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The Surface Oxidation Potential of Melanosomes Measured by Free Electron Laser‐Photoelectron Emission Microscopy
Author(s) -
Garguilo Jacob,
Hong Lian,
Edwards Glenn S.,
Nemanich Robert J.,
Simon John D.
Publication year - 2007
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 - 10.1562/2006-09-11-ra-1037
Subject(s) - sepia , melanosome , photoionization , chemistry , x ray photoelectron spectroscopy , scanning electron microscope , analytical chemistry (journal) , materials science , photochemistry , ionization , ion , chemical engineering , melanin , biochemistry , philosophy , linguistics , organic chemistry , chromatography , officinalis , engineering , composite material
A technique for measuring the photoionization spectrum and the photoelectron emission threshold of a microscopic structured material is presented. The theoretical underpinning of the experiment and the accuracy of the measurements are discussed. The technique is applied to titanium silicide nanostructures and melanosomes isolated from human hair, human and bovine retinal pigment epithelium cells, and the ink sac of Sepia officinalis . A common photothreshold of 4.5 ± 0.2 eV is found for this set of melanosomes and is attributed to the photoionization of the eumelanin pigment. The relationship between the photoionization threshold and the electrochemical potential referenced to the normal hydrogen electrode is used to quantify the surface oxidation potential of the melanosome. The developed technique is used to examine the effect of iron chelation on the surface oxidation potential of Sepia melanosomes. The surface oxidation potential is insensitive to bound Fe(III) up to saturation, suggesting that the metal is bound to the interior of the granule. This result is discussed in relation to the age‐dependent accumulation of iron in human melanosomes in both the eye and brain.