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Spectroscopy and Photoreactivity of Trichochromes: Molecular Components of Pheomelanins †
Author(s) -
Simon John D.,
Goldsmith Michael R.,
Hong Lian,
Kempf Valerie R.,
McGuckin Laura E. L.,
Ye Tong,
Zuber Gerard
Publication year - 2006
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/2005-06-17-ra-578
Subject(s) - chemistry , photochemistry , ground state , spectroscopy , solvation , absorption spectroscopy , picosecond , molecule , absorption (acoustics) , atomic physics , materials science , organic chemistry , laser , physics , quantum mechanics , optics , composite material
The trichochromes are a class of small molecules present in pheomelanin (the red melanin) and absent in eumelanin (the black melanin). Herein trichochrome F (TF) and decarboxy‐trichochrome C (dTC) are examined. Both trichochromes are characterized by a visible absorption band, which is shown to be the result of overlapping transitions of the cis and trans isomers. The temperature dependence of the absorption spectrum of dTC suggests the additional presence of equilibrium between the enol and keto forms of the molecule. These conclusions are supported by ground‐state energies of these isomers obtained using a continuum solvation model. Near‐infrared emission measurements were not able to detect photoproduction of 1 O 2 , and spin‐trapping experiments revealed formation of O 2 *‐ . DNA nicking assays also revealed a low level of light‐induced aerobic activity of dTC, suggesting a quantum efficiency of at most 5 × 10 ‐6 for the photo‐generation of O 2 *‐ . These results are consistent with pump‐probe optical experiments, which reveal efficient and nearly complete ground‐state recovery within a few picoseconds of excitation. Both trichochromes are efficient quenchers of 1 O 2 , exhibiting a bimolecular rate constant comparable with vitamin C. These results suggest that trichochromes could serve a protective role in pheomelanin pigments.