Photoionization Thresholds of Melanins Obtained from Free Electron Laser–Photoelectron Emission Microscopy, Femtosecond Transient Absorption Spectroscopy and Electron Paramagnetic Resonance Measurements of Oxygen Photoconsumption

Abstract Free electron laser–photoelectron emission microscopy (FEL-PEEM), femtosecond absorption spectroscopy and electron paramagnetic resonance (EPR) measurements of oxygen photoconsumption were used to probe the threshold potential for ionization of eumelanosomes and pheomelanosomes isolated from human hair. FEL-PEEM data show that both pigments are characterized by an ionization threshold at 282 nm. However, pheomelanosomes exhibit a second ionization threshold at 326 nm, which is interpreted to be reflective of the benzothiazine structural motif present in pheomelanin and absent in eumelanin. The lower ionization threshold for pheomelanin is supported by femtosecond transient absorption spectroscopy. Unlike photolysis at 350 nm, following excitation of solubalized synthetic pheomelanin at 303 nm, the transient spectrum observed between 500 and 700 nm matches that for the solvated electron, indicating the photoionization threshold for the solubalized pigment is between 350 and 303 nm. For the same synthetic pheomelanin, EPR oximetry experiments reveal an increased rate of oxygen uptake between 338 nm and 323 nm, narrowing the threshold for photoionization to sit between these two wavelengths. These results on the solubalized synthetic pigment are consistent with the FEL-PEEM results on the human melanosomes. The lower ionization potential observed for pheomelanin could be an important part of the explanation for the greater incidence rate of UV-induced skin cancers in red-haired individuals.

[1]  R. S. Sinclair,et al.  PHOTOCHEMISTRY OF BENZOTHIAZOLE MODELS OF PHEOMELANIN , 1984 .

[2]  G. Monfrecola,et al.  Phaeomelanin versus eumelanin as a chemical indicator of ultraviolet sensitivity in fair-skinned subjects at high risk for melanoma: a pilot study , 1998, Melanoma research.

[3]  T. Sarna,et al.  PHOTOSENSITIZATION OF MELANINS: A COMPARATIVE STUDY , 1985, Photochemistry and Photobiology.

[4]  P. Agin,et al.  PHOTOCHEMISTRY OF PHEOMELANIN: ACTION SPECTRUM FOR SUPEROXIDE PRODUCTION , 1980 .

[5]  T. Sarna,et al.  PHOTOLYSIS OF PHEOMELANIN PRECURSORS: AN ESR‐SPIN TRAPPING STUDY , 1986, Photochemistry and photobiology.

[6]  T. Sarna,et al.  PHOTOINDUCED OXYGEN CONSUMPTION IN MELANIN SYSTEMS. ACTION SPECTRA AND QUANTUM YIELDS FOR EUMELANIN AND SYNTHETIC MELANIN , 1984, Photochemistry and photobiology.

[7]  B. Kalyanaraman,et al.  Photoionization and photohomolysis of melanins: an electron spin resonance-spin trapping study , 1984 .

[8]  John D Simon,et al.  Isolation and biophysical studies of natural eumelanins: applications of imaging technologies and ultrafast spectroscopy. , 2003, Pigment cell research.

[9]  J. Simon,et al.  Comparison of the ultrafast absorption dynamics of eumelanin and pheomelanin , 2003 .

[10]  A. Welch,et al.  Diet and basal cell skin cancer: results from the EPIC–Norfolk cohort , 2002, The British journal of dermatology.

[11]  G. Edwards,et al.  Oxidation Potentials of Human Eumelanosomes and Pheomelanosomes¶ , 2005, Photochemistry and photobiology.

[12]  R. Fowler,et al.  The Analysis of Photoelectric Sensitivity Curves for Clean Metals at Various Temperatures , 1931 .

[13]  S. Ito,et al.  Optimization of conditions for preparing synthetic pheomelanin. , 1989, Pigment cell research.

[14]  Jonathan Rees,et al.  Eumelanin and pheomelanin concentrations in human epidermis before and after UVB irradiation. , 2005, Pigment cell research.

[15]  K. Wakamatsu,et al.  Advanced chemical methods in melanin determination. , 2002, Pigment cell research.

[16]  M. Chedekel,et al.  PHOTODESTRUCTION OF PHAEOMELANIN , 1977, Photochemistry and photobiology.

[17]  H. Adami,et al.  A prospective study of pigmentation, sun exposure, and risk of cutaneous malignant melanoma in women. , 2004, Journal of the National Cancer Institute.

[18]  E. Hart,et al.  The Hydrated Electron , 1963, Science.

[19]  J. Rees.  Genetics of hair and skin color. , 2003, Annual review of genetics.

[20]  G. Edwards,et al.  Photoionization threshold of eumelanosomes determined using uv free electron laser-photoelectron emission microscopy , 2004 .

[21]  T. Sarna,et al.  Free radicals from eumelanins: quantum yields and wavelength dependence. , 1984, Archives of biochemistry and biophysics.

[22]  K. Wakamatsu,et al.  Comparison of Structural and Chemical Properties of Black and Red Human Hair Melanosomes¶ , 2005, Photochemistry and photobiology.

[23]  A. Pokora,et al.  Photodestruction of pheomelanin: role of oxygen. , 1978, Proceedings of the National Academy of Sciences of the United States of America.

[24]  E. Wenczl,et al.  (Pheo)melanin photosensitizes UVA-induced DNA damage in cultured human melanocytes. , 1998, The Journal of investigative dermatology.

[25]  K. Wakamatsu,et al.  Pheomelanin as well as eumelanin is present in human epidermis. , 1991, The Journal of investigative dermatology.

[26]  J. Simon,et al.  Ultrafast Nonradiative Relaxation Dynamics of Eumelanin , 2001 .

[27]  Shosuke Ito,et al.  Melanin acts as a potent UVB photosensitizer to cause an atypical mode of cell death in murine skin. , 2004, Proceedings of the National Academy of Sciences of the United States of America.