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. 2025 Nov 7;10(45):54919-54928.
doi: 10.1021/acsomega.5c08896. eCollection 2025 Nov 18.

In-Depth Analysis of the Paramagnetic Properties in DHI/DHICA-Controlled Eumelanin

João V Paulin  1 , João P Cachaneski-Lopes  2 , Emanuele Carrella  3 , Alessandro Pezzella  4   5   6   7 , Augusto Batagin-Neto  2   8 , Carlos F O Graeff  1   2
Affiliations

In-Depth Analysis of the Paramagnetic Properties in DHI/DHICA-Controlled Eumelanin

João V Paulin et al. ACS Omega. .

Abstract

Eumelanin, a naturally occurring pigment derived from the oxidative polymerization of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), has been extensively studied for its role in human biology and its potential applications in biomedical and sustainable electronic fields. By employing X-band electron paramagnetic resonance spectroscopy, this study focuses on eumelanin with different DHI/DHICA ratios, directly comparing monomeric and polymeric forms. The analysis reveals significant differences in the paramagnetic profiles, with changes in spin dynamics and relaxation times closely linked to the distinct macrostructures formed during polymerization. These findings offer valuable insights into how the molecular architecture of eumelanin influences its paramagnetic environments.

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Figures

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(a) Oxidation and reduction forms of the eumelanin monomeric precursors. (b) Protonated semiquinone (SQa). (c) C4–C2′ and (d) C4–C7′ dimer structures. 5,6-Dihydroxyindole (DHI) is represented by R = H and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) by R = COOH.
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(a) CW X-band EPR spectra of different DHI and DHICA ratios measured at low microwave power (0.1 mW). The spectra are normalized to the positive peak. (b) Power saturation EPR spectra. The arrow indicates an increase in microwave power (from 0.1 to 31.6 mW). All spectra exhibit similar g iso, with the red arrow also indicating the growth of a shoulder-like feature as microwave power increases. (c) Normalized power saturation curves.
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X-band EPR spectra of the monomer and polymer forms of (a) DHI and (b) DHICA, along with their corresponding fits shown in (c,d).
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g-Values of various species derived from DHI and DHICA. (a) Deprotonated DHI species. (b) Deprotonated DHICA species. (c) Charged species resulting from oxidation and reduction processes. (d) Effect of increasing the number of repeating units on g-values, using SQ units at the initial position.
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References

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