Tunneling microscopy verification of an x-ray scattering-derived molecular model of tyrosine-based melanin

Abstract

The molecular structure of melanins has been a focus of study for many years and only recently has a molecular model (MM) been proposed based upon x-ray scattering results. In order to test the validity of the model, scanning tunneling microscopy (STM) and spectroscopy have been performed on the same tyrosine-derived melanin, which has been characterized in the x-ray scattering studies and has been shown to exhibit characteristics of natural melanins. The STM measurements of the lateral and vertical dimensions provide a characteristic fundamental size which agree well with the x-ray scattering-derived MM. We find that the compact three-dimensional structure is best described as a planar morphology of finite lateral extent (≊2 nm) with a vertical extent (<1 nm) which is believed to consist of stacks of a basic two-dimensional amorphous polymer of 5,6 indolequinone units. Molecular mechanics and orbital calculations of representative amorphous models support the x-ray scattering and STM results.