Abstract
Chiral porphyrin hetero-aggregates, produced from meso-tetrakis(4-N-methylpyridyl) porphyrin H2T4 and copper(II) meso-tetrakis(4-sulfonatophenyl)porphyrin CuTPPS by an imprinting effect in the presence of L-3,4-dihydroxyphenylalanine (L-DOPA), are shown herein to serve as templates for the generation of chiral structures during the oxidative conversion of the amino acid to melanin. This remarkable phenomenon is suggested to involve the initial role of L-DOPA and related chiral intermediates like dopachrome as templates for the production of chiral porphyrin aggregates. When the entire chiral pool from DOPA is lost, chiral porphyrin hetero-aggregate would elicit axially chiral oligomer formation from 5,6-dihydroxyindole intermediates in the later stages of melanin synthesis. These results, if corroborated by further studies, may open unprecedented perspectives for efficient strategies of asymmetric melanin synthesis with potential biological and technological applications.
Highlights
L-3,4-Dihydroxyphenylalanine (L-DOPA; Figure 1A) is an aromatic amino acid produced in various organisms by the oxidative modification of L-tyrosine (Raper, 1927; Mason and Wright, 1949; Haneda and Watanabe, 1971; Prota, 1995; Ito, 2003; Slominski et al, 2012; Marchiosi et al, 2020)
The process involves the oxidation of tyrosine to dopaquinone followed by intramolecular cyclization to an indoline intermediate, termed leucodopachrome or cyclodopa, which can enter a redox cycle by exchanging electrons with dopaquinone to produce L-DOPA and L-dopachrome
We show that porphyrin hetero-aggregates made up of 5,10,15,20tetrakis(4-N-methylpyridyl)porphyrin H2T4 (Figure 1B) and copper(II) 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin CuTPPS (Figure 1B) can drive the oxidative polymerization of DOPA to melanins with the unexpected generation of asymmetric structures
Summary
L-3,4-Dihydroxyphenylalanine (L-DOPA; Figure 1A) is an aromatic amino acid produced in various organisms by the oxidative modification of L-tyrosine (Raper, 1927; Mason and Wright, 1949; Haneda and Watanabe, 1971; Prota, 1995; Ito, 2003; Slominski et al, 2012; Marchiosi et al, 2020). The process involves the oxidation of tyrosine to dopaquinone followed by intramolecular cyclization to an indoline intermediate, termed leucodopachrome or cyclodopa, which can enter a redox cycle by exchanging electrons with dopaquinone to produce L-DOPA and L-dopachrome The latter, undergoes isomerization with or without decarboxylation and loss of the chiral center to give 5,6-dihydroxyindole (DHI). As a consequence of extensive network of interactions (electrostatic, solvophobic, etc.) that trap porphyrin aggregates in a quite deep local energy minimum ensuring kinetic inertia, the porphyrin supramolecular assembly is able to memorize the chiral information imprinted by the chiral template in aqueous solution (Mammana et al, 2007; Gaeta et al, 2016). We show that porphyrin hetero-aggregates made up of 5,10,15,20tetrakis(4-N-methylpyridyl)porphyrin H2T4 (Figure 1B) and copper(II) 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin CuTPPS (Figure 1B) can drive the oxidative polymerization of DOPA to melanins with the unexpected generation of asymmetric structures
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