title: Lack of visible chromophore development in the pulse radiolysis oxidation of 5,6-dihydroxyindole-2-carboxylic acid oligomers: DFT investigation and implications for eumelanin absorption properties.
abstract: The structural factors underlying the peculiar optical properties and visible chromophore of eumelanin biopolymers are largely uncharted. It is known that synthetic eumelanins from 5,6-dihydroxyindole are black and display a featureless UV-visible absorption spectrum, whereas those from 5,6-dihydroxyindole-2-carboxylic acid (1) are lighter in color and exhibit a distinct band around 310 nm, but the origin of this difference has never been addressed in detail. Recently, we showed that 5,6-dihydroxyindole dimers generate on pulse radiolysis oxidation strongly absorbing transients with intense maxima in the 500-600 nm region, which have been attributed to planar extended quinone methide species. We now report the unexpectedly different behavior of three oligomers from 1, namely, the 4,4'-biindolyl 2, the 4,7'-biindolyl 3, and the 4,7':4',7''-terindolyl 4. Pulse radiolysis oxidation of 2-4 led initially to semiquinone intermediates exhibiting similar absorption maxima at 360-380 nm. Semiquinone absorption decay followed second-order kinetics (2k = 1.4 x 10(8), 3.2 x 10(8), and 1.4 x 10(8) M(-1) s(-1) for 2, 3, and 4, respectively) but did not lead to significant chromophore development in the visible region. Similar absorption traces were obtained from monomer 1. DFT calculations predicted 5,6-dihydroxyindolyl-5,6-indolequinone structures with significant dihedral twists across the interunit single bonds for the most stable two-electron oxidation products of 2 and 3. The computed absorption spectra consistently featured strong bands around 310 nm but little or no absorption in the visible region. It is suggested that the effective conjugation length in oligomeric/polymeric eumelanin components from 1 may be controlled by hindered rotation around inter-ring bonds preventing planarization of the continuous array of indole units. This may provide an explanation for the difference in the absorption properties of polymers from the two key eumelanin monomers.
authors.affiliation: Department of Organic Chemistry and Biochemistry, University of Naples Federico II, Via Cinthia 4, I-80126 Naples, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Pezzella
authors.forename: Lucia
authors.initials: L
authors.lastname: Panzella
authors.forename: Orlando
authors.initials: O
authors.lastname: Crescenzi
authors.forename: Alessandra
authors.initials: A
authors.lastname: Napolitano
authors.forename: Suppiah
authors.initials: S
authors.lastname: Navaratnam
authors.forename: Ruth
authors.initials: R
authors.lastname: Edge
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Land
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Barone
authors.forename: Marco
authors.initials: M
authors.lastname: d'Ischia
mesh_terms: D000042:Absorption; D007211:Indoles / Q000737:chemistry*; D008543:Melanins / Q000737:chemistry*; D010084:Oxidation-Reduction; D011108:Polymers / Q000737:chemistry; D011675:Pulse Radiolysis; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D013997:Time Factors
chemical_list: D007211:Indoles; D008543:Melanins; D011108:Polymers; C041877:eumelanin; C030692:5,6-dihydroxy-2-indolylcarboxylic acid