| 1. |
- Corani, Alice, et al.
(författare)
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Bottom-Up Approach to Eumelanin Photoprotection: Emission Dynamics in Parallel Sets of Water-Soluble 5,6-Dihydroxyindole-Based Model Systems.
- 2012
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 116:44, s. 13151-13158
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Tidskriftsartikel (refereegranskat)abstract
- The molecular mechanisms by which the black eumelanin biopolymers exert their photoprotective action on human skin and eyes are still poorly understood, owing to critical insolubility and structural heterogeneity issues hindering direct investigation of excitation and emission behavior. Recently, we set up strategies to obtain water-soluble 5,6-dihydroxyindole (DHI)-based polymers as useful models for disentangling intrinsic photophysical properties of eumelanin components from aggregation and scattering effects. Herein, we report the absorption properties and ultrafast emission dynamics of two separate sets of DHI-based monomer-dimer-polymer systems which were made water-soluble by means of poly(vinyl alcohol) or by galactosyl-thio substitution. Data showed that dimerization and polymerization of DHI result in long-lived excited states with profoundly altered properties relative to the monomer and that glycosylation of DHI imparts monomer-like behavior to oligomers and polymers, due to steric effects hindering planar conformations and efficient interunit electron communication. The potential of S-glycation as an effective tool to probe and control emission characteristics of eumelanin-like polymers is disclosed.
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| 2. |
- Corani, Alice, et al.
(författare)
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Excited-State Proton-Transfer Processes of DHICA Resolved: From Sub-Picoseconds to Nanoseconds
- 2013
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:9, s. 1383-1388
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Tidskriftsartikel (refereegranskat)abstract
- Excited-state proton transfer has been hypothesized as a mechanism for UV energy dissipation in eumelanin skin pigments. By using time resolved fluorescence spectroscopy, we show that the previously proposed, but unresolved, excited-state intramolecular proton transfer (ESIPT) of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) occurs with a time constant of 300 fs in aqueous solution but completely stops in methanol. The previously disputed excited-state proton transfer involving the 5- or 6-OH groups of the DHICA anion is now found to occur from the 6-OH group to aqueous solvent with a rate constant of 4.0 x 10(8) s(-1).
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| 3. |
- Corani, Alice, et al.
(författare)
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Superior photoprotective motifs and mechanisms in eumelanins uncovered.
- 2014
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 136:33, s. 11626-11635
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Tidskriftsartikel (refereegranskat)abstract
- Human pigmentation is a complex phenomenon commonly believed to serve a photoprotective function through the generation and strategic localization of black insoluble eumelanin biopolymers in sun exposed areas of the body. Despite compelling biomedical relevance to skin cancer and melanoma, eumelanin photoprotection is still an enigma: What makes this pigment so efficient in dissipating the excess energy brought by harmful UV-light as heat? Why has Nature selected 5,6-dihydroxyindole-2-carboxylic acid (DHICA) as the major building block of the pigment instead of the decarboxylated derivative (DHI)? By using pico- and femtosecond fluorescence spectroscopy we demonstrate herein that the excited state deactivation in DHICA oligomers is 3 orders of magnitude faster compared to DHI oligomers. This drastic effect is attributed to their specific structural patterns enabling multiple pathways of intra- and interunit proton transfer. The discovery that DHICA-based scaffolds specifically confer uniquely robust photoprotective properties to natural eumelanins settles a fundamental gap in the biology of human pigmentation and opens the doorway to attractive advances and applications.
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| 4. |
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| 5. |
- Gauden, Magdalena, et al.
(författare)
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Ultrafast Excited State Dynamics of 5,6-Dihydroxyindole, A Key Eumelanin Building Block: Nonradiative Decay Mechanism.
- 2009
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 113, s. 12575-12580
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Tidskriftsartikel (refereegranskat)abstract
- As part of a program designed to elucidate the excited state properties of key eumelanin building blocks, we report herein a study of 5,6-dihydroxyindole (DHI) in phosphate buffer at pH 3 and pH 7 using femtosecond transient absorption spectroscopy. The transient absorption changes following excitation at 266 nm were used to directly monitor relaxation of the excited states. It was found that the initially generated excited state of DHI, exhibiting two main absorption bands at approximately 450 and approximately 550 nm, decays with a time constant of 5-10 ps to the equilibrated singlet excited state characterized by a very similar spectrum. This latter state then decays to the ground state and the triplet state with a characteristic time of approximately 140-180 ps. Concomitant with the singlet excited state decay of DHI, spectral features characteristic of the DHI cation radical (band at approximately 575 nm) and the triplet state (band at 440-450 nm) are detected. These species do not decay further since geminate recombination of the solvated electron and the DHI radical cation, as well as deprotonation of the cation to form the neutral semiquinone radical, occur on a time scale longer than that covered by the present experiments. These results offer novel insights into the mechanisms of nonradiative decay of eumelanin building blocks of possible relevance to the putative photoprotective and phototoxic roles of these biopolymers.
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| 6. |
- Huijser, Annemarie, et al.
(författare)
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Functionality of epidermal melanin pigments: current knowledge on UV-dissipative mechanisms and research perspectives.
- 2011
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 13, s. 9119-9127
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Tidskriftsartikel (refereegranskat)abstract
- So far it is not known whether epidermal melanins are solely photoprotective or also phototoxic. Also largely unknown are underlying UV-induced mechanisms and impact of the chemical structure. This perspective will focus on the current insights into the UV-dissipative mechanisms in melanin model systems and the implications for the in vivo pigments. We will also identify future research perspectives towards a full understanding of the functionality of epidermal pigments.
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| 7. |
- Migliaccio, Ludovico, et al.
(författare)
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Aqueous photo(electro)catalysis with eumelanin thin films
- 2018
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Ingår i: Materials Horizons. - : ROYAL SOC CHEMISTRY. - 2051-6347 .- 2051-6355. ; 5:5, s. 984-990
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Tidskriftsartikel (refereegranskat)abstract
- We report that eumelanin, the ubiquitous natural pigment found in most living organisms, is a photocatalytic material. Though the photoconductivity of eumelanin and its photochemical reactions with oxygen have been known for some time, eumelanins have not been regarded as photofaradaic materials. We find that eumelanin shows photocathodic behavior for both the oxygen reduction reaction and the hydrogen evolution reaction. Eumelanin films irradiated in aqueous solutions at pH 2 or 7 with simulated solar light photochemically reduce oxygen to hydrogen peroxide with accompanying oxidation of sacrificial oxalate, formate, or phenol. Autooxidation of the eumelanin competes with the oxidation of donors. Deposition of thin films on electrodes yields photoelectrodes with higher photocatalytic stability compared with the case of pure photocatalysis, implicating the successful extraction of positive charges from the eumelanin layer. These results open up new potential applications for eumelanin as a photocatalytically-active biomaterial, and inform the growing fundamental body of knowledge about the physical chemistry of eumelanins.
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| 8. |
- Nogueira, Juan J., et al.
(författare)
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Sequential Proton-Coupled Electron Transfer Mediates Excited-State Deactivation of a Eumelanin Building Block
- 2017
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:5, s. 1004-1008
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Tidskriftsartikel (refereegranskat)abstract
- Skin photoprotection is commonly believed to rely on the photochemistry of 5,6-dihydroxyindole (DHI)- and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-based eumelanin building blocks. Attempts to elucidate the underlying excited-state relaxation mechanisms have been partly unsuccessful due to the marked instability to oxidation. We report a study of the excited-state deactivation of DHI using steady-state and time-resolved fluorescence accompanied by high-level quantum-chemistry calculations including solvent effects. Spectroscopic data show that deactivation of the lowest excited state of DHI in aqueous buffer proceeds on the 100 ps time scale and is 20 times faster than in methanol. Quantum-chemical calculations reveal that the excited-state decay mechanism is a sequential proton-coupled electron transfer, which involves the initial formation of a solvated electron from DHI, followed by the transfer of a proton to the solvent. This unexpected finding would prompt a revision of current notions about eumelanin photophysics and photobiology.
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