Probing pheomelanin synthesis using thioflavin T fluorescence

Davy, Alastair Douglas and Birch, David (2019) Probing pheomelanin synthesis using thioflavin T fluorescence. In: SPIE Proceedings volume 10893. SPIE, Bellingham, Washington.

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    Abstract

    The fluorescent rotor probe thioflavin T (ThT) is used to investigate the presence of sheet structures in pheomelanin by studying the formation and disassembly of pheomelanin synthesized from L-cysteine, L-DOPA and the enzyme tyrosinase. Brown/black eumelanin and red/yellow pheomelanin are the most common forms of melanin with the former more extensively studied. The conclusion of decades of research is that both melanins possess ill-defined polymeric structures, which are reflected in their complex photophysics. The structural integrity and co-existence of these two forms has significant bearing on their functionality and indeed their spectroscopy offers possibilities as a melanoma biomarker. Extrinsic fluorescence probes have been little used to study melanin, but the evidence from ThT fluorescence is consistent with other techniques in finding that eumelanin’s constituent dihydroxyindoles form a stacked sheet structure akin to graphite. Whether or not such an equivalent sheet structure exists for pheomelanin’s constituent benzothiazines and benzothiazoles is unresolved. On investigating this possibility we find a sigmoidal increase in ThT fluorescence during pheomelanin synthesis is similar to that observed for eumelanin, implying a sheet structure. A red spectral shift in ThT fluorescence in pheomelanin and the emergence of a fluorescence decay component of ~ 5.5 ns, much longer than that of ThT, are interpreted in terms of sites wherein ThT is coupled to pheomelanin’s native structure. Such sites can act as fluorescent traps to which energy is funneled when melanin is excited and the composite fluorescence yield thus increased. However, we find the abundance of this decay component is significantly less for pheomelanin (6%) than eumelanin (11%), suggesting that ThT does not intercalate so readily in pheomelanin. This may reflect less sheet structure or tighter binding of pheomelanin’s sheets, consistent with its low fluorescence quantum yield. Disassembling pheomelanin upon raising pH provides further spectroscopic evidence that ThT is trapped in sheet-like aggregates of smaller oligomeric forms.