[Paper Review] Electronic Properties of Random Polymers: Modelling Optical Spectra of Melanins

Kinga Bochenek, Ewa Gudowska–Nowak|ArXiv.org|Jan 18, 2003

Spectroscopy and Chemometric Analyses2 references3 citations

TL;DR

This study models the optical absorption spectra of synthetic eumelanins using semiempirical INDO/CI calculations on random pentamers composed of hydroquinone (HQ), 5,6-indolequinone (IQ), and semiquinone (SQ) monomers. The model successfully reproduces the broad, featureless absorption in the 500–800 nm range observed experimentally, with the main contribution to this band attributed to SQ-containing oligomers, and the overall shape matching experimental spectra when considering mixed, non-uniform compositions (HQ:IQ:SQ = 3:1:0).

ABSTRACT

Melanins are a group of complex pigments of biological origin, widely spread in all species from fungi to man. Among diverse types of melanins, the human melanins, eumelanins,are brown or black nitrogen-containing pigments, mostly known for their photoprotective properties in human skin. We have undertaken theoretical studies aimed to understand absorption spectra of eumelanins and their chemical precursors. The structure of the biopigment is poorly defined, although it is believed to be composed of cross-linked heteropolymers based on indolequinones. As a basic model of the eumelanin structure, we have chosen pentamers containing hydroquinones (HQ) and/or 5,6-indolequinones (IQ) and/or semiquinones (SQ) often listed as structural melanin monomers. The eumelanin oligomers have been constructed as random compositions of basic monomers and opitimized for the energy of bonding. Absorption spectra of model assemblies have been calculated within the semiempirical intermediate neglect of differential overlap (INDO) approximation. Model spectrum of eumelanin has been further obtained by sum of independent spectra of singular polymers. By comparison with experimental data it is shown that the INDO/CI method manages to reproduce well characteristic properties of experimental spectrum of synthetic eumelanins.

Motivation & Objective

• To understand the electronic and optical properties of eumelanins, which are broadband absorbers with poorly defined structures.
• To address the challenge of modeling melanin’s complex, amorphous, and heterogeneous polymer structure using theoretical methods.
• To reproduce the experimentally observed broad absorption spectrum of synthetic eumelanins through a statistical model of random oligomers.
• To investigate the role of different redox states (HQ, IQ, SQ) and their mixtures in shaping the optical response of melanin.
• To assess the influence of aggregation, conformational variations, and spectral broadening on the simulated absorption profiles.

Proposed method

• Constructed random pentamers from three monomeric units: hydroquinone (HQ), 5,6-indolequinone (IQ), and semiquinone (SQ), representing key structural motifs in eumelanin.
• Optimized the geometry and electronic structure of each pentamer using the intermediate neglect of differential overlap (INDO) approximation.
• Calculated electronic transitions and absorption spectra using the INDO/CASCI (configuration interaction) method to include electron correlation effects.
• Simulated the overall absorption spectrum as a direct sum (mixture) of individual oligomer spectra, assuming no inter-oligomer interactions.
• Applied Lorentzian line broadening to mimic experimental inhomogeneous broadening, varying the width parameter to assess its effect on spectral shape.
• Compared simulated spectra with experimental data for synthetic DOPA melanin, particularly focusing on the 200–800 nm range and peak positions.

Experimental results

Research questions

• RQ1 How do different monomeric units (HQ, IQ, SQ) contribute to the optical absorption spectrum of eumelanin?
• RQ2 Can a random, heterogeneous oligomeric model of eumelanin reproduce the broad, featureless absorption bands observed experimentally?
• RQ3 What is the role of aggregation and stacking of oligomers in shaping the visible and near-IR absorption of melanin?
• RQ4 How do spectral broadening and conformational variations affect the simulated absorption profiles?
• RQ5 Why is the simulated spectrum red-shifted compared to experiment, and what environmental factors might explain the discrepancy?

Key findings

• The simulated absorption spectrum for a mixture of 45 heteropolymers with HQ:IQ:SQ = 3:1:0 closely resembles the experimental spectrum of synthetic DOPA melanin, particularly in the 500–800 nm range.
• The main contribution to the broad absorption band in the 500–800 nm region arises from the presence of semiquinone (SQ) units in the oligomeric structures.
• The peak intensity of the model spectrum is shifted to ~300 nm, while the experimental peak is at ~200 nm, indicating a blue shift in the model relative to experiment.
• Increasing the Lorentzian line width parameter (from 10 to 40 units) leads to a broader, more structureless spectrum, with a single humped tail, but does not significantly alter absorbance at 200 nm.
• Conformational deformations of individual oligomers have only a minor effect on the overall shape of the simulated absorption spectrum.
• The model fails to reproduce the 250–270 nm absorption feature seen in experimental spectra, which is attributed to the absence of a protein coat in the synthetic melanin model.