One-electron oxidation of methoxylated and hydroxylated indoles by azide.  1.  Characterization of the primary indolic radicals

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

doi:10.1021/j100380a028

Cited by 51 publications

(59 citation statements)

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“…Our results for HQ, IQ and SQ are consistent with the calculations of Il'ichev and Simon [22]. The concentrations at 300 K are in direct contradiction to the experimental evidence [23,24,25,26,27,28] …”

Section: Calculation Detailssupporting

confidence: 84%

“…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: Resultscontrasting

confidence: 60%

“…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%

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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

155

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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Section: Calculation Detailssupporting

confidence: 84%

Section: Resultscontrasting

confidence: 60%

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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

155

127

View full text Add to dashboard Cite

show abstract

“…This would allow for extensive de-localization of both the highest occupied molecular orbital (HOMO -the electronic ground state) and lowest occupied molecular orbital (LUMO -the first excited state) wavefunctions since they both have predominant p-character. Clearly the reactivity of both the 4 and 7 positions is critically dependent upon the oxidation state of the molecule (Al-Kazwinin et al, 1990). Deprotonation and 1-electron oxidation in the 5-hydroxy position favours coupling in the 4 position.…”

Section: Macromolecular Structurementioning

confidence: 99%

The physical and chemical properties of eumelanin

Meredith

Sarna

2006

Pigment Cell Research

906

1,045

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SummaryIn this article, we review the current state of knowledge concerning the physical and chemical properties of the eumelanin pigment. We examine properties related to its photoprotective functionality, and draw the crucial link between fundamental molecular structure and observable macroscopic behaviour. Where necessary, we also briefly review certain aspects of the pheomelanin literature to draw relevant comparison. A full understanding of melanin function, and indeed its role in retarding or promoting the disease state, can only be obtained through a full mapping of key structure-property relationships in the main pigment types. We are engaged in such an endeavor for the case of eumelanin.

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“…Furthermore, decarboxylation at C2, C17 and both had minor effect on the chemical shift of H4 and H7 . Chemical shifts of H4 and H7 atoms in 5,6‐dihydroxyindole (DHI), the product of oxidative decarboxylation of the cyclo ‐DOPA‐5‐ O ‐β‐ d ‐glucoside aglycone, are 6.89 and 6.82 ppm, respectively, in CDCl 3 , and 6.87 and 6.78 ppm in DMSO‐d 6 and olefinic H2 and H3 show a 3 J = 8.0 Hz and chemical shifts of 6.20 and 6.98 in CDCl 3 . Also, the chemical shifts H4 and H7 atoms of 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) are 6.80 and 6.76 ppm in DMSO‐d 6 .…”

Section: Resultsmentioning

confidence: 99%

Revisiting the Mechanism of Hydrolysis of Betanin

Esteves

Pinheiro

Pioli

et al. 2018

Photochem & Photobiology

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Betanin (betanidin 5-O-β-D-glucoside) is a water-soluble plant pigment used as a color additive in food, drugs and cosmetic products. Despite its sensitivity to light and heat, betanin maintains appreciable tinctorial strength in low acidic and neutral conditions, where the color of other plant pigments, such as anthocyanins, quickly fades. However, betanin is an iminium natural product that experiences acid- and base-catalyzed hydrolysis to form the fairly stable betalamic acid and cyclo-DOPA-5-O-β-D-glucoside. Here, we show that the decomposition of betanin in aqueous phosphate solution pH 2-11 is subject to general base catalysis by hydrogen phosphate ion and intramolecular general acid and base catalysis, providing new insights on the mechanism of betanin hydrolysis. UV/Vis absorption spectrophotometry, H NMR spectroscopy and mass spectrometry were used to investigate product formation. Furthermore, theoretical calculations support the hypothesis that the nitrogen atom of the tetrahydropyridine ring of betanin is doubly protonated, as observed for structurally simpler amino dicarboxylic acids. Our results contribute to the study of betanin and other pigments belonging to the class of betalains and to deepen the knowledge on the chemical properties of imino acids as well as on iminium-catalyzed modifications of carbonyl compounds in water.

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One-electron oxidation of methoxylated and hydroxylated indoles by azide. 1. Characterization of the primary indolic radicals

Cited by 51 publications

References 0 publications

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

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

The physical and chemical properties of eumelanin

Revisiting the Mechanism of Hydrolysis of Betanin

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