The supramolecular structure of melanin

Abstract

The melanins are an important class of multifunctional bio-macromolecules with fascinating and potentially useful electronic and optoelectronic properties. They are one of the last major bio-macromolecular systems where we do not know how the constituent molecules organise. This knowledge gap hinders attempts to map their properties and function and is a particular issue in understanding their roles in human photoprotection and melanoma cancer. In this article we provide strong and direct evidence of supramolecular organisation in both natural and synthetic eumelanins. Using low voltage–high resolution transmission electron microscopy (LVHRTEM) we show that sheets of protomolecules stack to form onion-like nanostructures. The inter-sheet spacings within these structures are between 3.7 and 4.0 Å consistent with non-covalent π–π stacking in heteroaromatic systems. This type of arrangement has previously been suggested by reciprocal space scattering studies, but our real-space microscopic analysis provides the definitive evidence and dimensions. Furthermore, we show that key photophysical properties of melanin related to their role as photoprotectants are derived from the system's primary chemical structure rather than supramolecular organisation. We now have a framework for the secondary structure of melanins upon which biologically relevant structure–function relationships can be built and new bio-inspired optoelectronic materials and multifunctional coatings can be designed and realised.