Dimeric Products Research Articles

Degradation and Detoxification of Chlorophenols with Different Structure by LAC-4 Laccase Purified from White-Rot Fungus Ganoderma lucidum.

A laccase named LAC-4 was purified from Ganoderma lucidum. Firstly, the enzymatic properties of purified LAC-4 laccase, and the degradation of three chlorophenol pollutants 2,6-dichlorophenol (2,6-DCP), 2,3,6-trichlorophenol (2,3,6-TCP) and 3-chlorophenol (3-CP) by LAC-4 were systematically studied. LAC-4 had a strong ability for 2,6-DCP and 2,3,6-TCP degradation. The degradation ability of LAC-4 to 3-CP was significantly lower than that of 2,6-DCP and 2,3,6-TCP. LAC-4 also had a good degradation effect on the chlorophenol mixture (2,6-DCP + 2,3,6-TCP). The results of kinetics of degradation of chlorophenols by LAC-4 suggested that the affinity of LAC-4 for 2,6-DCP was higher than 2,3,6-TCP. The catalytic efficiency and the catalytic rate of LAC-4 on 2,6-DCP were also significantly higher than 2,3,6-TCP. During degradation of 2,6-DCP and 2,3,6-TCP, LAC-4 had a strong tolerance for high concentrations of different metal salts (such as MnSO4, ZnSO4, Na2SO4, MgSO4, CuSO4, K2SO4) and organic solvents (such as ethylene glycol and glycerol). Next, detoxification of chlorophenols by LAC-4 was also systematically explored. LAC-4 treatment had a strong detoxification ability and a good detoxification effect on the phytotoxicity of individual chlorophenols (2,6-DCP, 2,3,6-TCP) and chlorophenol mixtures (2,6-DCP + 2,3,6-TCP). The phytotoxicities of 2,6-DCP, 2,3,6-TCP and chlorophenol mixtures (2,6-DCP + 2,3,6-TCP) treated with LAC-4 were considerably reduced or eliminated. Finally, we focused on the degradation mechanisms and pathways of 2,6-DCP and 2,3,6-TCP degradation by LAC-4. The putative transformation pathway of 2,6-DCP and 2,3,6-TCP catalyzed by laccase was revealed for the first time. The free radicals formed by LAC-4 oxidation of 2,6-DCP and 2,3,6-TCP produced dimers through polymerization. LAC-4 catalyzed the polymerization of 2,6-DCP and 2,3,6-TCP, forming dimer products. LAC-4 catalyzed 2,6-DCP into two main products: 2,6-dichloro-4-(2,6-dichlorophenoxy) phenol and 3,3′,5,5′-tetrachloro-4,4′-dihydroxybiphenyl. LAC-4 catalyzed 2,3,6-TCP into two main products: 2,3,6-trichloro-4-(2,3,6-trichlorophenoxy) phenol and 2,2′,3,3′,5,5′-hexachloro-[1,1′-biphenyl]-4,4′-diol.

Revisiting the condensation reaction of lignin in alkaline pulping with quantitativity. Part 2: evaluation of self-condensation of vanillyl alcohol compared with its condensation with creosol under soda cooking conditions

Condensation of lignin is believed to interfere with delignification in alkaline pulping processes. The lack of clear evidence motivated us to quantitatively revisit it. This article evaluates the self-condensation of 4-hydroxymethyl-2-methoxyphenol (vanillyl alcohol, Va) in the simplest model system using Va and 2-methoxy-4-methylphenol (creosol, Cr) under the same soda cooking conditions employed in our previous paper. Dimeric and trimeric self-condensation products of Va consisting of the α-5 bond (VaVa, 2-(4-hydroxy-3-methoxybenzyl)-4-hydroxymethyl-6-methoxyphenol), α-1 bond (VaVa’, bis(4-hydroxy-3-methoxyphenyl)methane), and α-5 & α-1 bonds (VaVaVa’, 2,4-bis(4-hydroxy-3-methoxy-benzyl)-6-methoxyphenol) were identified and quantified. The formation rate of VaVa from Va was suggested to be about twice of that of VaVa’, and to be similar to that of the condensation product between the α-carbon of Va and the aromatic C5-carbon of Cr (VaCr), which was identified in the first paper. Among all these detected condensation products, only VaVa that can be converted to the corresponding quinone methide (QM) readily underwent further condensation to be VaVaVa’ and others. Only the monomers (Va and Cr) possessing the small steric factors condense with electrophilic QM’s as nucleophiles. These results suggest that condensation of lignin in actual soda cooking processes, with steric factors much larger than the model system, progresses much less frequently than generally believed.

An Efficient Route to 3,3'-Biindolinylidene-diones by Iron-Catalyzed Dimerization of Isatins.

Iron-catalyzed dimerization of various isatin derivatives is described for the efficient synthesis of 3,3'-biindolinylidene-diones (isoindigos). The reaction provides easy access to self-coupled and cross-coupled 3,3'-indolinylidene-diones that have high relevance to biology and materials. This Fe(0)- or Fe(II)-catalyzed dimerization reaction tolerates a wide range of functionalities, such as fluoro, chloro, bromo, alkenyl, nitrile, ether, ester, pyrrolyl, indolyl and carbazolyl groups, including cyclic and acyclic alkyls as well as an alkyl-bearing fatty-alcohol moiety. Especially, the coupling between two distinct isatins provided excellent selectivity for the cross-dimerization with trace of self-couplings. The single-crystal X-ray diffraction study established the molecular structure of eight dimerized products. A preliminary mechanistic study of the Fe-catalyzed dimerization supported the radical pathway for the reaction.

Independent Subsystems of Atomic Hydrogen in Silicon Responsible for Boron Passivation and for Dimer Production

Herein, the reported data on boron–hydrogen defects HB are analyzed to conclude that there are at least two independent hydrogen ions, H+(1) and H+(2), that passivate boron by forming the HB(1) and HB(2) defects, respectively. The two H+(i)/HB(i) subsystems differ remarkably by the value of the transport parameter DK (a product of the H+ diffusivity and the equilibrium dissociation constant of HB). There are numerous pairing reactions involving different single‐hydrogen species in positive and neutral charge states, and producing various kinds of dimeric hydrogen H2. Within this model, a good fit to plasma‐induced hydrogen profiles (including a complicated shape found in boron‐implanted sample) is obtained.

Maresin 1 alleviates sevoflurane-induced neuroinflammation in neonatal rats via JAK2/STAT3/IL-6 pathways

Developmental neurotoxicity is a serious consequence of repeated exposure to sevoflurane during the postnatal period. Neuroinflammation is closely related to sevoflurane-induced developmental neurotoxicity. The present study explored the potential effect of maresin 1 (MaR1), an anti-inflammatory agent, on neuroinflammation and neurotoxicity induced by repeated sevoflurane exposure. In this study, MaR1 ameliorated sevoflurane-induced cognitive dysfunction, Tau hyperphosphorylation, and synaptic protein reduction. The protein level of the proinflammatory factor interleukin-6 (IL-6) and its upstream regulator Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway were substantially upregulated by sevoflurane in rat brains, but downregulated after MaR1 administration. To confirm the involvement of this pathway, we used coumermycin A1 (CA1), a chemical dimeric natural product that induced dimerization and activation of JAK2, and observed that the protective effect of MaR1 against sevoflurane-induced toxicity was completely blocked, as shown by decreased synaptic protein levels and increased Tau phosphorylation. Furthermore, it confirmed by dual luciferase reporter assay that there existed a STAT3′s binding motif on the promoter sequence of IL-6. These results suggest that the protective effect of MaR1 against developmental neurotoxicity is mediated through the JAK2/STAT3/IL-6 signaling pathway and provide an experimental basis for clinical guidance on the use of sevoflurane.

S-Nitrosothiol-Impregnated Silicone Catheter for Colorimetric Sensing of Indole and E. coli: Toward On-Body Detection of Urinary Tract Infections.

Although there are many techniques to detect pathogenic bacteria, most of them are only suited for in vitro diagnostics. We report a urinary catheter-based colorimetric sensor for potential on-body detection of E. coli, the most prevalent bacterial species in urinary tract infections associated with the use of urinary catheters. In urine, indole is secreted by E. coli and reacts with a nitrosating agent incorporated in a silicone catheter. A red dimeric product, indoxyl red, is generated within silicone rubber to allow for color-based indole sensing with high sensitivity, linearity, and specificity. This reaction is initiated by the nitrosation reaction of indole at its C-3 position via reagents like sodium nitrite or S-nitroso-N-acetyl-penicillamine under aerobic conditions. The generated 3-nitrosoindole undergoes tautomerization, dimerization, and deoximation to form indoxyl red with high absorbance at 537 nm. In contrast to other indole sensors, the presented method can be applied in real catheters to detect indole and E. coli in biofluids such as urine. The is because (1) S-nitroso-N-acetyl-penicillamine, the nitrosating agent, can be impregnated into silicone elastomers, (2) indole from urine is extracted into silicone due to its hydrophobicity, and (3) the high acidity and oxygen solubility of silicone facilitates the sensing reaction within the silicone matrix. This silicone-based colorimetric sensor clearly differentiates E. coli below and above 105 CFU/mL, which is the threshold concentration of bacteriuria. We expect that early diagnosis of urinary tract infections using the naked eye is possible by functionalizing an exposed section of urinary catheters with the proposed molecular probe.

Variable hydrazine coordination modes from reactions with dichlorotris(triphenylphosphine)ruthenium(II)

The reactivity of the prominent ruthenium(II) catalyst and precursor, RuCl2(PPh3)3, was explored with hydrazine and its derivatives. The reactions produced 6-coordinate ruthenium(II) products that bond with the hydrazine moiety in bridging (NH2NH2 or NH2NHMe), end-on (NH2NHMe, NH2NHPh, or NH2NMe2), or side-on (NH2NMe2) fashions. The products have been characterized by single crystal X-ray diffraction and 31P NMR spectroscopy. With NH2NH2, the dimeric product, [RuCl2(PPh3)2]2(μ-N2H4)2, was formed under all conditions used. For other hydrazines, the reaction stoichiometry governed product formation to yield either bimetallic or monomeric products. When the RuCl2(PPh3)3 to hydrazine derivative ratio was 1:1 or lower, bimetallic complexes were formed while a 1:2 ratio produced monomeric compounds. The exception to the latter occurs with the production of the monomeric compound, RuCl2(PPh3)2(η2-NH2NMe2), that forms employing a 1:1 ratio. In addition to stoichiometry, steric factors and hydrogen bonding played a significant role in dictating the bonding mode of hydrazines. The bimetallic complexes were further reacted to yield the corresponding monomeric products, adopting the form RuCl2(PPh3)2(L)2 (L = NH2NHMe, NH2NHPh, or NH2NMe2). Reactions utilizing less than a 1:2 ratio of RuCl2(PPh3)3 to NH2NMe2 produced a mixture of products that appear to be in equilibrium. DFT calculations were performed to gain insight into the reaction energetics and molecular stability.

Consequences of Intrapore Liquids on Reactivity, Selectivity, and Stability for Aldol Condensation Reactions on Anatase TiO2 Catalysts

AbstractThis study provides evidence and mechanistic interpretations for the significant consequences of intrapore non‐polar liquids on acetone aldol condensation turnover rates, selectivity to primary dimer products, and catalyst stability for reactions at Lewis acid‐base site pairs on TiO2 surfaces. These non‐polar liquids confer such benefits through the preferential stabilization of transition states (TS) for adsorption (“entry”) and desorption (“exit”) steps, which place their respective reactants or products within a solvating outer sphere environment. The extent to which non‐polar fluids (n‐heptane) form an intrapore liquid phase within TiO2 voids was obtained from N2 uptakes using established formalisms that consider the different molal volume, surface tension, and volatility between N2 and n‐heptane. Acetone condensation rates are limited by C−H activation, an “entry” step that forms bound prop‐1‐en‐2‐olates via a TS stabilized by intrapore liquids, leading to higher aldol condensation turnover rates as n‐heptane pressure increases and active TiO2 surfaces become increasingly immersed within a non‐polar liquid phase. These liquids solvate the late TS structures that mediate the desorption of primary C6 condensation products even more effectively than those involved in prop‐1‐en‐2‐olate formation or in nucleophilic attack events that later form C−C bonds. Such preferential solvation favors desorption over C−C coupling events, thus disfavoring the formation of larger oligomers that become stranded at active sites, thus leading to much slower deactivation. Moreover, solvation by non‐polar liquids also leads to C6 alkanones as the sole products formed in a single surface sojourn. These effects of a non‐polar dense phase circumvent the inherent stability, reactivity, and selectivity hurdles that have precluded practical aldol condensation catalysis on Lewis acid‐base pairs at oxide surfaces; these consequences are demonstrated here for TiO2 catalysts, acetone aldol condensation reactions, and n‐heptane as the non‐polar liquid but through strategies, concepts, and mechanistic features that extend to other systems. More generally, these observations and their mechanistic origins demonstrate how a contacting liquid preferentially solvates TS structures for elementary steps that involve either reactants arriving from or products entering into an outer sphere environment that contains a dense non‐polar phase.

One-electron oxidation of guanine derivatives: Detection of 2,5-diaminoimidazolone and novel guanine-guanine cross-links as major end products

Oxidation of guanine derivatives initiated by electron transfer is different from the process initiated by hydroxyl radical attack in many aspects. The initially generated radical cation undergoes rapid deprotonation in neutral solutions to form a neutral radical G(-H)•, which is well known for its low reactivity. The focus of the present study is the fate of G(-H)•- in a non-reducing environment that prevents it from reduction back to guanine. The oxidation of guanosine and 2′-deoxyguanosine was studied using a series of well-characterized and selectively generated one-electron oxidants including free radicals, metal complexes, and the triplet photosensitizers. 2.5-Diaminoimidazolone, formed with up to 20% yield, was found as the most abundant oxidation product in systems where simultaneous production of G(-H)•- and O2•- occurred, in agreement with the mechanism that involves the recombination of these two species. Less efficient anoxic and O2•- -independent pathways, however, were also shown to exist. Two other major components D1 and D2, found in all investigated systems, were identified as dimerization products by their molecular masses and fragmentation patterns in the mass-spectra. The molecular structures of D1 and D2 were suggested based of their 1H-NMR spectra. Two diastereomers of D1, stable within the pH range 2-9, were isolated and shown to serve as precursors to D2 in an oxidizing environment. The stability of D2 drops sharply with increasing pH with the half-lives of 10.4 hr (pH 4.6, 37oC), 4.8 hr (pH 6.9, 37oC), and 1.1 hr (pH 8.9, 4oC). The mechanism behind the dimer formation is not yet clear, although experimental evidence suggests that 8-oxoguanine is not involved in the process.

Diphosphorus Release and Heterocumulene Oligomerisation by Nickel Complexes.

The generation of diphosphorus molecules P2 under mild conditions in solution is a useful strategy to generate diphosphines via [4+2] cycloadditions. We recently described the release of P2 units from the nickel butterfly complex [{(IMes)Ni(CO)}2(μ2,η2:η2‐P2)] (IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene) upon addition of CO gas. Herein, we developed an alternative protocol for the same process using heterocumulenes. In addition to formation of P4 (the dimerisation product of P2), the reactions afford nickel complexes of novel pincer‐type ligands. Aryl isothiocyanates undergo a trimerisation within the coordination sphere of nickel and afford square planar nickel complexes with S−C−S pincer‐ligand frameworks. Carbon disulfide coordinates to the [(IMes)Ni]‐fragment in an η2‐fashion, affording a dinuclear complex. Similar products are formed when the N‐heterocyclic carbene nickel(0) complex [(IMes)Ni(vtms)2] is used as a precursor (vtms=vinyltrimethylsilane).

Mechanistic Study of the Effects of Agricultural Amendments on Photochemical Processes in Paddy Water during Rice Growth.

The photochemical properties of paddy water might be affected by the commonly used amendments in rice fields owing to the associated changes in water chemistry; however, this important aspect has rarely been explored. We examined the effects of agricultural amendments on the photochemistry of paddy water during rice growth. The amendments significantly influenced the photogenerated reactive intermediates (RIs) in paddy water, such as triplet dissolved organic matter (3DOM*), singlet oxygen, and hydroxyl radicals. Compared with control experiments without amendments, the application of straw and lime increased the RI concentrations by up to 16.8 and 11.1 times, respectively, while biochar addition had limited effects on RI generation from paddy water in in situ experiments under sunlight. Fluorescence emission-excitation matrix spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and structural equation modeling revealed that upon the addition of straw and lime amendments, humified DOM substances contained lignins, proteins, and fulvic acids, which could produce more RIs under irradiation. Moreover, the amendments significantly accelerated the degradation rate of 2,4-dichlorophenol but led to the 3DOM*-mediated formation of more toxic and stable dimeric products. This study provides new insights into the effects of amendments on the photochemistry of paddy water and the pathways of abiotic degradation of organic contaminants in paddy fields.

Sonochemical degradation of DEET in aqueous medium: Complex by-products from synergistic effect of sono-Fenton – New insights from a HRMS study

A detailed investigation on the sonochemical degradation of DEET (N,N-diethyl-m-toluamide), a widely used broad spectrum insect repellant and an ubiquitous contaminant of emerging concern (CEC) in aquatic environments, was conducted in aqueous medium. The degradation followed a pseudo-first-order kinetics. The effects of pH, inorganic anions, organic matter and additives such as surfactants and ferrous ion (Fe2+) were evaluated. The degradation was favored under alkaline conditions, while comparable rates were found at acidic and near-neutral conditions. Presence of inorganic ions, organic matter and surfactants showed a positive effect on the degradation. Significant TOC reduction (≈96%) was achieved at 90 min of sonolysis which is the highest figure compared to all other reported AOP based degradation of DEET. The addition of Fe2+ (5 × 10−5 M) exhibited a positive synergy and leads to remarkable enhancement in the degradation efficiency up to 99%. The degradation products of both the sonolysis and sono-Fenton were analyzed by using LC-Q-ToF-MS. Fourteen sonochemical and six sono-Fenton degradation products were identified. Based on the identified intermediates, possible degradation pathways are proposed. Sonolysis mainly proceeds through hydroxylation (mono and poly) along with oxidation, demethylation, aromatic ring cleavage and hydrolysis. Whereas, majority of the sono-Fenton products are observed to be the breakdown products after dimerization followed by radical reactions. Studies characterizing such products are rather limited. This study provides new insights on complex breakdown products of DEET dimers formed during sono-Fenton, which has high relevance in its degradation protocol in natural environment as well as waste water treatment.

Mechanofluorochromism and mechanical force-triggered solid-state [2+2] photocycloaddition in α-cyanostilbene derivatives

α-Cyanostilbene derivatives are known as fluorescent photochromic molecules that could be used to realize emission colour change or on/off switching via photo-induced isomerization or cycloaddition. Comparing with isomerization, photocycloaddition is less studied due to its critical molecular position and alignment requirement for molecular packing. In this work, biphenyl-based cyanostilbenic compounds 1–3 were synthesized, which exhibit characteristic mechanofluorochromic (MFC) response behaviour. Under force stimuli, the luminescent colour has bathochromic shift from blue to green, together with the phase transition from crystal to amorphous state. Photochromic fluorescent response attributed to photo-induced [2 + 2] cycloaddition could take place instantly in amorphous state, rather than crystals. The MFC response can be successfully used to monitor the photoreaction process, giving rise to quantitative dimerized product. The single crystal structure of compound 2 revealed its photostable feature. The formation of excimers in ground amorphous state, together with the mechanical shearing, facilitates the occurrence of cycloaddition. Subsequently, the recovery process could be achieved thermally or under UV irradiation.

UHPLC-Q-Orbitrap /MS2 identification of (+)-Catechin oxidation reaction dimeric products in red wines and grape seed extracts

B-type procyanidin dimers and (+)-catechin dimeric oxidation products were analyzed in grape seed extracts and red wines (UHPLC-Q-Orbitrap MS). The different dimers had different fragmentation patterns according to their interflavan linkage position. Oxidation dimeric compounds had a specific fragment ion at m/z 393, missing for B-Type dimers fragmentations. A fragment ion at m/z 291 occurred and was specific for oxidation dimeric compounds with a COC linkage. Higher level oxidation products had abundant specific fragments: m/z 425, 397 and 245. These fragmentations were useful to identify them in complex samples such as grape seed extracts and wines. Three grape varieties and three ripening stages were selected and the corresponding seed extracts were obtained. The analyses revealed an increasing trend for the oxidation markers during grape ripening. The analysis of Syrah wines (2018, 2014, 2010) showed a decreasing trend of these molecules during wine ageing which might be due to further oxidation.

Biosynthetic machineries of anthraquinones and bisanthraquinones in Talaromyces islandicus.

Talaromyces islandicus is a unique fungus that produces more than 20 numbers of anthraquinones (AQs) and their dimeric natural products, bisanthraquinones (BQs). These compounds share a 9,10-anthracenedione core derived from emodin. The biosynthetic pathway of emodin has been firmly established, while that of other AQs and BQs is still unclear. In this study, we identified the biosynthetic gene clusters for chrysophanol and skyrin. The function of key modification enzymes was examined by performing biotransformation experiments and in vitro enzymatic reactions with emodin and its derivatives, allowing us to propose a mechanism for the modification reactions. The present study provides insight into the biosynthesis of AQs and BQs in T. islandicus.

Closing the loop: Valorizing pyrolyzed waste tyre residue into functional carbon materials, SiO2 with exceptionally high silanol groups, and Zn salt

This study aims to identify suitable processing conditions for converting pyrolytic solid residue from off-the-road tyres (OTR) to improve carbon materials properties that can be used in multiple applications and the recovery of minerals from OTR. Pyrolysis of OTR at 800 °C and a heating rate 2 °C.min−1 gave a carbon material with the highest surface area, most defective carbon structures, and the highest micro-porosity. This operating condition was used to compare the conventional three-step carbonization approach, which involves a demineralization stage that produces high volumes of toxic wastewater, with a two-step approach that bypasses this stage. Analysis of the carbon structures showed that the quality of the carbon material from the two-step approach is similar to the three-step approach. This two-step approach resulted in a solid and a liquid phase, in which ∼ 93.4% of Zn was selectively fractionated to the liquid phase. The wastewater from the acid wash of the carbonized OTR was neutralized to recover the SiO2, of which 55.5% was reactive SiO2. The SiO2 was found to have an exceptionally high cross-linking ratio of 5.94, achievable only when SiO2 is reacted with silane groups. The study demonstrated that the engineered carbon material from OTR has a H2 uptake of 1.03 wt% at 77 K and 1.2 bar, and the sulfonated counterpart was an effective catalyst (64% conversion) for the Aldol condensation of levunilic acid to two dimer products [tetrahydro-2- methyl-5,γ-dioxo-2-furanpentanoic acid (TMDFA) and 3-(2-methyl-5-oxo- tetrahydrofuran-2-yl)-4-oxopentanoic acid (MOTOA)] that are precursors for fuels and chemicals.

Protein Dimerization via Tyr Residues: Highlight of a Slow Process with Co-Existence of Numerous Intermediates and Final Products.

Protein dimerization via tyrosine residues is a crucial process in response to an oxidative attack, which has been identified in many ageing-related pathologies. Recently, it has been found that for isolated tyrosine amino acid, dimerization occurs through three types of tyrosine–tyrosine crosslinks and leads to at least four final products. Herein, considering two protected tyrosine residues, tyrosine-containing peptides and finally proteins, we investigate the dimerization behavior of tyrosine when embedded in a peptidic sequence. After azide radical oxidation and by combining UPLC-MS and H/D exchange analyzes, we were able to evidence: (i) the slow kinetics of Michael Addition Dimers (MAD) formation, i.e., more than 48 h; (ii) the co-existence of intermediates and final cyclized dimer products; and (iii) the probable involvement of amide functions to achieve Michael additions even in proteins. This raises the question of the possible in vivo existence of both intermediates and final entities as well as their toxicity and the potential consequences on protein structure and/or function.

Effect of Active Groups and Oxidative Dimerization on the Antimelanogenic Activity of Catechins and Their Dimeric Oxidation Products.

Some catechins and their dimeric oxidation products are well known to possess antimelanogenic activity, which could be influenced by their structures and oxidative dimerization. This study compared the antimelanogenic activity of different catechins and dimeric oxidation products and clarified the mechanism using an α-MSH-stimulated B16F10 cell model. It was found that 100 μg/mL (-)-gallocatechin gallate, (-)-epigallocatechin gallate, theasinensin A, and theaflavine-3,3'-digallate could significantly inhibit melanin synthesis without cytotoxicity. The tyrosinase (TYR) activities were 26.24 ± 4.97, 31.57 ± 5.37, 66.10 ± 9.62, and 78.19 ± 5.14%, respectively, and the melanin contents were 38.29 ± 3.50, 41.21 ± 7.62, 62.13 ± 9.80, and 68.82 ± 11.62%, respectively. These compounds inhibit melanin production by attenuating the mRNA levels of TYR, TRP1, and TRP2 gene. The structure-activity relationship showed that geometrical isomerism was not the key factor affecting catechins' antimelanogenic activity. Compared with the catechol, catechins with B-ring pyrogallol inhibited melanin synthesis more effectively. The number of galloyl groups was positively correlated with antimelanogenic activity. Compared with 3-galloyl, 3'-galloyl was a stronger active group in antimelanogenesis. Interestingly, the contribution of B-ring pyrogallol to the antimelanogenic activity was significantly stronger than that of 3-galloyl in catechins. Additionally, the antimelanogenic activity of the dimeric oxidation product at 100 μM was more than or equal to that of individual substrate-catechin, while being significantly less than that of the substrate-catechin mixture. Results indicated that pyrogallol and galloyl were the active groups inhibiting melanin synthesis. The oxidative dimerization weakened the antimelanogenic activity of the substrate-catechin mixture.

Insights into the Interactions that Trigger the Primary Nucleation of Polymorphic α-Synuclein Dimers.

Parkinson's disease is associated with the accumulation of α-synuclein (AS) aggregates that include polymorphic AS oligomers and polymorphic fibrils. There have been advances in solving the polymorphic state of AS fibrils, both by experimental techniques and molecular modeling tools. Yet, the polymorphic AS oligomers are now considered as the neurotoxic species, thus current and future studies making efforts to solve their structures at the molecular level. Importantly, it is crucial to explore the specific interactions between AS monomers within the dimer that stabilize the dimer and yield nucleation. Herein, we present a first work that probes at the molecular level the specific interactions between monomers in polymorphic AS dimers are derived from AS fibrils by applying molecular modeling tools. Our work reveals that both N-terminal and the non-amyloidogenic component domains play a role in the dimerization of all polymorphic AS dimers. In addition, helices along the N-terminal of AS monomers impede the contacts between AS monomers, thus preventing the nucleation or the dimerization of AS. This work provides insights into several mechanisms of the production of polymorphic AS dimers. Thus, the findings obtained in this work may assist in developing new therapeutic strategies for inhibiting the formation of the early-stage neurotoxic AS dimers.

MgO(111) Nanocatalyst for Biomass Conversion: A Study of Carbon Coating Effects on Catalyst Faceting and Performance

Solid base metal oxide catalysts such as MgO offer utility in a wide variety of syntheses from pharmaceuticals to fuels. The (111) facet of MgO shows enhanced, unique properties relative to the other facets. Carbon coatings have emerged as a promising modification to impart metal oxide catalyst stability. Here, we report the synthesis, characterization, and catalytic properties of commercial MgO, MgO(111), and carbon coated derivatives thereof for 2-pentanone condensation. The dimer and trimer products of this reaction can be used as precursors for biofuels upon oxygen removal and thus have relevance in environmental sustainability. MgO(111) maintained impressive selectivity towards the dimer product after carbon coating, whereas the other catalysts experienced a decrease in conversion and selectivity as a consequence of the carbon coating. Our findings highlight the catalytic efficacy of MgO(111), provide insight into carbon coating for catalyst stability, and pave the way for continued mechanistic investigations.