Physical Sciences, University of Queensland, Brisbane, Queensland, Australia
      *Corresponding author email: snighrem@physics.uq.edu.au (Stephen P. Nighswander-Rempel)
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  • Seth Olsen,

    1. Centre for Computational and Molecular Science, Australian Institute for Bioengineering and Nanotechnology, Brisbane, Queensland, Australia
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  • Indumathy B. Mahadevan,

    1. Centre for Organic Photonics and Electronics, School of Physical Sciences, University of Queensland, Brisbane, Queensland, Australia
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  • George Netchev,

    1. Division of Biophysics and Imaging, Ontario Cancer Institute, Princess Margaret Hospital, Toronto, Ontario, Canada
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  • Brian C. Wilson,

    1. Division of Biophysics and Imaging, Ontario Cancer Institute, Princess Margaret Hospital, Toronto, Ontario, Canada
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  • Sean C. Smith,

    1. Centre for Computational and Molecular Science, Australian Institute for Bioengineering and Nanotechnology, Brisbane, Queensland, Australia
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  • Halina Rubinsztein-Dunlop,

    1. Centre for Biophotonics and Laser Science, School of Physical Sciences, University of Queensland, Brisbane, Queensland, Australia
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  • Paul Meredith

    1. Centre for Organic Photonics and Electronics, School of Physical Sciences, University of Queensland, Brisbane, Queensland, Australia
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  • This invited paper is part of the Symposium-in-Print: Melanins.

*Corresponding author email: snighrem@physics.uq.edu.au (Stephen P. Nighswander-Rempel)

ABSTRACT

We have synthesized a compound ideally suited to the study of structure-function relationships in eumelanin synthesis. N-methyl-5-hydroxy-6-methoxy-indole (MHMI) has key functional groups strategically placed on the indole framework to hinder binding in the 2, 5, 6 and 7 positions. Thus, the dimer bound exclusively in the 4-4′ positions was isolated and characterized. In order to study the difference in vibrational structure between the MHMI monomer and dimer, Raman spectra were acquired of both compounds, as well as indole, indole-2-carboxylic acid and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Peaks were assigned to particular vibrational modes using B3LYP density functional theory calculations, and experimental and theoretical spectra displayed good agreement. Addition of functional groups to either benzene or pyrrole rings in the indole framework impacted vibrational spectra attributed to vibrations in either ring, and in some cases, peaks appearing unchanged between two compounds corresponded to different contributing vibrations. Dimerization resulted in an expected increase in the number of vibrational modes, but not a significant increase in the number of apparent peaks, as several modes frequently contributed to an individual observed peak. Comparison of spectral features of the monomer and dimer provides insight into eumelanin photochemistry, but final conclusions depend on the planarity of oligomeric structure in vivo.

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