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Invited for this month's cover is the group of Bert M. Weckhuysen at the Utrecht University (The Netherlands). The cover picture shows an attenuated Total Reflection Infrared spectroscopy (ATR-IR) probe monitoring a chemical reaction in the liquid phase, particularly water, at demanding conditions. The light penetrates few microns deep in proximity of the ATR-IR probe and thus any IR-active entity in that range is probed. This research systematically addresses the challenges associated with acquiring operando ATR-IR spectra for liquid/aqueous phase reactions, which is essential for better understanding of chemical reactions in the liquid phase. Read the full text of their Full Paper at 10.1002/cmtd.202100041.

The Front Cover shows an attenuated Total Reflection Infrared spectroscopy (ATR-IR) probe monitoring a chemical reaction in the liquid phase, particularly water, at demanding conditions. The light penetrates few microns deep in proximity of the ATR-IR probe and thus any IR-active entity in that range is probed. This research systematically addresses the challenges associated with acquiring operando ATR-IR spectra for liquid/aqueous phase reactions, which is essential for better understanding of chemical reactions in the liquid phase. More information can be found in the Full Paper by Khaled N. M. Khalili et al.

Additional file 1: Fig S1. Visualization of VdhA, VhyA and MhdA by SDS-PAGE. Fig. S2. Conversion of aromatic compounds by the reference (a), ∆vdhA (b), ∆vhyA (c), and ∆mhdA (d) after 24 h of incubation. Concentrations of the detected compounds can be found in Table 1. Error bars represent the standard deviation between three biological replicates. Fig. S3. Maximum likelihood (ML; 500 bootstraps) phylogenetic tree of A. niger VdhA compared to selected fungal genomes. The scale bar shows a distance equivalent to 0.2 amino acid substitutions per site. Values over 50% bootstrap support are shown with ML values in black, Neighbor Joining values in purple and Minimum Evolution values in blue. Characterized enzymes are in bold. Blue font represents ascomycete fungi, red font basidiomycete fungi, green font bacteria, and orange font Saccharomycetes. Fungal species names are followed by protein IDs from JGI ( http://genome.jgi-psf.org/programs/fungi/index.jsf ). Fig. S4. Maximum likelihood (ML; 500 bootstraps) phylogenetic tree of A. niger MhdA compared to selected fungal genomes. The scale bar shows a distance equivalent to 0.2 amino acid substitutions per site. Values over 50% bootstrap support are shown with ML values in black, Neighbor Joining values in purple and Minimum Evolution values in blue. In bold are characterized enzymes. Blue font represents ascomycete fungi, red font basidiomycete fungi, green font bacteria, black font plants and pink font Homo sapiens. Fungal species names are followed by protein IDs from JGI ( http://genome.jgi-psf.org/programs/fungi/index.jsf ). Fig. S5. 1H-NMR spectrum of 4-oxo-monomethyl adipate (D2O). The structure corresponds to 4-oxo-monomethyl adipate with D2 at C5 position.

The Diels-Alder (DA) reaction of furans is a versatile tool in synthetic organic chemistry and in the production of sustainable building blocks and smart materials. Numerous experimental and theoretical investigations suggest that the diene scope is effectively limited to electron-rich furans, which excludes the most abundant and readily accessible renewable derivatives: furfural and its 5-hydroxymethyl homologue. Herein we show for the first time that electron-poor 2-formylfurans can also directly engage in Diels-Alder couplings. The key to success is the use of aqueous medium, which supplies an additional thermodynamic driving force by coupling the unfavorable DA equilibrium to the exergonic hydration of the carbonyl functionality in the adducts to form geminal diols. This finding enables the direct access to various novel DA adducts derived from renewable furfurals and maleimides, via a mild, simple and environmentally-friendly synthetic protocol.

PD was supported by a grant of the Netherlands Scientific Organization NWO 824.15.023 to RPdV. The Academy of Finland grant no. 308284 to MRM is acknowledged. Part of the research was performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research, located at the Pacific Northwest National Laboratory in Richland, WA, USA. The work conducted by the U.S. Department of Energy Joint Genome Institute (JGI), was supported by the Office of Science of the U.S. Department of Energy under Contract No. DE‐AC02‐05CH11231. CSL was supported by the CatchBio program. White‐rot fungi, such as Dichomitus squalens, degrade all wood components and inhabit mixed‐wood forests containing both soft‐ and hardwood species. In this study, we evaluated how D. squalens responded to the compositional differences in softwood [guaiacyl (G) lignin and higher mannan content] and hardwood [syringyl/guaiacyl (S/G) lignin and higher xylan content] using semi‐natural solid cultures. Spruce (softwood) and birch (hardwood) sticks were degraded by D. squalens as measured by oxidation of the lignins using 2D‐NMR. The fungal response as measured by transcriptomics, proteomics and enzyme activities showed a partial tailoring to wood composition. Mannanolytic transcripts and proteins were more abundant in spruce cultures, while a proportionally higher xylanolytic activity was detected in birch cultures. Both wood types induced manganese peroxidases to a much higher level than laccases, but higher transcript and protein levels of the manganese peroxidases were observed on the G‐lignin rich spruce. Overall, the molecular responses demonstrated a stronger adaptation to the spruce rather than birch composition, possibly because D. squalens is mainly found degrading softwoods in nature, which supports the ability of the solid wood cultures to reflect the natural environment. Publisher PDF

Detailed structural analysis of industrial and model kraft lignins reveals an important new reaction intermediate and condensation pathway operating during kraft pulping., Kraft lignin, the main by-product of the pulping industry, is an abundant, yet highly underutilized renewable aromatic polymer. During kraft pulping, the lignin undergoes extensive structural modification, with many labile native bonds being replaced by new, more recalcitrant ones. Currently little is known about the nature of those bonds and linkages in kraft lignin, information that is essential for its efficient valorization to renewable fuels, materials or chemicals. Here, we provide detailed new insights into the structure of softwood kraft lignin, identifying and quantifying the major native as well as kraft pulping-derived units as a function of molecular weight. De novo synthetic kraft lignins, generated from (isotope labelled) dimeric and advanced polymeric models, provided key mechanistic understanding of kraft lignin formation, revealing different process dependent reaction pathways to be operating. The discovery of a novel kraft-derived lactone condensation product proved diagnostic for the identification of a previously unknown homovanillin based condensation pathway. The lactone marker is found in various different soft- and hardwood kraft lignins, suggesting the general pertinence of this new condensation mechanism for kraft pulping. These novel structural and mechanistic insights will aid the development of future biomass and lignin valorization technologies.