Eumelanin broadband absorption develops from aggregation-modulated chromophore interactions under structural and redox control

Micillo, Raffaella; Panzella, Lucia; Iacomino, Mariagrazia; Prampolini, Giacomo; Cacelli, Ivo; Ferretti, Alessandro; Crescenzi, Orlando; Koike, Kenichi; Napolitano, Alessandra; d’Ischia, Marco

doi:10.1038/srep41532

Cited by 78 publications

(95 citation statements)

References 33 publications

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“…27 It is worth noting that the presence of increased amounts of carboxylated monomers into MN structures leads to materials with significantly reduced visible light absorption. [49][50] Our observation that the various L-DOPA-based MN materials are capable of inducing IL-1 or IL-6 release from immune cells (see Figure 4) is in accordance with our previous observations and other reports. [19][20][21]23 Despite their differences in mode of synthesis, PS-mediated, base-mediated or Fe 2+ /H 2 O 2 -mediated oxidation, all L-DOPA-based materials we have evaluated thus far appeared to enhance the ILrelease from immune cells.…”

Section: Discussionsupporting

confidence: 81%

Light- or Dark-Colored, L-DOPA-Based Melanins

Vercruysse¹,

Whalen²

2018

Preprint

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This report is a continuation of previous research on the H 2 O 2 -mediated synthesis of melanin-like pigments. We synthesized and characterized L-DOPA-based pigments using air-or H 2 O 2 -mediated oxidation. We compared their physic-chemical properties and evaluated their capacity to affect the interleukin release from immune cells. The use of higher concentrations of H 2 O 2 resulted in melanin-like materials with a distinct chemical signature in their FT-IR spectra and a lighter color. All pigments enhanced the interleukin release from immune cells. The possibility that lighter-colored melanins can be generated is discussed in the context of the importance of melanin-based pigmentation in human physiology.

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Section: Discussionsupporting

confidence: 81%

Light- or Dark-Colored, L-DOPA-Based Melanins

Vercruysse¹,

Whalen²

2018

Preprint

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“…Aggregated domains of dihydroxy-containing molecules play a photoprotective role in melanin chemistry (14), but the precise mechanisms remain elusive. Uncertainty about the chemical structures found in eumelanins and the likely presence of significant heterogeneity (1,15,18) complicate the interpretation of spectroscopic measurements (19)(20)(21)(22). For these reasons, a number of investigators have studied simplified model systems containing chemical groups in arrangements thought to be present in eumelanin (20,21,23).…”

Section: Introductionmentioning

confidence: 99%

Intermolecular Hydrogen Bonding Modulates O‐H Photodissociation in Molecular Aggregates of a Catechol Derivative

Grieco

Kohl

Zhang

et al. 2018

Photochem & Photobiology

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The catechol functional group plays a major role in the chemistry of a wide variety of molecules important in biology and technology. In eumelanin, intermolecular hydrogen bonding between these functional groups is thought to contribute to UV photoprotective and radical buffering properties, but the mechanisms are poorly understood. Here, aggregates of 4‐t‐butylcatechol are used as model systems to study how intermolecular hydrogen bonding influences photochemical pathways that may occur in eumelanin. Ultrafast UV‐visible and mid‐IR transient absorption measurements are used to identify the photochemical processes of 4‐t‐butylcatechol monomers and their hydrogen‐bonded aggregates in cyclohexane solution. Monomer photoexcitation results in hydrogen atom ejection to the solvent via homolytic O‐H bond dissociation with a time constant of 12 ps, producing a neutral semiquinone radical with a lifetime greater than 1 ns. In contrast, intermolecular hydrogen bonding interactions within aggregates retard O‐H bond photodissociation by over an order of magnitude in time. Excited state structural relaxation is proposed to slow O‐H dissociation, allowing internal conversion to the ground state to occur in hundreds of picoseconds in competition with this channel. The semiquinone radicals formed in the aggregates exhibit spectral broadening of both their electronic and vibrational transitions.

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“…Although for many reaction mixtures no visible precipitations were observed and the aliquots of the reaction mixtures could be analyzed using SEC, we can not exclude the possibility that the pigment materials generated through the oxidation reactions may exist as aggregates of finely-dispersed small molecules as is discussed for MN pigments. [23][24][25] Fig.4, panel C, correlates the extent of the reactivity of the compound and the intensity of the color of the reaction mixture as evaluated through absorbance readings at 400nm. For all compounds, with the exception of (8), the absorbance increased with declining % of compound remaining up to the point when about 0% of the compound remained; typically when 0.075% v/v H 2 O 2 was present.…”

Section: Small Scale Experiments: Partmentioning

confidence: 99%

Dark- or Light-Colored Melanins: Generating Pigments Using Fe2+ and H2O2

Vercruysse

Russell²,

Knight³

et al. 2017

Preprint

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We have studied the formation of melanin-like pigments from catechol or pyrogallol and a wide range of other phenolic compounds using Fe 2+ and H 2 O 2 . Combining UV_Vis spectroscopic measurements and size-exclusion chromatography analyses we evaluated the impact of the intensity of the oxidation reaction by varying the concentration of H 2 O 2 present in the reaction mixtures. All compounds tested, except tyrosine, reacted readily leading to mixtures that were black, brown or yellow-orange in color. For many compounds tested, the use of increasing concentrations of H 2 O 2 resulted in either precipitation of the pigment or the formation of a soluble, lighter-colored pigment. With catechol or pyrogallol as model compounds, and using different concentrations of H 2 O 2 , several materials were synthesized, purified and dried. The physic-chemical properties of these materials were compared to the properties of melanin-like pigments synthesized from the same precursors using air-oxidation in an alkaline environment. For both precursors, a distinct chemical change, as judged from FT-IR spectroscopy, was introduced in the melanin structures when using H 2 O 2 as the oxidizing agent and the relative intensity of this distinct signal strengthened with increasing concentration of H 2 O 2 used in the reaction. In general, our results suggest that depending on the precursor molecule and the intensity of the oxidizing reaction conditions involved, light-or dark-colored melanin-like pigments can be generated. This may be an important factor when evaluating the visible outlook of histological or archeological specimens: the presence of a lighter color or the absence of a dark color may not necessarily mean the absence of melanin-like biomolecules.

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Eumelanin broadband absorption develops from aggregation-modulated chromophore interactions under structural and redox control

Cited by 78 publications

References 33 publications

Light- or Dark-Colored, L-DOPA-Based Melanins

Light- or Dark-Colored, L-DOPA-Based Melanins

Intermolecular Hydrogen Bonding Modulates O‐H Photodissociation in Molecular Aggregates of a Catechol Derivative

Dark- or Light-Colored Melanins: Generating Pigments Using Fe2+ and H2O2

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