Oxidation-State-Controlled Janus Behavior in Eumelanin: Nonadiabatic Branching between Photoprotection and
Maryam Farmani1, Cheol Ho Choi1
1Department of Chemistry, Kyungpook National University, Daegu 41566, South Korea.
The Journal of Physical Chemistry Letters
|May 26, 2026
Summary
Eumelanin
Area of Science:
- Photochemistry
- Biophysics
- Materials Science
Background:
- Eumelanin's dual role as photoprotectant and photoreactive agent is known.
- The molecular basis for this Janus character, especially concerning oxidation state, is unclear.
Purpose of the Study:
- Investigate the impact of oxidation state on eumelanin's excited-state dynamics.
- Elucidate the mechanisms governing photoprotection versus photoreactivity in eumelanin monomers.
Main Methods:
- Simulated excited-state dynamics of eumelanin monomers (DHI, MKI, DKI).
- Employed Multireference Second-Order Perturbation Theory Time-Dependent Density Functional Theory (MRSF-TDDFT) with surface-hopping molecular dynamics.
Main Results:
- Hydroquinone (DHI) shows extensive branching, favoring radical formation (62%) via bond dissociation.
- Semiquinone (MKI) primarily undergoes ultrafast internal conversion with some tautomerization (20.1%) but no radical formation.
- Fully oxidized quinone (DKI) exclusively relaxes via internal conversion with 100% structural recovery.
Conclusions:
- Oxidation state dictates the balance between photoprotective (internal conversion) and photoreactive (radical formation) pathways.
- Reduced eumelanin forms promote radical generation, while oxidized forms favor structural recovery.
- This oxidation-controlled dynamics provides a microscopic explanation for eumelanin's Janus behavior and its photobiological functions.
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