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Probing eumelanin photoprotection using a catechol:quinone heterodimer model system.

Christopher Grieco1, Jennifer M Empey1, Forrest R Kohl1

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Area of Science:

  • Photochemistry
  • Biophysics
  • Polymer Science

Background:

  • Eumelanin, a natural pigment, offers photoprotection and radical scavenging vital for organisms.
  • Its complex polymer structure and obscure molecular mechanisms hinder understanding of its functions.
  • Quinones in various oxidation states are proposed functional units of eumelanin.

Purpose of the Study:

  • To investigate the photochemistry of a catechol:o-quinone heterodimer as a model for eumelanin's photoprotective mechanisms.
  • To elucidate the ultrafast photochemical processes following o-quinone excitation in the model system.

Main Methods:

  • Ultrafast transient absorption spectroscopy in the UV to near-IR spectral regions.
  • Selective excitation of the o-quinone moiety at 395 nm.
  • Analysis of photochemical pathways and reaction kinetics.

Main Results:

  • Excited singlet and triplet o-quinone states induced hydrogen atom transfer from catechol.
  • Formation of persistent semiquinone radical pairs observed, lasting beyond 2.5 ns.
  • Hydrogen atom transfer occurred 1000 times faster via the singlet excited state pathway.

Conclusions:

  • Ultrafast excited-state hydrogen atom transfer mechanisms, particularly via the singlet channel, are proposed as key to eumelanin's photoprotection.
  • These findings provide insights into the functional role of catechol and o-quinone groups in eumelanin's protective capabilities.