The UV-Absorption Spectrum of Human Iridal Melanosomes: A New Perspective on the Relative Absorption of Eumelanin and Pheomelanin and its Consequences

Photoemission electron microscopy is used to measure the absorption coefficients, εc, of intact iridal stroma melanosomes isolated from dark brown and blue–green human irides for the spectral range λ = 244–310 nm. These iridal stroma melanosomes were chosen because different colored irides produce organelles of varying eumelanin:pheomelanin ratios with similar size and morphology. Similar absorption spectra are found for the two types of melanosomes. The experimental spectra measured within are compared with both the extinction coefficient spectra obtained on soluble synthetic model systems and the monomeric precursors to each pigment. The approach for collecting absorption spectra of intact melanosomes using photoemission electron microscopy is shown. Light enters the microscope at an angle of 77o from the surface. Similar to an absorption spectrometer, regions of the substrate correspond to I0 when the incident light arrives unobstructed by melanosomes, while the intensity of the photoemission in the shadow regions created by transmission through the melanosomes correspond to I. Analysis of the image then generates the absorption coefficient for the melanosomes at the incident wavelength.