Melanin Chemistry Revealed by Excited State Dynamics and the Resulting Biological Implications

Abstract

Dermatopathologists need more reliable tools for analyzing biopsies of lesions that are potentially melanomas and determining the best treatment plan for the patient. Previously inaccessible, the chemical and physical properties of melanin provide insight into melanoma biochemistry. Two-color, near-infrared pump-probe microscopy of unstained, human pathology slides reveals differences in the type of melanins and the distribution of melanins between melanomas and benign nevi. Because the pump-probe response of melanin is resilient to aging, even for hundreds of millions of years, this tool could prove useful in retrospective studies to correlate melanin characteristics with patient outcome, thus eliminating the pathologist's uncertainty from the development of this classification method.Pump-probe spectroscopy of a variety of melanin preparations including melanins with varying amounts of metal ions and toxins, those that have been photo-damaged or chemically oxidized, and melanins with a homogeneous size distribution shows that the pump-probe response is sensitive to these chemical and physical differences, not just melanin type as previously hypothesized. When sampling the response at several pump wavelengths, the specificity of this technique is derived from the absorption spectra of the underlying chromophores. Therefore, hyperspectral pump-probe microscopy of melanin could serve as an indicator of the chemical environment in a variety of biological contexts. For example, the melanin chemistry of macrophages suggests that these cells oxidize, homogenize, and compact melanin granules; whereas melanocytes produce heterogeneous melanins.