Evidence for pheomelanin sheet structure

Melanin remains one of the most enigmatic of pigments. It occurs in a variety of forms, but is perhaps best known for its role in providing ultra-violet protection of skin as brown/black eumelanin and red/yellow pheomelanin. Despite decades of research many questions remain about the structure, spectroscopy and biology of both forms. For example, their unusually broad optical absorption spectra have attracted different explanations, no protomolecule has ever been identified and pheomelanin has been implicated in melanoma, the most virulent form of skin cancer. Knowing more about the structure and spectroscopy of melanin is of paramount importance, not only in biology and medicine, but also in the design of biomimetic functional devices. There is general consistency across a variety of techniques that eumelanin’s building blocks arrange in π - stacked sheets analogous to graphite. By comparison pheomelanin has been the neglected sibling and here we present evidence from fluorescence spectroscopy for pheomelanin also displaying sheet-like behavior. As pheomelanin is synthesized the temporal response of the fluorescence intensity of the sheet-sensing probe thioflavin T (ThT) follows a similar sigmoidal increase as previously reported for eumelanin. Consistent with such intercalation fluorescence decay measurements reveal evidence for close coupling between melanin and ThT excited states.