Abstract

Eumelanins, the chief photoprotective pigments in man and mammals, owe their black color to an unusual broadband absorption spectrum whose origin is still a conundrum. Excitonic effects from the interplay of geometric order and disorder in 5,6-dihydroxyindole (DHI)-based oligomeric/polymeric structures play a central role, however the contributions of structural (scaffold-controlled) and redox (π-electron-controlled) disorder have remained uncharted. Herein, we report an integrated experimental-theoretical entry to eumelanin chromophore dynamics based on poly(vinyl alcohol)-controlled polymerization of a large set of 5,6-dihydroxyindoles and related dimers. The results a) uncover the impact of the structural scaffold on eumelanin optical properties, disproving the widespread assumption of a universal monotonic chromophore; b) delineate eumelanin chromophore buildup as a three-step dynamic process involving the rapid generation of oxidized oligomers, termed melanochromes (phase I), followed by a slow oxidant-independent band broadening (phase II) leading eventually to scattering (phase III); c) point to a slow reorganization-stabilization of melanochromes via intermolecular redox interactions as the main determinant of visible broadband absorption.

Highlights

  • Preliminary experiments were directed to compare the generation and evolution of chromophores from the eumelanin precursors and derivatives in the presence and in the absence of 1% poly(vinyl alcohol) (PVA) using three different oxidizing systems, namely potassium ferricyanide, which operates by an outer sphere one-electron transfer mechanism, and sodium periodate, which induces two-electron oxidations via cyclic esters with catechols, both in phosphate buffer at pH 7.0, or ceric ammonium nitrate (CAN), a one-electron oxidant active at pH 3

  • The integrated experimental and computational bottom-up approach reported was aimed at obtaining a chemical background in which to frame currently accepted theories about eumelanin chromophore buildup and the origin of broadband absorption spectrum

  • The main findings of this study can be summarized as follows: (1) Eumelanin chromophore development from DHI involves complex dynamics which reflects the fast generation of visible chromophores (“melanochromes”, phase I), up to a point beyond which further addition of oxidants has no detectable effect

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Summary

Introduction

An ab initio wave-function study of the absorption behavior of DHI oligomers and of doubly and triply π-stacked species of these oligomers by the MP2 and the linear-response CC2 methods demonstrated the effect of an increasing degree of oligomerization of DHI and of an increasing degree of π-stacking of DHI oligomers on the onset of the absorption spectra and on the degree of red-shift toward the visible region of the spectrum[16] These results reinforced the view that the optical properties of biological eumelanins can be simulated only by including catechol, semiquinone and quinone building blocks[19]. In relation to chemical disorder, two major contributions were considered separately, namely structural disorder, reflecting the variety of σ-scaffolds, and redox disorder, as determined by catechol-semiquinone-quinone mixing and π-electron perturbations

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