Ultraviolet Absorption Coefficients of Melanosomes Containing Eumelanin As Related to the Relative Content of DHI and DHICA

Central to understanding the photochemical function(s) of melanosomes is the determination of their absorption properties and an understanding of how the absorption varies with the molecular composition of melanin. Herein, the absorption coefficients for melanosomes containing predominantly eumelanin, a polymeric pigment derived from the molecular precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), are reported for) lambda = 244 nm. The absorption coefficient varies with the DHICA/DHI ratio, determined from chemical degradation analyses. With increasing DHICA content, the absorption coefficient of the melanosome increases. This observation is consistent with reported extinction coefficients, which reveal that at 244 nm, the extinction coefficient of DHICA is a factor of similar to 2.1 greater than that of DHI. The melanosome absorption coefficients are compared to predicted values based on a linear combination of the absorption of the constituent monomers.