Applications of Photoemission Electron Microscopy to Melanin and Melanosomes
Melanin is a biological pigment that is ubiquitous in nature and generally produced within melanosomes, specialized organelles. Typically, melanin is categorized into two distinct classes, based on color and molecular precursor: eumelanin (brown-black) and pheomelanin (yellow-red). Whereas much is known regarding the molecular precursors to the two pigments, an understanding of their resulting molecular structure remains elusive. Despite this lack of knowledge, several functions are attributed to the pigments, including photoprotection and photosensitization. Epidemiological data for skin and ocular cancers have observed an increased incidence for increased relative concentrations of pheomelanin. Furthermore, eumelanin is generally identified as photoprotective and antioxidant, whereas pheomelanin is generally identified as photoreactive and pro-oxidant. This thesis describes the photophysical properties of the naturally-occuring melanin pigments and presents new insights into their roles within the context of skin and ocular cancers.
Photoemission electron microscopy provides a unique opportunity to probe the complex photoproperties of melanins contained within intact melanosomes isolated from tissues of bovine and human eyes. Photoionization threshold potentials characteristic of eumelanin and pheomelanin have been determined and are used to investigate the molecular architecture of the pigments within the melanosome. Furthermore, a novel approach to photoemission electron microscopy is used to obtain the first direct measurements of the absorption coefficients from intact melanosomes.
Human iridal stroma melanosomes are comprised of both eumelanin and pheomelanin in various ratios according to iris color; dark brown and blue-green iris melanosomes are characterized by a eumelanin:pheomelanin ratio of 14.8 and 1.3, respectively. Despite the significant difference in the overall pigment composition, a common eumelanin surface photoionization threshold is obtained for both melanosomes. This data indicates that within the melanosome, the phototoxic pheomelanin pigment is encased by eumelanin. This structure mitigates the adverse photochemical properties of pheomelanin. However, damage to the eumelanic exterior and or significant reduction in the amount of eumelanin present could compromise the protective ability of eumelanin, providing mechanisms for exposure of pheomelanin and consequently contributing to oxidative stress.
The absorption spectra of intact melanosomes of varying melanin compositions were determined over the spectral range from 244 to 310 nm. The absorption spectra of eumelanic melanosomes are similar regardless of monomer composition or embryonic origin. Furthermore, the absorption spectra of melanosomes containing a mixture of pigments were similar to those containing pure eumelanin, arguing that the absorption properties of the melanosome are maintained regardless of increased pheomelanin composition. Therefore, the correlation between epidemiological data and the eumelanin:pheomelanin ratio is not predicted to be a reflection of the melanosome's decreased ability to attenuate biologically relevant wavelengths, but instead is predicted to be a reflection of the different photoreactivities of the melanin pigments contained within.
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