SummaryLaccases are oxidases that contain several copper atoms, and catalyse single‐electron oxidations of phenolic compounds with concomitant reduction of oxygen to water. The enzymes are particularly widespread in ligninolytic basidiomycetes, but also occur in certain prokaryotes, insects and plants. Depending on the species, laccases are involved in various biosynthetic processes contributing to carbon recycling in land ecosystems and the morphogenesis of biomatrices, wherein low‐molecular‐weight naturally occurring phenols serve as key enzyme substrates. Studies of these in vivo synthetic pathways have afforded new insights into fungal laccase applicability in green synthetic chemistry. Thus, we here review fungal laccase‐catalysed oxidations of naturally occurring phenols that are particularly relevant to the synthesis of fine organic chemicals, and we discuss how the discovered synthetic strategies mimic laccase‐involved in vivo pathways, thus enhancing the green nature of such reactions. Laccase‐catalysed in vivo processes yield several types of biopolymers, including those of cuticles, lignin, polyflavonoids, humus and the melanin pigments, using natural mono‐ or poly‐phenols as building blocks. The in vivo synthetic pathways involve either phenoxyl radical‐mediated coupling or cross‐linking reactions, and can be adapted to the design of in vitro oxidative processes involving fungal laccases in organic synthesis; the laccase substrates and the synthetic mechanisms reflect in vivo processes. Notably, such in vitro synthetic pathways can also reproduce physicochemical properties (e.g. those of chromophores, and radical‐scavenging, hydration and antimicrobial activities) found in natural biomaterials. Careful study of laccase‐associated in vivo metabolic pathways has been rewarded by the discovery of novel green applications for fungal laccases. This review comprehensively summarizes the available data on laccase‐catalysed biosynthetic pathways and associated applications in fine chemical syntheses.

Permanent dyeing of gray hair has become an increasingly active area in the cosmetics industry because of the increasingly aging population in developed countries. So far, p-phenylenediamine (PPD) and related diamine-based monomeric compounds have been widely used for the dyeing processes, but toxicological studies have revealed such compounds to be carcinogenic and allergenic. Here, we for the first time demonstrated that polydopamine, a mimic of human eumelanin, gives rise within a commercially acceptable period of time (i.e., 1 h) to deep black colors (i.e., natural Asian hair colors) in human keratin hairs in the presence of ferrous ions. The dyed hairs showed excellent resistance to conventional detergents, and the detailed color was readily varied by changing the kind of metal ion used. SEM images and FT-IR-ATR spectra suggested that the extent of polydopamine aggregation was crucial for the dyeing efficiency. High-resolution (15 T) FT-ICR mass spectrometry performed on the products detached from hairs with either 0.1 N HCl or NaOH indicated that similar polydopamine products were recruited into the hair matrices whether in the presence or absence of metal-based chelating. Polydopamine chains were determined using EPR and ICP-OES to use chelation of ferrous ions to self-assemble as well as to bind keratin surfaces in the dyeing conditions. Also, mice subjected to skin toxicity tests showed much greater viability and much less hair loss with our dyeing agents than with PPD. In conclusion, this study showed that a safe eumelanin mimic may be used to permanently dye gray hair, and showed three kinds of deposition mechanisms (i.e., innate binding ability of polydopamine, metal-assisted self-assembly of polydopamine, and metal-related bridging between keratin surface and polydopamine) to be involved.

No abstract

No abstract

No abstract

Fungal laccases have been highlighted as a catalytic tool for transforming phenols. Here we demonstrate that fungal laccase-catalyzed oxidations can transform naturally occurring phenols into plant fertilizers with properties very similar to those of commercial humic acids. Treatments of Arabidopsis thaliana with highly cross-linked polyphenolic products obtained from a mixture of catechol and vanillic acid were able to enhance the germination and salt tolerance of this plant. These results revealed that humic-like organic fertilizers can be produced via in vitro enzymatic oxidation reactions. In particular, the root elongation pattern resulting from the laccase products was comparable to that resulting from an auxin-like compound. A detailed structural comparison of the phenol variants and commercial humic acids revealed their similarities and differences. Analyses based on SEM, EFM, ERP, and zeta-potential measurement showed that they both formed globular granules bearing various hydrophilic/polar groups in aqueous and solid conditions. Solid-phase C NMR, FT-IR-ATR, and elemental analyses showed that more nitrogen-based functional and aliphatic groups were present in the commercial humic acids. Significant differences were also identifiable with respect to particle size and specific surface area. High-resolution (15 T) FT-ICR mass spectrometry-based van Krevelen diagrams showed the compositional features of the variants to be a subset of those of the humic acids. Overall, our study unraveled essential structural features of polyaromatics that affect the growth of plants, and also provided novel bottom-up ecofriendly and finely tunable pathways for synthesizing humic-like fertilizers.

The use of salt- or macro-sized NPK fertilizers is typically associated with low nutrient use efficiency and water eutrophication. Nanotechnology can overcome such drawbacks, but its practical application on a large scale is limited by (i) high costs and difficult scale-up of nanoparticle synthesis, (ii) questionable advantages over traditional methods, and (iii) health hazards related to nanomaterial introduction in the food stream and the environment. Here, we report on a novel biocompatible and multifunctional P nanofertilizer obtained by self-assembling natural or synthetic humic substances and hydroxyapatite nanoparticles using a simple and straightforward dipping process, exploiting the interaction between the polyphenolic groups of humic substances and the surface of nanohydroxyapatite. Pot tests using the as-prepared materials were performed on Zea mays as a model crop, and the results were compared to those obtained using commercial fused superphosphate and bare nanohydroxyapatites. A significant improvement, in terms of early plant growth, corn productivity, rhizosphere bacteria, and the resistance to NaCl-induced abiotic stresses, was achieved using hydroxyapatite nanoparticles assembled with humic substances. These effects were ascribed to the synergistic co-release of phosphate ions and humic substances, which are two types of plant-beneficial agents for crop nutrition and stimulation, respectively. The release patterns were proven to be tunable with the amount of humic substances adsorbed on the nanoparticles, inducing competition between humic-substance-driven phosphorous dissolution and block of water contact. Such positive effects on plant growth in association with its intrinsic biocompatibility, simple synthesis, and multifunctionality qualify this novel nanofertilizer as a promising material for large-scale use in the agronomic field.

Commercial humic acids mainly obtained from leonardite are in increasing demand in agronomy, and their market size is growing rapidly because these materials act as soil conditioners and direct stimulators of plant growth and development. In nature, fungus-driven nonspecific oxidations are believed to be a key to catabolizing recalcitrant plant lignins, resulting in lignin humification. Here we demonstrated the effective transformation of technical lignins derived from the Kraft processing of woody biomass into humic-like plant fertilizers through one-pot Fenton oxidations (i.e., artificially accelerated fungus reactions). The lignin variants resulting from the Fenton reaction, and manufactured using a few different ratios of FeSO4 to H2O2, successfully accelerated the germination of Arabidopsis thaliana seeds and increased the tolerance of this plant to NaCl-induced abiotic stress; moreover, the extent of the stimulation of the growth of this plant by these manufactured lignin variants was comparable or superior to that induced by commercial humic acids. The results of high-resolution (15 T) Fourier transform-ion cyclotron resonance mass spectrometry, electrostatic force microscopy, Fourier transform-infrared spectroscopy, and elemental analyses strongly indicated that oxygen-based functional groups were incorporated into the lignins. Moreover, analyses of the total phenolic contents of the lignins and their sedimentation kinetics in water media together with scanning electron microscopy- and Brunauer–Emmett–Teller-based surface characterizations further suggested that polymer fragmentation followed by modification of the phenolic groups on the lignin surfaces was crucial for the humic-like activity of the lignins. A high similarity between the lignin variants and commercial humic acids also resulted from autonomous deposition of iron species into lignin particles during the Fenton oxidation, although their short-term effects of plant stimulations were maintained whether the iron species were present or absent. Finally, we showed that lignins produced from an industrial-scale acid-induced hydrolysis of wood chips were transformed with the similar enhancements of the plant effects, indicating that our fungus-mimicking processes could be a universal way for achieving effective lignin humification.