Reactions of indolic radicals produced upon one-electron oxidation of 5,6-dihydroxyindole and its N(1)-methylated analogue

Al-Kazwini, Akeel; O’Neill, Peter; Adams, G. E.; Cundall, R. B.; Lang, Gérard; Junino, A.

doi:10.1039/p29910001941

Cited by 25 publications

(31 citation statements)

References 21 publications

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“…Much broader and longer‐lived absorption spectra of the products of disproportionation of the semiquinones of both DHI and DHICA were found here using Br as the initial one‐electron oxidant, than were obtained previously using azide (Al‐Kazwini et al., 1991; Lambert et al., 1989). Although, as noted in the introduction, we cannot yet distinguish between the various quinonoid tautomers as likely products of DHI and DHICA semiquinone decay, the spectrum of the quinonoid species from DHI observed in this study does not appear to correspond with any of the predicted tautomer spectra obtained theoretically by Il'ichev and Simon (2003).…”

Section: Discussionsupporting

confidence: 55%

Dopaquinone redox exchange with dihydroxyindole and dihydroxyindole carboxylic acid

Edge

d’Ischia

Land

et al. 2006

Pigment Cell Research

103

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A pulse radiolytic investigation has been conducted to establish whether a redox reaction takes place between dopaquinone and 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA) and to measure the rate constants of the interactions. To obviate possible confounding reactions, such as nucleophilic addition, the method employed to generate dopaquinone used the dibromide radical anion acting on dopa to form the semiquinone which rapidly disproportionates to dopaquinone. In the presence of DHI the corresponding indole-5,6-quinone (and/or tautomers) was also formed directly but, by judicious selection of suitable relative concentrations of initial reactants, we were able to detect the formation of additional indolequinone from the redox exchange reaction of DHI with dopaquinone which exhibited a linear dependency on the concentration of DHI. Computer simulation of the experimental time profiles of the absorption changes showed that, under the conditions chosen, redox exchange does proceed but not quite to completion, a forward rate constant of 1.4 x 10(6)/M/s being obtained. This is in the same range as the rate constants previously established for reactions of dopaquinone with cyclodopa and cysteinyldopa. In similar experiments carried out with DHICA, the reaction more obviously does not go to completion and is much slower, k (forward) =1.6 x 10(5)/M/s. We conclude that, in the eumelanogenic pathway, DHI oxidation may take place by redox exchange with dopaquinone, although such a reaction is likely to be less efficient for DHICA.

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Section: Discussionsupporting

confidence: 55%

Dopaquinone redox exchange with dihydroxyindole and dihydroxyindole carboxylic acid

Edge

d’Ischia

Land

et al. 2006

Pigment Cell Research

103

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“…Indeed we find that at 300 K there should be less than 0.1% SQ. Although we should stress that this calculation is for a single molecule in vacuo it is, of course, in direct contradiction to the evidence [23,24,25,26,27,28] that SQ is one of the building blocks of eumelanin. However, since SQ is widely thought to play an important role in eumelanin, we also consider it here.…”

Section: Resultsmentioning

confidence: 73%

“…However, several pulse radiolysis studies [23,24,25,26,27,28] have indicated the presence of the semiquinone (SQ -also shown in figure 1), a tautomer of IQ, and so we also consider SQ here.…”

Section: Introductionmentioning

confidence: 99%

A first-principles density-functional calculation of the electronic and vibrational structure of the key melanin monomers

Powell

Baruah

Bernstein

et al. 2004

The Journal of Chemical Physics

156

127

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We report first principles density functional calculations for hydroquinone (HQ), indolequinone (IQ) and semiquinone (SQ). These molecules are believed to be the basic building blocks of the eumelanins, a class of bio-macromolecules with important biological functions (including photoprotection) and with potential for certain bioengineering applications. We have used the ∆SCF (difference of self consistent fields) method to study the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), ∆HL. We show that ∆HL is similar in IQ and SQ but approximately twice as large in HQ. This may have important implications for our understanding of the observed broad band optical absorption of the eumelanins. The possibility of using this difference in ∆HL to molecularly engineer the electronic properties of eumelanins is discussed. We calculate the infrared and Raman spectra of the three redox forms from first principles. Each of the molecules have significantly different infrared and Raman signatures, and so these spectra could be used in situ to non-destructively identify the monomeric content of macromolecules. It is hoped that this may be a helpful analytical tool in determining the structure of eumelanin macromolecules and hence in helping to determine the structure-property-function relationships that control the behaviour of the eumelanins.

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“…The possible radical intermediates under dis- cussion are shown in Scheme 1. In organic solvents, the protonated form of the semiquinone radicals needs to be considered, which is also dominant in water below pH 5.2 (49,50).…”

Section: Radical Productsmentioning

confidence: 99%

Absolute Rate Constants for the Quenching of Reactive Excited States by Melanin and Related 5,6-Dihydroxyindole Metabolites: Implications for Their Antioxidant Activity

Zhang

Erb²,

Flammer³

et al. 2000

Photochem Photobiol

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The triplet-excited state of benzophenone and the singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-P) have been employed as kinetic probes to obtain information on the antioxidant activity of the skin and eye pigment melanin and its biogenetic precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). The excited states were generated by the laser-flash photolysis technique and their reaction kinetics was examined by time-resolved transient absorption or fluorescence spectroscopy, respectively. The reaction between triplet benzophenone and DHI produced with unit efficiency the corresponding 6O-centered semiquinone radical, which was characterized by its characteristic transient absorption. The quenching rate constants for DHI (3.1-8.4 x 10(9) M-1 s-1) and DHICA (3.3-5.5 x 10(9) M-1 s-1) were near the diffusion-controlled limit, indicating excellent antioxidant properties. Kinetic solvent effects were observed. The reactivity of synthetic melanin, assessed through the quenching rate constant of Fluorazophore-P and normalized to the number of monomer units, was more than one order of magnitude lower (2.7 x 10(8) M-1 s-1) than that of its precursors. The trend of the quenching rate constants, i.e. DHI > DHICA approximately alpha-tocopherol > melanin, along with the preferential solubility of DHICA in aqueous environments, serves to account for several experimental results from biochemical studies on the inhibition of lipid peroxidation by these natural antioxidants.

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Reactions of indolic radicals produced upon one-electron oxidation of 5,6-dihydroxyindole and its N(1)-methylated analogue

Cited by 25 publications

References 21 publications

Dopaquinone redox exchange with dihydroxyindole and dihydroxyindole carboxylic acid

Dopaquinone redox exchange with dihydroxyindole and dihydroxyindole carboxylic acid

A first-principles density-functional calculation of the electronic and vibrational structure of the key melanin monomers

Absolute Rate Constants for the Quenching of Reactive Excited States by Melanin and Related 5,6-Dihydroxyindole Metabolites: Implications for Their Antioxidant Activity

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