Phenolic Subunits Research Articles

The application of unsupervised machine learning for the elucidation of the burial effect of lignin in peatland: Case of TMAH thermochemolysis

Lignin is one of the major components of organic macromolecules in peatland. Analysis of lignin in sediments is considered as a valuable tool for understanding peat vegetation as well as carbon cycle. It provides valuable perspectives on different sides such as occurrence of various vegetation types and the extent of decay in terrestrial organic material. Thermochemolysis technique using tetramethyl ammonium hydroxide (TMAH) was used for lignin analysis, in an ombrotrophic peatland. It specifically breaks of O-aryl bonds followed by methylation of the lignin components yielding phenolic subunits. Several discrepancies exist between TMAH thermochemolysis findings and those obtained from previous investigations with CuO oxidation. This could be explained by the specific pool of aryl ether, specifically targeted by the thermochemolysis, with discarding of C-C bonds. Hence, this would show the capacity of TMAH thermochemolysis to cleave oxidised lignin leaving lignin of fresh and preserved organic matter intact. The TMAH thermochemolysis method's efficiency in molecular characterization and lignin degradation was evaluated using principal component analysis (PCA). PCA reduced dimensionality and redundancy of component contributions, with the first two principal components accounting for 70.33 % of the total variance. PCA has reinforced the selectively of the thermochemolytic approach in targeting O-aryl bonds while maintaining C-C bonding of polyphenolic sub-units. Hence, two distinct oxidation phases: recent (acrotelm and mesotelm) and older (catotelm) have been revealed. PCA was applied to diagenetic and source vegetation proxies, revealing strong agreement among variables which is supported by the use of molecular ratios as indicators of organic matter source and dynamics in a peat core.

Application of Unsupervised Learning for the Evaluation of Burial Behavior of Geomaterials in Peatlands: Case of Lignin Moieties Yielded by Alkaline Oxidative Cleavage

Tropical Peatlands accumulate organic matter (OM) and a significant source of carbon dioxide (CO2) and methane (CH4) under anoxic conditions. However, it is still ambiguous where in the peat profile these OM and gases are produced. The composition of organic macromolecules that are present in peatland ecosystems are mainly lignin and polysaccharides. As greater concentrations of lignin are found to be strongly related to the high CO2 and CH4 concentrations under anoxic conditions in the surface peat, the need to study the degradation of lignin under anoxic and oxic conditions has emerged. In this study, we found that the "Wet Chemical Degradation" approach is the most preferable and qualified to evaluate the lignin degradation in soils accurately. Then, we applied PCA for the molecular fingerprint consisting of 11 major phenolic sub-units produced by alkaline oxidation using cupric oxide (II) along with alkaline hydrolysis of the lignin sample presented in the investigated peat column called "Sagnes". The development of various characteristic indicators for lignin degradation state on the basis of the relative distribution of lignin phenols was measured by chromatography after CuO-NaOH oxidation. In order to achieve this aim, the so-called Principal Component Analysis (PCA) has been applied for the molecular fingerprint composed of the phenolic sub-units, yielded by CuO-NaOH oxidation. This approach aims to seek the efficiency of the already available proxies and potentially create new ones for the investigation of lignin burial along a peatland. Lignin phenol vegetation index (LPVI) is used for comparison. LPVI showed a higher correlation with PC1 rather than PC2. This confirms the potential of the application of LPVI to decipher vegetation change, even in a dynamic system as the peatland. The population is composed of the depth peat samples, and the variables are the proxies and relative contributions of the 11 yielded phenolic sub-units.

Low-Energy Electron Interactions with Resveratrol and Resorcinol: Anion States and Likely Dissociation Pathways.

We report a computational study of the anion states of the resveratrol (RV) and resorcinol (RS) molecules, also investigating dissociative electron attachment (DEA) pathways. RV has well-known beneficial effects in human health, and its antioxidant activity was previously associated with DEA reactions producing H2. Our calculations indicate a valence bound state and four resonances ( to ) for that system. While the computed thermodynamic thresholds are compatible with DEA reactions producing H2 at 0 eV, the well-known mechanism involving vibrational Feshbach resonances built on a dipole bound state should not be present in RV. Our results suggest that the shallow valence bound state is expected to account for H2 elimination, probably involving / couplings along the vibration dynamics. The RS molecule is also an oxidant and a subunit of RV. Because two close-lying hydroxyl groups are found in the RS moiety, the H2-elimination reaction in RV should take place at the RS site. Our calculations point out a correspondence between the anion states of RV and RS and even between the thresholds. Nevertheless, the absence of bound anion states in RS, indicated by our calculations, is expected to suppress the H2-formation channel at 0 eV. One is led to conclude that the ethene and phenol subunits in RV stabilize the state, thus switching on the DEA mechanism producing H2.

Lignin Biodegradation and Its Valorization

Lignin, a rigid polymer composed of phenolic subunits with high molecular weight and complex structure, ranks behind only cellulose in the contribution to the biomass of plants. Therefore, lignin can be used as a new environmentally friendly resource for the industrial production of a variety of polymers, dyes and adhesives. Since laccase was found to be able to degrade lignin, increasing attention had been paid to the valorization of lignin. Research has mainly focused on the identification of lignin-degrading enzymes, which play a key role in lignin biodegradation, and the potential application of lignin degradation products. In this review, we describe the source, catalytic specificity and enzyme reaction mechanism of the four classes of the lignin-degrading enzymes so far discovered. In addition, the major pathways of lignin biodegradation and the applications of the degradative products are also discussed. Lignin-degrading bacteria or enzymes can be used in combination with chemical pretreatment for the production of value-added chemicals from lignin, providing a promising strategy for lignin valorization.

Polythiacalixarene-Embedded Gold Nanoparticles for Visible-Light-Driven Photocatalytic CO2 Reduction.

Metal nanoparticles are potent reaction catalysts, but they tend to aggregate, thereby limiting their catalytic efficiency. Their coordination with specific functional groups within a porous structure prevents their aggregation and facilitates the mass flow of catalytic starting materials and products. Herein, we use a thiacalix[4]arene-based polymer as a porous support with abundant docking sites for Au nanoparticles. The sulfur atoms bridging the phenolic subunits of thiacalix[4]arene serve as Lewis basic sites that coordinate Au atoms. Therefore, this approach takes advantage of the functional groups inherent in the monomer and avoids laborious postsynthetic modifications of the polymer. The presented system was tested for visible-light-driven photocatalytic CO2 reduction, where it showed adequate ability to generate 6.74 μmol g–1 CO over the course of 4 h, while producing small amounts of the CH4 product. This study aims to stimulate interest in the design and development of synthetically simpler porous polymer supports for various metal nanoparticles in catalytic and other applications.

Phloroglucinol-enhanced whey protein isolate hydrogels with antimicrobial activity for tissue engineering

Aging populations in developed countries will increase the demand for implantable materials to support tissue regeneration. Whey Protein Isolate (WPI), derived from dairy industry by-products, can be processed into hydrogels with the following desirable properties for applications in tissue engineering: (i) ability to support adhesion and growth of cells; (ii) ease of sterilization by autoclaving and (iii) ease of incorporation of poorly water-soluble drugs with antimicrobial activity, such as phloroglucinol (PG), the fundamental phenolic subunit of marine polyphenols.In this study, WPI hydrogels were enriched with PG at concentrations between 0 and 20% w/v. PG solubilization in WPI hydrogels is far higher than in water. Enrichment with PG did not adversely affect mechanical properties, and endowed antimicrobial activity against a range of bacteria which occur in healthcare-associated infections (HAI). WPI-PG hydrogels supported the growth of, and collagen production by human dental pulp stem cells and - to a lesser extent - of osteosarcoma-derived MG-63 cells. In summary, enrichment of WPI with PG may be a promising strategy to prevent microbial contamination while still promoting stem cell attachment and growth.

Exploring how structural changes to new Licarin A derivatives effects their bioactive properties against rapid growing mycobacteria and biofilm formation

Several species of rapidly growing mycobacteria (RGM) have been associated with biofilms in areas such as biomedical devices, water distribution systems, cosmetic surgery, and catheter-related blood infections. Biofilms which exhibit antimicrobial resistance such as those formed by the genus Mycobacterium pose a significant risk to health and are of particular interest to researchers. Licarin A (a neolignan found in numerous plant species e.g. nutmeg) has been reported to show a wide range of biological actions including anti-inflammatory, antioxidant, and antibacterial properties. The aim of this study was to prepare a set of Licarin A derivatives and investigate the impact of specific structural changes on its antimycobacterial ability, and its effect on the biofilm formation of RGM species. Initially, the phenolic sub-unit and alkenyl side chain of Licarin A were modified to create derivatives with a higher partition coefficient; as the activity of a compound against mycobacteria seems to be strongly influenced by its hydrophobicity. Further, polar groups were inserted into the side chain to change the hydrophilic-lipophilic profile of the molecules. Results showed variability in the susceptibility profile of mycobacteria against the Licarin A derivatives under analysis. A number of the derivatives showed significant inhibitory activity of planktonic growth of the three strains of mycobacteria used, with even lower MIC values than those observed with reference drugs and Licarin A itself. Cytotoxicity assays showed they also have low toxicity, confirming that structural modifications to the Licarin A have made improvements to its antimycobacterial properties.

Rearrangement of meta-Bridged Calix[4]arenes Promoted by Internal Strain.

The meta-bridged calixarenes possess a rigidified and highly distorted cavity, where the additional single-bond bridge imposes an extreme internal strain on the whole system. As a consequence, these compounds exhibit a reasonably amended reactivity, compared with common calix[4]arene derivatives, which is governed by the release of internal strain. This can be documented by the reaction of the bridged calix[4]arene with P2O5 or Nafion-H, leading (apart from polymers) to a macrocyclic product with a rearranged basic skeleton. The methylene bridge next to the fluorene moiety is intramolecularly shifted from position 2 to position 4 of the phenolic subunit to minimize the tension. As revealed by single-crystal X-ray analysis and by application of the residual dipolar coupling method, the rearrangement occurs without altering the original conformation.

The synthesis and characterization of giant Calixarenes

Calixarenes are cyclic oligomers obtained by condensation of suitable p-functionalised phenols with formaldehyde, usually allowing for the synthesis of the well known small calixarenes (including up to eight phenolic subunits). We report here the discovery of much larger members of this family, exhibiting sizes up to 90 phenolic subunits: the giant calixarenes. These macrocycles are obtained according to simple, easily scalable processes, in yields up to 65%. We show that the formation of these giant macrocycles is favored by an oxygen-containing-group at the para-position of the starting phenol, high concentrations of heavy alkaline bases (rubidium or cesium hydroxides) and long reaction times. A mechanism is proposed to rationalize these observations. These giant macrocycles can also be obtained in the quasi-solid state, opening interesting perspectives in the field of calixarenes chemistry. Along with their intrinsic fundamental interest, these objects are also opening interesting applicative potentialities.

Fluorescent pH-Responsive Probes Based on Water-Soluble Boron-Dipyrromethene (BODIPY) Derivatives, Featuring Long-Wavelength Emission.

We describe here the synthesis of water-soluble red/NIR-emissive, boron-dipyrromethene (BODIPY) derivatives displaying optical (absorption and emission) responses in pH range of 4-8. Substitution close to the tertiary aniline or the phenol subunits selected as the proton-sensitive sites allowed us to finely tune the pH ranges. Furthermore, the introduction of sulfobetaine functions at the boron centre of these pH-responsive BODIPYs afforded valuable fluorescent dyes in the red/NIR region in aqueous media, for which the steric hindrance and electrostatic repulsions prevent their non-emissive aggregation. All the absorption and emission studies, as well as the protonation properties were investigated in aqueous, ethanolic and saline solutions (mimicking physiological conditions). Interestingly, the systems present a fluorescent ratiometric protonation response in EtOH, but the non-protonated form is almost a non-fluorescent species under quasi-physiological conditions (saline aqueous solutions) due to the fading of the emissive character of the low-lying charge-transfer transition in the presence of a supporting electrolyte.

Laccase/Mediator Systems: Their Reactivity toward Phenolic Lignin Structures.

Laccase-mediator systems (LMS) have been widely studied for their capacity to oxidize the nonphenolic subunits of lignin (70–90% of the polymer). The phenolic subunits (10–30% of the polymer), which can also be oxidized without mediators, have received considerably less attention. Consequently, it remains unclear to what extent the presence of a mediator influences the reactions of the phenolic subunits of lignin. To get more insight in this, UHPLC-MS was used to study the reactions of a phenolic lignin dimer (GBG), initiated by a laccase from Trametes versicolor, alone or in combination with the mediators HBT and ABTS. The role of HBT was negligible, as its oxidation by laccase occurred slowly in comparison to that of GBG. Laccase and laccase/HBT oxidized GBG at a comparable rate, resulting in extensive polymerization of GBG. In contrast, laccase/ABTS converted GBG at a higher rate, as GBG was oxidized both directly by laccase but also by ABTS radical cations, which were rapidly formed by laccase. The laccase/ABTS system resulted in Cα oxidation of GBG and coupling of ABTS to GBG, rather than polymerization of GBG. Based on these results, we propose reaction pathways of phenolic lignin model compounds with laccase/HBT and laccase/ABTS.

Insertion of the o-Aminophenol Core into Ninhydrin–Phenol Adducts: Migration of Ninhydrin Carbon Leading to N-Phenyl­benzoate-Substituted Phthalimides

An unexpected migration of a ninhydrin carbon bearing a phenolic subunit has been observed when phenolic adducts of ninhydrin reacted with 2-aminophenol in butan-1-ol at the reflux temperature. The products were unambiguously assigned as 2-(1,3-dioxoisoindolin-2-yl)phenyl benzoates on the basis of NMR spectroscopy and X-ray crystallographic analysis.

Syntheses of triblock bottlebrush polymers through sequential ROMPs: Expanding the functionalities of molecular brushes

Syntheses of triblock bottlebrush polymers through sequential ROMPs: Expanding the functionalities of molecular brushes

Synthesis of different isoindolone embedded heterocycles with phenolic subunits from a common intermediate, 3-(2′-hydroxyaroyl)-2,3-dihydroisoindol-1-ones

A series of biologically important isoindolone embedded heterocycles such as tetracyclic 2,4-diamino-5-aryl-10-oxo-10H-1,10a-diazaindeno[2,1-a]indene-3-carbonitriles and tricyclic 1-aryl-3,5-dioxo-1H-imidazo-[3,4-b]isoindoles have been synthesized from an easily derived common intermediate, 3-(2′-hydroxyaroyl)-2,3-dihydroisoindol-1-ones. The significant advantages of the present methodologies are the use of simple and easily available starting materials and reagents, operational simplicity, and good yields of the products with high atom-economy.

Synthesis and radical scavenging activity of phenol–imidazole conjugates

Novel hydroxylated benzylideneamino imidazole derivatives were synthesized and their radical scavenging activity was assessed against DPPH and hydroxyl radicals. In the DPPH assay, most of the synthesized compounds showed an IC50 in the range 3.2μM⩽IC50⩽8.4μM, lower than the reference compound trolox (IC50=9.5μM) or the parent aldehydes (5.4μM⩽IC50⩽11.6μM). The activity depends mainly on the phenolic subunit (number and position of the hydroxyl groups) and the extent of conjugation with the imidazole ring. In the deoxyribose assay, all the compounds, including parent imidazoles and aldehydes, showed high activity against the hydroxyl radical and the ability to chelate iron ions. At 5μM concentration, the compounds protected the deoxyribose from degradation by hydroxyl radical between 62% and 38%.

Sorption of Tannin and Related Phenolic Compounds and Effects on Extraction of Soluble-N in Soil Amended with Several Carbon Sources

Some tannins sorb to soil and reduce soluble-N. However, we know little about how they interact with organic amendments in soil. Soil (0–5 cm) from plots, which were amended annually with various carbon substances, was treated with water (control) or solutions containing tannins or related phenolic subunits. Treatments included a proanthocyanidin, catechin, tannic acid, β-1,2,3,4,6-penta-O-galloyl-D-glucose (PGG), gallic acid, and methyl gallate. We applied solutions of each of these materials to soil and measured soluble-C and -N in supernatants after application and following extraction with hot water (16 h, 80 °C). Sorption was low for non-tannin phenolics, methyl gallate, gallic acid, and catechin, and unaffected by amendment. Sorption of tannins, proanthocyanidin, tannic acid, and PGG, was higher and greater in plots amended with biosolids or manure. Extraction of soluble-N was not affected by amendment or by catechin, proanthocyanidin, or methyl gallate, but was reduced with PGG, tannic acid and gallic acid. Soil cation exchange capacity increased following treatment with PGG but decreased with gallic acid, irrespective of amendment. Tannins entering soil may thus influence soil organic matter dynamics and nutrient cycling but their impact may be influenced by the composition of soil organic matter.

Repeated applications of tannins and related phenolic compounds are retained by soil and affect cation exchange capacity

Abstract Retention of tannins, produced by plants, could be important for managing soil organic matter and nutrient cycling. However, we know little about the comparative retention of different classes of tannins and related compounds or if soils have a maximum storage capacity for them. To address these questions, forest, and pasture loam soils, collected at 0–5 cm (surface) and 10–20 cm (subsurface), were repeatedly treated with water (Control) or solutions containing condensed and hydrolyzable tannins or related phenolic subunits (10 mg g−1 soil). Treatments included a polymeric flavonoid-based procyanidin from sorghum, catechin, tannic acid, β-1,2,3,4,6-penta-O-galloyl- d -glucose (PGG), gallic acid, and methyl gallate. After each application, soluble-C in supernatants was determined by oxidative-combustion infrared analysis and retention of treatment-carbon by soil was calculated as the difference between added and recovered soluble-C. An interaction between soil depth and treatment was evident through all applications with highest retention of both hydrophobic (PGG) and hydrophilic (procyanidin) tannins, compared to other phenolic compounds. For all treatments except gallic acid and methyl gallate, higher sorption occurred in surface soil, which contained more organic matter than subsurface soil. With each successive application, less additional treatment-C was retained by soil and the amount of C remaining in supernatants was correlated with the presence of phenolic substances. Cumulative retention by surface soil was more than 10.3, 8.5 and 6.4 mg C g−1 soil for PGG, tannic acid, and procyanidin, several times higher than the other compounds. Soluble-C extracted from treated soil, with cool water (23 °C), was 1–2 orders of magnitude greater than Control samples and highly correlated with Prussian Blue (PB) phenolics, indicating some retained treatment-C was only weakly held on the soil. The final extraction, with hot water (80 °C), removed more soluble-C, particularly from surface samples, that contained fewer PB phenolics per unit soluble-C than cool water extracts. After all extractions more than 85% of sorbed procyanidin-C was retained by samples compared to 81% of methyl gallate, 79% of PGG, 74% of tannic acid, 50% of catechin, and 40% of the gallic acid. Total C, measured in soil after all extractions, was close to expected values, confirming tannins and phenolic compounds had remained in soil and were not otherwise lost. Cation exchange capacity was increased about 30% in subsurface and forest samples by PGG, a hydrolyzable tannin, but decreased by 30% and 35% in surface and pasture soil, respectively, by its monomer, gallic acid.

In vitro inhibitory effect of tetrahydrocurcuminoids on Fusarium proliferatum growth and fumonisin B1 biosynthesis

Many plant pathogens produce toxic metabolites when growing on food and feed. Some antioxidative components seem to prevent fungal growth and mycotoxin formation. Recently, we synthesized a new class of powerful antioxidative compounds, i.e. tetrahydrocurcuminoids, and its structure/antioxidant activity relationships have been established. The South West of France produces large amounts of corn, which can be infected by Fusarium species, particularly F. proliferatum. In this context, the efficiency of tetrahydrocurcuminoids, which can be obtained from natural curcuminoids, was investigated to control in vitro the growth of F. proliferatum and the production of its associated mycotoxin, fumonisin B1. The relation between structure and antifungal activity was studied. Tetrahydrocurcumin (THC1), with two guaiacyl phenolic subunits, showed the highest inhibitory activity (measured as radial growth on agar medium) against the F. proliferatum development (67% inhibition at a concentration of 13.6 µmol ml−1). The efficiencies of THC2 (36% at a concentration of 11.5 µmol ml−1), which contains syringyl phenolic units, and THC3 (30% at a concentration of 13.6 µmol ml−1), which does not have any substituent on the aromatic rings, were relatively close. These results indicate that the simultaneous presence of guaiacyl phenols and the enolic function of the β-diketone moiety play an important role in the inhibition mechanisms. The importance of this combination was confirmed using n-propylguaiacol and acetylacetone as molecular models. Under the same conditions, ferulic acid and eugenol, other natural phenolic antioxidants, were less efficient in inhibiting fungal growth. THC1 also reduced fumonisin B1 production in liquid medium by approximately 35, 50 and 75% at concentrations of 0.8, 1.3, and 1.9 µmol ml−1, respectively. These very low inhibitory concentrations show that tetrahydrocurcuminoids could be one of the most promising biobased molecules for the control of mycotoxinogen fungal strains.

The effect of lignin model compound structure on the rate of oxidation catalyzed by two different fungal laccases

Laccases (EC 1.10.3.2) are multicopper oxidases able to oxidize various substrates, such as phenolic subunits of lignin. The substrate range can be widened to non-phenolic units by the use of mediators. Since discovery of the laccase-mediator system, direct reactions of lignin and laccase without mediated electron-transfer have gained much less attention. The objective of this study was to investigate lignin as a substrate for fungal laccases by using lignin model compounds. These model compounds contained guaiacylic and syringylic moieties and also compounds of guaiacylic origin at a higher oxidation level. Some of these compounds are commercially available, but most of them were synthesized. The oxidation reaction rates of the lignin model compounds were studied by monitoring consumption of the co-substrate oxygen, in reactions catalyzed by laccases from two different fungi; Melanocarpus albomyces and Trametes hirsuta, possessing different molecular and catalytic properties. These reaction rate studies were compared to physicochemical properties of the lignin model compounds: relative redox potentials determined using cyclic voltammetry and p K a-values. Docking of syringylic and biphenylic compounds to the active sites of both laccases was performed and the resulting model complex structures were used to further interpret the reaction rate results. Reaction rates of laccases are mainly affected by the ability of a lignin model compound to be oxidized and the p K a-value of the substrate seems to be less important. As a consequence, syringylic compounds are oxidized with the highest rates and compounds at a higher oxidation level and redox-potential, such as vanillin, are oxidized at a much lower rate. Both guaiacylic and syringylic type compounds fit well in the active sites of both laccases. Only for a biphenylic compound, steric clashes were observed, and they are likely to have an effect on the reaction rate. When the oxidation rates on the selected model compounds with the two different laccases were compared, the redox-potential difference between laccases T1 copper and the lignin model compound (Δ E) was not the only property that determined the oxidation rate. In the case of lignin model substrates, also the selectivity of a specific laccase, reflected in the k cat/ K m value, plays an important role.

An unprecedented condensation pathway leading to the formation of phenolic C 40 bis-diterpenoids in sediments from the Lower Oligocene of the Rhine Rift Valley

In the course of a detailed palaeoenvironmental study based on biomarker distributions in sediments of Rupelian age (Lower Oligocene from the Rhine Rift Valley), an outcrop sample rich in higher plant fossil remains was collected from a quarry near the city of Burnhaupt-le-Haut (South Alsace, France). In agreement with the macrofossil assemblage, most of the biomarkers were related to terrigenous organic matter, as evidenced by the presence of di- and triterpenoid derivatives from vascular plants. The aromatic hydrocarbon fraction was also shown to contain two predominant series of unknown C 40 oxygenated compounds, as indicated by high resolution mass spectrometry. Isolation of the predominant compound from the first series and of two isomers from the second was undertaken to determine the structures from nuclear magnetic resonance (NMR) studies. Thus, we determined that both series of compounds consist of bis-diterpenoids made up of phenolic sub-units related to totarol and sempervirol, possibly originating from Podocarpaceae. Their formation comprises a first step involving oxidative coupling between the two phenolic monomeric sub-units via a free radical mechanism and leading to the formation of an ether bond (first series of compounds), or a carbon–carbon bond (second series of compounds). In the case of the compounds from the second series, a second, acid-catalysed cyclisation step likely results in the formation of a central dibenzofuranyl moiety. It remains unclear, however, whether these condensation/cyclization mechanisms are biologically-mediated, or result from diagenetic reactions.