Neuromelanins in brain aging and Parkinson's disease: synthesis, structure, neuroinflammatory, and neurodegenerative role.

Neuromelanins are compounds accumulating in neurons of human and animal brain during aging, with neurons of substantia nigra and locus coeruleus having the highest levels of neuromelanins. These compounds have melanic, lipid, peptide, and inorganic components and are contained inside special autolysosomes. Neuromelanins can participate in neuroprotective or toxic processes occurring in Parkinson's disease according to cellular environment. Their synthesis depends on the concentration of cytosolic catechols and is a protective process since it prevents the toxic accumulation of catechols-derived reactive compounds. Neuromelanins can be neuroprotective also by binding reactive/toxic metals to produce stable and non-toxic complexes. Extraneuronal neuromelanin released by dying dopamine neurons in Parkinson's disease activates microglia which generate reactive oxygen species, reactive nitrogen species, and proinflammatory molecules, thus producing still neuroinflammation and neuronal death. Synthetic neuromelanins have been prepared with melanic, protein structure, and metal content closely mimicking the natural brain pigment, and these models are also able to activate microglia. Neuromelanins have different structure, synthesis, cellular/subcellular distribution, and role than melanins of hair, skin, and other tissues. The main common aspect between brain neuromelanin and peripheral melanin is the presence of eumelanin and/or pheomelanin moieties in their structure.

Photoactivated Gallium Porphyrin Reduces Staphylococcus aureus Colonization on the Skin and Suppresses Its Ability to Produce Enterotoxin C and TSST-1.

Staphylococcus aureus is a key pathogen in atopic dermatitis (AD) pathogenicity. Over half of AD patients are carriers of S. aureus. Clinical isolates derived from AD patients produce various staphylococcal enterotoxins, such as staphylococcal enterotoxin C or toxic shock syndrome toxin. The production of these virulence factors is correlated with more severe AD. In this study, we propose cationic heme-mimetic gallium porphyrin (Ga3+CHP), a novel gallium metalloporphyrin, as an anti-staphylococcal agent that functions through dual mechanisms: a light-dependent mechanism (antimicrobial photodynamic inactivation, aPDI) and a light-independent mechanism (suppressing iron metabolism). Ga3+CHP has two additive quaternary ammonium groups that increase its water solubility. Furthermore, Ga3+CHP is an efficient generator of singlet oxygen and can be recognized by heme-target systems such as Isd, which improves the intracellular accumulation of this compound. Ga3+CHP activated with green light effectively reduced the survival of clinical S. aureus isolates derived from AD patients (>5 log10 CFU/mL) and affected their enterotoxin gene expression. Additionally, there was a decrease in the biological functionality of studied toxins regarding their superantigenicity. In aPDI conditions, there was no pronounced toxicity in HaCaT keratinocytes with both normal and suppressed filaggrin gene expression, which occurs in ∼50% of AD patients. Additionally, no mutagenic activity was observed. Green light-activated gallium metalloporphyrins may be a promising chemotherapeutic to reduce S. aureus colonization on the skin of AD patients.

Iron and copper ions accelerate and modify dopamine oxidation to eumelanin: implications for neuromelanin genesis.

Dopamine (DA) is a precursor of neuromelanin (NM) synthesized in the substantia nigra of the brain. NM is known to contain considerable levels of Fe and Cu. However, how Fe and Cu ions affect DA oxidation to DA-eumelanin (DA-EM) and modify its structure is poorly understood. EMs were prepared from 500 µM DA, dopaminechrome (DAC), or 5,6-dihydroxyindole (DHI). Autoxidation was carried out in the absence or presence of 50 µM Fe(II) or Cu(II) at pH 7.4 and 37 â„ƒ. EMs were characterized by Soluene-350 solubilization analyzing absorbances at 500 nm (A500) and 650 nm (A650) and alkaline hydrogen peroxide oxidation (AHPO) yielding various pyrrole carboxylic acids. Pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA) served as a molecular marker of cross-linked DHI units. Importantly, Fe and Cu accelerated DA oxidation to DA-EM and DHI oxidation to DHI-EM several-fold, whereas these metals only weakly affected the production of DAC-EM. The A500 values indicated that DA-EM contains considerable portions of uncyclized DA units. Analysis of the A650/A500 ratios suggests that Fe and Cu caused some degradation of DHI units of DA-EM during 72-h incubation. Results with AHPO were consistent with the A500 values and additionally revealed that (1) DA-EM is less cross-linked than DAC-EM and DHI-EM and (2) Fe and Cu promote cross-linking of DHI units. In conclusion, Fe and Cu not only accelerate the oxidation of DA to DA-EM but also promote cross-linking and degradation of DHI units. These results help to understand how Fe and Cu in the brain affect the production and properties of NM.

Scavenging of Cation Radicals of the Visual Cycle Retinoids by Lutein, Zeaxanthin, Taurine, and Melanin.

In the retina, retinoids involved in vision are under constant threat of oxidation, and their oxidation products exhibit deleterious properties. Using pulse radiolysis, this study determined that the bimolecular rate constants of scavenging cation radicals of retinoids by taurine are smaller than 2 × 107 M-1s-1 whereas lutein scavenges cation radicals of all three retinoids with the bimolecular rate constants approach the diffusion-controlled limits, while zeaxanthin is only 1.4-1.6-fold less effective. Despite that lutein exhibits greater scavenging rate constants of retinoid cation radicals than other antioxidants, the greater concentrations of ascorbate in the retina suggest that ascorbate may be the main protectant of all visual cycle retinoids from oxidative degradation, while α-tocopherol may play a substantial role in the protection of retinaldehyde but is relatively inefficient in the protection of retinol or retinyl palmitate. While the protection of retinoids by lutein and zeaxanthin appears inefficient in the retinal periphery, it can be quite substantial in the macula. Although the determined rate constants of scavenging the cation radicals of retinol and retinaldehyde by dopa-melanin are relatively small, the high concentration of melanin in the RPE melanosomes suggests they can be scavenged if they are in proximity to melanin-containing pigment granules.

Gallium Mesoporphyrin IX-Mediated Photodestruction: A Pharmacological Trojan Horse Strategy To Eliminate Multidrug-Resistant Staphylococcus aureus.

One of the factors determining efficient antimicrobial photodynamic inactivation (aPDI) is the accumulation of a light-activated compound, namely, a photosensitizer (PS). Targeted PS recognition is the approach based on the interaction between the membrane receptor on the bacterial surface and the PS, whereas the compound is efficiently accumulated by the same mechanism as the natural ligand. In this study, we showed that gallium mesoporphyrin IX (Ga3+MPIX) provided dual functionality─iron metabolism disruption and PS properties in aPDI. Ga3+MPIX induced efficient (>5log10 reduction in CFU/mL) bacterial photodestruction with excitation in the area of Q band absorption with relatively low eukaryotic cytotoxicity and phototoxicity. The Ga3+MPIX is recognized by the same systems as haem by the iron-regulated surface determinant (Isd). However, the impairment in the ATPase of the haem detoxification efflux pump was the most sensitive to the Ga3+MPIX-mediated aPDI phenotype. This indicates that changes within the metalloporphyrin structure (vinyl vs ethyl groups) did not significantly alter the properties of recognition of the compound but influenced its biophysical properties.

Fine Particulate Matter-Induced Oxidative Stress Mediated by UVA-Visible Light Leads to Keratinocyte Damage.

The human skin is exposed to various environmental factors including solar radiation and ambient air pollutants. Although, due to its physical and biological properties, the skin efficiently protects the body against the harm of environmental factors, their excessive levels and possible synergistic action may lead to harmful effects. Among particulate matter present in ambient air pollutants, PM2.5 is of particular importance for it can penetrate both disrupted and intact skin, causing adverse effects to skin tissue. Although certain components of PM2.5 can exhibit photochemical activity, only a limited amount of data regarding the interaction of PM2.5 with light and its effect on skin tissue are available. This study focused on light-induced toxicity in cultured human keratinocytes, which was mediated by PM2.5 obtained in different seasons. Dynamic Light Scattering (DLS) and Atomic Force Microscopy (AFM) were employed to determine sizes of the particles. The ability of PM2.5 to photogenerate free radicals and singlet oxygen was studied using EPR spin-trapping and time-resolved singlet oxygen phosphorescence, respectively. Solar simulator with selected filters was used as light source for cell treatment to model environmental lightning conditions. Cytotoxicity of photoexcited PM2.5 was analyzed using MTT assay, PI staining and flow cytometry, and the apoptotic pathway was further examined using Caspase-3/7 assay and RT-PCR. Iodometric assay and JC-10 assay were used to investigate damage to cell lipids and mitochondria. Light-excited PM2.5 were found to generate free radicals and singlet oxygen in season-dependent manner. HaCaT cells containing PM2.5 and irradiated with UV-Vis exhibited oxidative stress features-increased peroxidation of intracellular lipids, decrease of mitochondrial membrane potential, enhanced expression of oxidative stress related genes and apoptotic cell death. The data indicate that sunlight can significantly increase PM2.5-mediated toxicity in skin cells.

Photoreactivity of Hair Melanin from Different Skin Phototypes-Contribution of Melanin Subunits to the Pigments Photoreactive Properties.

Photoreactivity of melanin has become a major focus of research due to the postulated involvement of the pigment in UVA-induced melanoma. However, most of the hitherto studies were carried out using synthetic melanin models. Thus, photoreactivity of natural melanins is yet to be systematically analyzed. Here, we examined the photoreactive properties of natural melanins isolated from hair samples obtained from donors of different skin phototypes (I, II, III, and V). X-band and W-band electron paramagnetic resonance (EPR) spectroscopy was used to examine the paramagnetic properties of the pigments. Alkaline hydrogen peroxide degradation and hydroiodic acid hydrolysis were used to determine the chemical composition of the melanins. EPR oximetry and spin trapping were used to examine the oxygen photoconsumption and photo-induced formation of superoxide anion, and time-resolved near infrared phosphorescence was employed to determine the singlet oxygen photogeneration by the melanins. The efficiency of superoxide and singlet oxygen photogeneration was related to the chemical composition of the studied melanins. Melanins from blond and chestnut hair (phototypes II and III) exhibited highest photoreactivity of all examined pigments. Moreover, melanins of these phototypes showed highest quantum efficiency of singlet oxygen photogeneration at 332 nm and 365 nm supporting the postulate of the pigment contribution in UVA-induced melanoma.

The role of hydrogen peroxide and singlet oxygen in the photodegradation of melanin.

When aging, melanin in human retinal pigment epithelium (RPE) undergoes oxidative modifications, which increase its photoreactivity and reduce its antioxidant capacity, elevating the risk of chronic phototoxicity to the retina. The aim of this research was to examine the effect of iron on the degradation of melanin induced by hydrogen peroxide and light, and to elucidate the role of hydrogen peroxide and singlet oxygen in the photodegradation of melanin. A water-soluble synthetic model of eumelanin with and without iron ions was treated either with exogenous hydrogen peroxide or with intense violet light. Oxidative modifications of melanin were analyzed by electron paramagnetic resonance (EPR) spectroscopy, absorption spectrophotometry, dynamic light scattering (DLS) and by chemical analysis of melanin subunits. The results showed that although iron strongly accelerated melanin degradation induced by hydrogen peroxide, it had very little influence on the rate of photodegradation of melanin. On the other hand, the photodegradation of melanin was partly inhibited by NaN3. The determination of hydrogen peroxide together with oxygen uptake indicates that irradiated melanin generates similar amounts of singlet oxygen and hydrogen peroxide. Analysis of melanin samples exhibiting comparable reduction of their EPR signal revealed that the loss of the representative melanin subunits was much higher in irradiated samples than in those treated with hydrogen peroxide in the dark. In conclusion, hydrogen peroxide, formed during the aerobic photolysis of melanin, is not responsible for the accompanying oxidative modifications of melanin. On the other hand, singlet oxygen can be considered as a key oxidizing agent involved in the photodegradation of melanin.

Quercetin protects ARPE-19 cells against photic stress mediated by the products of rhodopsin photobleaching.

Although the primary biological function of retina photoreceptors is to absorb light and provide visual information, exposure to intense light could increase the risk of phototoxic reactions mediated by rhodopsin photobleaching products (RPBP) that might accumulate in photoreceptor outer segments (POS). Here we investigated whether quercetin can modify the phototoxic potential of RPBP under in vitro photic stress conditions. ARPE-19 cells or quercetin enriched cultures pre-loaded with rhodopsin-rich POS isolated from bovine retinas were irradiated with green light to photobleach rhodopsin, and subsequently with blue light. Survival of cells was determined by MTT assay and propidium iodide staining. Changes in mitochondrial membrane potential (MMP) were assessed by JC-1 staining. Protein hydroperoxides, formed by photosensitized oxidation, mediated by RPBP, were analyzed in cells and in a model system with bovine serum albumin (BSA), using the coumarin boronic acid fluorogenic probe. The effect of photic stress on specific phagocytosis of RPE cells was determined by flow cytometry. Photoreactivity of POS with and without quercetin was analyzed by EPR oximetry and EPR spin trapping. Cytotoxicity measurements and MMP analyses confirmed that supplementation with quercetin protected ARPE-19 cells against photic stress mediated by rhodopsin-rich POS. Quercetin significantly reduced the inhibitory effect of RPBP-mediated stress on POS phagocytosis and the RPBP ability to photooxidize cellular proteins or BSA. The data support the hypothesis that quercetin may efficiently diminish the phototoxic action of retinoids, necessary for restoring the phagocytic function of ARPE-19 cells.

Sodium nitrite potentiates antimicrobial photodynamic inactivation: possible involvement of peroxynitrate.

We have recently shown that a wide range of different inorganic salts can potentiate antimicrobial photodynamic inactivation (aPDI) and TiO2-mediated antimicrobial photocatalysis. Potentiation has been shown with azide, bromide, thiocyanate, selenocyanate, and most strongly, with iodide. Here we show that sodium nitrite can also potentiate broad-spectrum aPDI killing of Gram-positive MRSA and Gram-negative Escherichia coli bacteria. Literature reports have previously shown that two photosensitizers (PS), methylene blue (MB) and riboflavin, when excited by broad-band light in the presence of nitrite could lead to tyrosine nitration. Addition of up to 100 mM nitrite gave 6 logs of extra killing in the case of Rose Bengal excited by green light against E. coli, and 2 logs of extra killing against MRSA (eradication in both cases). Comparable results were obtained for other PS (TPPS4 + blue light and MB + red light). Some bacterial killing was obtained when bacteria were added after light using a functionalized fullerene (LC15) + nitrite + blue light, and tyrosine ester amide was nitrated using both "in" and "after" modes with all four PS. The mechanism could involve formation of peroxynitrate by a reaction between superoxide radicals and nitrogen dioxide radicals; formation of the latter species was demonstrated by spin trapping with nitromethane.

Scavenging of Retinoid Cation Radicals by Urate, Trolox, and α-, ß-, γ-, and δ-Tocopherols.

Retinoids are present in human tissues exposed to light and under increased risk of oxidative stress, such as the retina and skin. Retinoid cation radicals can be formed as a result of the interaction between retinoids and other radicals or photoexcitation with light. It has been shown that such semi-oxidized retinoids can oxidize certain amino acids and proteins, and that α-tocopherol can scavenge the cation radicals of retinol and retinoic acid. The aim of this study was to determine (i) whether ß-, γ-, and δ-tocopherols can also scavenge these radicals, and (ii) whether tocopherols can scavenge the cation radicals of another form of vitamin A-retinal. The retinoid cation radicals were generated by the pulse radiolysis of benzene or aqueous solution in the presence of a selected retinoid under oxidizing conditions, and the kinetics of retinoid cation radical decays were measured in the absence and presence of different tocopherols, Trolox or urate. The bimolecular rate constants are the highest for the scavenging of cation radicals of retinal, (7 to 8) × 109 M-1·s-1, followed by retinoic acid, (0.03 to 5.6) × 109 M-1·s-1, and retinol, (0.08 to 1.6) × 108 M-1·s-1. Delta-tocopherol is the least effective scavenger of semi-oxidized retinol and retinoic acid. The hydrophilic analogue of α-tocopherol, Trolox, is substantially less efficient at scavenging retinoid cation radicals than α-tocopherol and urate, but it is more efficient at scavenging the cation radicals of retinoic acid and retinol than δ-tocopherol. The scavenging rate constants indicate that tocopherols can effectively compete with amino acids and proteins for retinoid cation radicals, thereby protecting these important biomolecules from oxidation. Our results provide another mechanism by which tocopherols can diminish the oxidative damage to the skin and retina and thereby protect from skin photosensitivity and the development and/or progression of changes in blinding retinal diseases such as Stargardt's disease and age-related macular degeneration (AMD).

Potassium Iodide Potentiates Antimicrobial Photodynamic Inactivation Mediated by Rose Bengal in In Vitro and In Vivo Studies.

Rose bengal (RB) is a halogenated xanthene dye that has been used to mediate antimicrobial photodynamic inactivation for several years. While RB is highly active against Gram-positive bacteria, it is largely inactive in killing Gram-negative bacteria. We have discovered that addition of the nontoxic salt potassium iodide (100 mM) potentiates green light (540-nm)-mediated killing by up to 6 extra logs with the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, the Gram-positive bacterium methicillin-resistant Staphylococcus aureus, and the fungal yeast Candida albicans The mechanism is proposed to be singlet oxygen addition to iodide anion to form peroxyiodide, which decomposes into radicals and, finally, forms hydrogen peroxide and molecular iodine. The effects of these different bactericidal species can be teased apart by comparing the levels of killing achieved in three different scenarios: (i) cells, RB, and KI are mixed together and then illuminated with green light; (ii) cells and RB are centrifuged, and then KI is added and the mixture is illuminated with green light; and (iii) RB and KI are illuminated with green light, and then cells are added after illumination with the light. We also showed that KI could potentiate RB photodynamic therapy in a mouse model of skin abrasions infected with bioluminescent P. aeruginosa.

The nanomechanical role of melanin granules in the retinal pigment epithelium.

Nanomechanical properties of cells and tissues, in particular their elasticity, play an important role in different physiological and pathological processes. Recently, we have demonstrated that melanin granules dramatically modify nanomechanical properties of melanoma cells making them very stiff and, as a result, less aggressive. Although the mechanical effect of melanin granules was demonstrated in pathological cells, it was never studied in the case of normal cells. In this work, we analyzed the impact of melanin granules on nanomechanical properties of primary retinal pigment epithelium tissue fragments isolated from porcine eyes. The obtained results clearly show that melanin granules are responsible for the exceptional nanomechanical properties of the tissue. Our findings suggest that melanin granules in the retinal pigment epithelium may play an important role in sustaining the stiffness of this single cell layer, which functions as a natural mechanical barrier separating the retina from the choroid.

Antimicrobial photodynamic therapy with fulleropyrrolidine: photoinactivation mechanism of Staphylococcus aureus, in vitro and in vivo studies.

A family of N-methylpyrrolidinium fullerene iodide salts has been intensively studied to determine their applicability in antimicrobial photodynamic therapy (APDT). This study examined in vitro the efficacy of a C60 fullerene functionalized with one methylpyrrolidinium group to kill upon irradiation with white light gram-negative and gram-positive bacteria, as well as fungal cells, and the corresponding mechanism of the fullerene bactericidal action. The in vitro studies revealed that the high antistaphylococcal efficacy of functionalized fullerene could be linked to their ability to photogenerate singlet oxygen and superoxide anion. Following Staphylococcus aureus photoinactivation, no modifications of its genomic DNA were detected. In contrast, photodamage of the cell envelope seemed to be a dominant mechanism of bactericidal action. In in vivo studies, a 2 log10 reduction in the average bioluminescent radiance between treated and non-treated mice was reached. One day post APDT treatment, moist and abundant growth of bacteria could be observed on wounds of non-fulleropyrrolidine and dark control mice. APDT-treated wounds stayed visibly clear up to the third day. Moreover, cytotoxicity test on human dermal keratinocytes revealed great safety of using the sensitizer toward eukaryotic cells. These data indicate potential application of functionalized fullerene as antistaphylococcal sensitizer for superficial infections.

Role of semiconductivity and ion transport in the electrical conduction of melanin.

Melanins are pigmentary macromolecules found throughout the biosphere that, in the 1970s, were discovered to conduct electricity and display bistable switching. Since then, it has been widely believed that melanins are naturally occurring amorphous organic semiconductors. Here, we report electrical conductivity, muon spin relaxation, and electron paramagnetic resonance measurements of melanin as the environmental humidity is varied. We show that hydration of melanin shifts the comproportionation equilibrium so as to dope electrons and protons into the system. This equilibrium defines the relative proportions of hydroxyquinone, semiquinone, and quinone species in the macromolecule. As such, the mechanism explains why melanin at neutral pH only conducts when "wet" and suggests that both carriers play a role in the conductivity. Understanding that melanin is an electronic-ionic hybrid conductor rather than an amorphous organic semiconductor opens exciting possibilities for bioelectronic applications such as ion-to-electron transduction given its biocompatibility.

Cell elasticity is an important indicator of the metastatic phenotype of melanoma cells.

The relationship between melanin pigmentation and metastatic phenotype of melanoma cells is an intricate issue, which needs to be unambiguously determined to fully understand the process of metastasis of malignant melanoma. Despite significant research efforts undertaken to solve this problem, the outcomes are far from being satisfying. Importantly, none of the proposed explanations takes into consideration biophysical aspects of the phenomenon such as cell elasticity. Recently, we have demonstrated that melanin granules dramatically modify elastic properties of pigmented melanoma cells. This prompted us to examine the mechanical effects of melanosomes on the transmigration abilities of melanoma cells. Here, we show for the first time that melanin granules inhibit transmigration abilities of melanoma cells in a number of granules dependent manner. Moreover, we demonstrate that the inhibitory effect of melanosomes is mechanical in nature. Results obtained in this study demonstrate that cell elasticity may play a key role in the efficiency of melanoma cells spread in vivo. Our findings may also contribute to better understanding of the process of metastasis of malignant melanoma.

Potentiation of photoinactivation of Gram-positive and Gram-negative bacteria mediated by six phenothiazinium dyes by addition of azide ion.

Antimicrobial photodynamic inactivation (APDI) using phenothiazinium dyes is mediated by reactive oxygen species consisting of a combination of singlet oxygen (quenched by azide), hydroxyl radicals and other reactive oxygen species. We recently showed that addition of sodium azide paradoxically potentiated APDI of Gram-positive and Gram-negative bacteria using methylene blue as the photosensitizer, and this was due to electron transfer to the dye triplet state from azide anion, producing azidyl radical. Here we compare this effect using six different homologous phenothiazinium dyes: methylene blue, toluidine blue O, new methylene blue, dimethylmethylene blue, azure A, and azure B. We found both significant potentiation (up to 2 logs) and also significant inhibition (>3 logs) of killing by adding 10 mM azide depending on Gram classification, washing the dye from the cells, and dye structure. Killing of E. coli was potentiated with all 6 dyes after a wash, while S. aureus killing was only potentiated by MB and TBO with a wash and DMMB with no wash. More lipophilic dyes (higher log P value, such as DMMB) were more likely to show potentiation. We conclude that the Type I photochemical mechanism (potentiation with azide) likely depends on the microenvironment, i.e. higher binding of dye to bacteria. Bacterial dye-binding is thought to be higher with Gram-negative compared to Gram-positive bacteria, when unbound dye has been washed away, and with more lipophilic dyes.

Can l-ascorbic acid and trans-resveratrol protect HaCaT cells from fine particulate matter toxicity?

Continuous exposure of human skin to air pollution can result in a range of undesirable skin conditions. In our recent study, UV and visible light were found to increase cytotoxicity of fine particulate matter (PM2.5 ) against human keratinocytes. Since it is impossible to avoid exposure of human skin to PM2.5 , effective strategies are needed to reduce their damaging effects. l-ascorbic acid and resveratrol were tested as potential topical agents against pollution-related skin impairment. Although these agents were previously found to ameliorate PM-dependent damage, the effect of light and seasonal variation of particles were not previously studied. EPR spin-trapping, DPPH assay, and singlet oxygen phosphorescence were used to determine the scavenging activities of the antioxidants. MTT, JC-10 and iodometric assays were used to analyze the effect on PM2.5 -induced cytotoxicity, mitochondrial damage and oxidation of lipids. Live-cell imaging was employed to examine wound-healing properties of cells. Light-induced, PM2.5 -mediated oxidative damage was examined by immunofluorescent staining. Both antioxidants effectively scavenged free radicals and singlet oxygen produced by PM2.5 , reduced cell death and prevented oxidative damage to HaCaT cells. l-ascorbic acid and resveratrol, especially when applied in combination, can protect HaCaT cells against the dark and light induced toxicity of PM2.5 .

The effect of oxidative degradation of Dopa-melanin on its basic physicochemical properties and photoreactivity.

Melanin, particularly eumelanin, is commonly viewed as an efficient antioxidant and photoprotective pigment. Nonetheless, the ability of melanin to photogenerate reactive oxygen species and sensitize the formation of cyclobutane pyrimidine dimers may contribute to melanin-dependent phototoxicity. The phototoxic potential of melanin depends on a variety of factors, including molecular composition, redox state, and degree of aggregation. Using complementary spectroscopic and analytical methods we analyzed the physicochemical properties of Dopa-melanin, a synthetic model of eumelanin, subjected to oxidative degradation induced by aerobic photolysis or exposure to 0.1 M hydrogen peroxide. Both modes of oxidative degradation were accompanied by dose-dependent bleaching of melanin and irreversible modifications of its paramagnetic, ion- and electron-exchange and antioxidant properties. Bleached melanin exhibited enhanced efficiency to photogenerate singlet oxygen in both UVA and short-wavelength visible light. Although chemical changes of melanin subunits, including a relative increase of DHICA content and disruption of melanin polymer induced by oxidative degradation were considered, these two mechanisms may not be sufficient for a satisfactory explanation of the elevated photosensitizing ability of the bleached eumelanin. This study points out possible adverse changes in the photoprotective and antioxidant properties of eumelanin that could occur in pigmented tissues after exposure to high doses of intense solar radiation.

Photodynamics of Melanin Radicals: Contribution to Photoprotection by Melanin†.

The mechanism of very efficient relaxation of the melanin-photoexcited states, responsible for the photoprotective action of the pigment, remains a subject for intense investigation. The most recent study by C. Grieco, F. Kohl, and B. Kohler, entitled "Ultrafast radical photogeneration pathways in eumelanin," addresses key issues of melanin photophysics and photochemistry. By using femtosecond broad-band pump probe-transient absorption measurements, the researchers were able to identify the absorption spectrum of DOPA melanin radicals for the first time and proposed two distinct mechanisms of radical formation-photoionization and photoinduced charge separation. The observed photodynamic of melanin radicals suggests a new paradigm in which the ultrafast excited state deactivation is due to the efficient recombination of melanin radicals created promptly by photoexcitation.