title: Intermolecular π-electron perturbations generate extrinsic visible contributions to eumelanin black chromophore in model polymers with interrupted interring conjugation.
abstract: The key structural factors underlying the unique black chromophore of eumelanin biopolymers have so far defied elucidation. Capitalizing on the ability of 1% polyvinylalcohol (PVA) to prevent pigment precipitation during melanogenesis in vitro, we have investigated the visible chromophore properties of soluble eumelanin-like polymers produced by biomimetic oxidation of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) in 1% PVA-containing buffer at pH 7. Upon dilution DHI-eumelanin solutions exhibited almost linear visible absorbance changes, whereas DHICA-eumelanin displayed a remarkable deviation from linearity in simple buffer, but not in PVA-containing buffer. It is suggested that in DHICA polymers, exhibiting repeated interruptions of interring conjugation due to lack of planar conformations, the black chromophore is not due to an overlap of static entities defined intrinsically by the conjugation length across the carbon frame, but results largely from aggregation-related intermolecular perturbations of the π-electron systems which are extrinsic in character.
authors.affiliation: Department of Chemical Sciences, University of Naples Federico II, Naples, Italy.
authors.forename: Laura
authors.initials: L
authors.lastname: Ascione
authors.forename: Alessandro
authors.initials: A
authors.lastname: Pezzella
authors.forename: Veronica
authors.initials: V
authors.lastname: Ambrogi
authors.forename: Cosimo
authors.initials: C
authors.lastname: Carfagna
authors.forename: Marco
authors.initials: M
authors.lastname: d'Ischia
mesh_terms: D001704:Biopolymers / Q000737:chemistry*; D004583:Electrons; D006863:Hydrogen-Ion Concentration; D007211:Indoles / Q000737:chemistry*; D008543:Melanins / Q000737:chemistry*; D010084:Oxidation-Reduction; D011142:Polyvinyl Alcohol / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D001704:Biopolymers; D007211:Indoles; D008543:Melanins; C041877:eumelanin; C030692:5,6-dihydroxy-2-indolylcarboxylic acid; D011142:Polyvinyl Alcohol; C033871:5,6-dihydroxyindole