Near-Infrared Excited State Dynamics of Melanins: The Effects of Iron Content, Photo-Damage, Chemical Oxidation, and Aggregate Size

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• 作者：Mary Jane Simpson ; Jesse W. Wilson ; Francisco E. Robles ; Christopher P. Dall ; Keely Glass ; John D. Simon ; Warren S. Warren
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：February 13, 2014
• 年：2014
• 卷：118
• 期：6
• 页码：993-1003
• 全文大小：592K
• ISSN：1520-5215

文摘

Ultrafast pump鈥損robe measurements can discriminate the two forms of melanin found in biological tissue (eumelanin and pheomelanin), which may be useful for diagnosing and grading melanoma. However, recent work has shown that bound iron content changes eumelanin鈥檚 pump鈥損robe response, making it more similar to that of pheomelanin. Here we record the pump鈥損robe response of these melanins at a wider range of wavelengths than previous work and show that with shorter pump wavelengths the response crosses over from being dominated by ground-state bleaching to being dominated by excited-state absorption. The crossover wavelength is different for each type of melanin. In our analysis, we found that the mechanism by which iron modifies eumelanin鈥檚 pump鈥損robe response cannot be attributed to Raman resonances or differences in melanin aggregation and is more likely caused by iron acting to broaden the unit spectra of individual chromophores in the heterogeneous melanin aggregate. We analyze the dependence on optical intensity, finding that iron-loaded eumelanin undergoes irreversible changes to the pump鈥損robe response after intense laser exposure. Simultaneously acquired fluorescence data suggest that the previously reported 鈥渁ctivation鈥?of eumelanin fluorescence may be caused in part by the dissociation of metal ions or the selective degradation of iron-containing melanin.