Melanins are common pigments with a non-repeating primary structure that is generally accepted to be composed of dihydroxyindoles. However, despite intensive research the secondary structure defining the minimum functional unit (protomolecule) remains elusive. We have revisited eumelanin formation in-situ during the non-enzymatic auto-oxidation of 3,4-dihydroxy-L-phenylalanine by using the fluorescence of thioflavin T; an extrinsic probe known to report on sheet structure. This approach obviates the complex intrinsic fluorescence and reveals a sigmoidal temporal dependence of assembly that is consistent with protomolecule formation and assembly into a stacked sheet structure rather than a randomized heteropolymer formed by monomer addition.
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