Different Molecular Constituents in Pheomelanin are Responsible for Emission, Transient Absorption and Oxygen Photoconsumption


  • This paper is part of a special issue dedicated to Professor Hasan Mukhtar on the occasion of his 60th birthday.

*Corresponding author email: jsimon@duke.edu (John D. Simon)


Steady-state absorption and emission spectroscopies, oxygen activation and transient spectroscopy on a single sample of synthetic pheomelanin are compared. The absorption, emission and excitation spectra of pheomelanin depend on the molecular weight (MW) of the dissolved pigment constituents. While weakly-depending on MW, the maximum of the emission excitation spectrum is ∼400 nm. The electron paramagnetic resonance oximetry measurements show a clear increase in oxygen uptake between 338 and 323 nm, and also reveal a local enhancement around ∼370 nm. Pump-probe absorption spectroscopy reveals that photoexcitation of pheomelanin by UVA light generates a transient absorption peak in the visible and UV regions within the instrument response. The action spectrum for the formation of the 780 nm transient species peaks at ∼360 nm. While both transient absorption bands show the same ultrafast decay component, the 780 nm peak completely vanishes on the 10s of picosecond time scale, but the UV band shows a kinetic evolution to a subsequent intermediate. While in a similar wavelength range, the maximum of the action spectrum derived from the transient data, the emission excitation spectrum and the action spectrum for photoconsumption all differ from one another, suggesting that the chromophore responsible for each is not that same. This raises concern about comparing the results from different photochemical methodologies for melanin, which is a specific case of comparing data on systems where molecular constituents are not well defined.