Abstract
A detailed computational investigation of the 5,6-dihydroxyindole (DHI)-based porphyrin-type tetramer first described by Kaxiras as a theoretical structural model for eumelanin biopolymers is reported herein, with a view to predicting the technological potential of this unique bioinspired tetracatechol system. All possible tautomers/conformers, as well as alternative protonation states, were explored for the species at various degrees of oxidation and all structures were geometry optimized at the density functional theory (DFT) level. Comparison of energy levels for each oxidized species indicated a marked instability of most oxidation states except the six-electron level, and an unexpected resilience to disproportionation of the one-electron oxidation free radical species. Changes in the highest energy occupied molecular orbital (HOMO)–lowest energy unoccupied molecular orbital (LUMO) gaps with oxidation state and tautomerism were determined along with the main electronic transitions: more or less intense absorption in the visible region is predicted for most oxidized species. Data indicated that the peculiar symmetry of the oxygenation pattern pertaining to the four catechol/quinone/quinone methide moieties, in concert with the NH centers, fine-tunes the optical and electronic properties of the porphyrin system. For several oxidation levels, conjugated systems extending over two or more indole units play a major role in determining the preferred tautomeric state: thus, the highest stability of the six-electron oxidation state reflects porphyrin-type aromaticity. These results provide new clues for the design of innovative bioinspired optoelectronic materials.
Highlights
Eumelanins, according to a recent consensus definition, are “a black-to-brown insoluble subgroup of melanin pigments derived at least in part from the oxidative polymerization of L-dopa via5,6-dihydroxyindole (DHI) intermediates” [1]
In view of the faster convergence, a scaled van der Waals cavity based on universal force field (UFF) radii [15] was used, and polarization charges were modeled by spherical Gaussian functions [16,17]; nonelectrostatic contributions to the solvation free energy were disregarded at this stage; these terms were accounted for in single-point polarizable continuum model (PCM) calculations employing radii and nonelectrostatic terms of the SMD solvation model [18]
A simple procedure to generate them would consist in defining the number of heteroatom-bound hydrogens that corresponds to the target oxidation state (e.g., 10 hydrogens for Kaxiras’s porphyrin (KP)-2e, 8 hydrogens for KP-4e, etc.), and placing them in all possible different ways on the 12 available positions of the porphyrin skeleton (i.e., N1, O5, and O6 for each of the four rings)
Summary
Eumelanins, according to a recent consensus definition, are “a black-to-brown insoluble subgroup of melanin pigments derived at least in part from the oxidative polymerization of L-dopa via5,6-dihydroxyindole (DHI) intermediates” [1]. Despite growing interest in eumelanin-type functional materials and systems, a major gap hindering progress toward melanin-based technology relates to the highly insoluble and heterogeneous character of these polymers, which has so far hindered detailed insights into the structural basis of their physicochemical. In 2006 Kaxiras et al [2] In proposed on a theoretical that eumelanin properties could be their physicochemical properties. 2006 Kaxiras et al [2]basis proposed on a theoretical basis that interpreted in terms of porphyrin-type building blocks derived from the oxidative cyclization of DHI eumelanin properties could be interpreted in terms of porphyrin-type building blocks derived from tetramers built via 2,70 -coupling (Scheme 1). The oxidative cyclization of DHI tetramers built via 2,7′-coupling (Scheme 1).
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