Fast Knoevenagel Condensations Catalyzed by an Artificial Schiff-Base-Forming Enzyme

Self-Healing Dynamic Hydrogel Microparticles with Structural Color for Wound Management

Chronic diabetic wounds confront a significant medical challenge because of increasing prevalence and difficult-healing circumstances. It is vital to develop multifunctional hydrogel dressings, with well-designed morphology and structure to enhance flexibility and effectiveness in wound management. To achieve these, we propose a self-healing hydrogel dressing based on structural color microspheres for wound management. The microsphere comprised a photothermal-responsive inverse opal framework, which was constructed by hyaluronic acid methacryloyl, silk fibroin methacryloyl and black phosphorus quantum dots (BPQDs), and was further re-filled with a dynamic hydrogel. The dynamic hydrogel filler was formed by Knoevenagel condensation reaction between cyanoacetate and benzaldehyde-functionalized dextran (DEX-CA and DEX-BA). Notably, the composite microspheres can be applied arbitrarily, and they can adhere together upon near-infrared irradiation by leveraging the BPQDs-mediated photothermal effect and the thermoreversible stiffness change of dynamic hydrogel. Additionally, eumenitin and vascular endothelial growth factor were co-loaded in the microspheres and their release behavior can be regulated by the same mechanism. Moreover, effective monitoring of the drug release process can be achieved through visual color variations. The microsphere system has demonstrated desired capabilities of controllable drug release and efficient wound management. These characteristics suggest broad prospects for the proposed composite microspheres in clinical applications.

Developing Isomeric Peptides for Mimicking the Sequence-Activity Landscapes of Enzyme Evolution

Enzymes catalyze almost all material conversion processes within living organisms, yet their natural evolution remains unobserved. Short peptides, derived from proteins and featuring active sites, have emerged as promising building blocks for constructing bioactive supramolecular materials that mimic native proteins through self-assembly. Herein, we employ histidine-containing isomeric tetrapeptides KHFF, HKFF, KFHF, HFKF, FKHF, and FHKF to craft supramolecular self-assemblies, aiming to explore the sequence-activity landscapes of enzyme evolution. Our investigations reveal the profound impact of peptide sequence variations on both assembly behavior and catalytic activity as hydrolytic simulation enzymes. During self-assembly, a delicate balance of multiple intermolecular interactions, particularly hydrogen bonding and aromatic-aromatic interactions, influences nanostructure formation, yielding various morphologies (e.g., nanofibers, nanospheres, and nanodiscs). Furthermore, the analysis of the structure-activity relationship demonstrates a strong correlation between the distribution of the His active site on the nanostructures and the formation of the catalytic microenvironment. This investigation of the sequence-structure-activity paradigm reflects how natural enzymes enhance catalytic activity by adjusting the primary structure during evolution, promoting fundamental research related to enzyme evolutionary processes.

Biomimetic Catalytic Retro-Aldol Reaction Using a Cation-Binding Catalyst: A Promising Route to Axially Chiral Biaryl Aldehydes

Here we describe a biomimetic catalytic retro-aldol reaction of racemic α-substituted β-hydroxy ketones utilizing a chiral oligoEG cation-binding catalyst as a type-II aldolase mimic. Our investigation of various aldol substrates has demonstrated that our biomimetic retro-aldol protocol enables rapid access to highly enantiomerically enriched aldols with a selectivity factor (s) of up to 70. Additionally, we have demonstrated the synthetic strategy's feasibility for accessing diverse and valuable axially chiral aldehydes.

Color-Switchable Nanosilicon Fluorescent Probes

Fluorescent probes are vital to cell imaging by allowing specific parts of cells to be visualized and quantified. Color-switchable probes (CSPs), with tunable emission wavelength upon contact with specific targets, are particularly powerful because they not only eliminate the need to wash away all unbound probe but also allow for internal controls of probe concentrations, thereby facilitating quantification. Several such CSPs exist and have proven very useful, but not for all key cellular targets. Here we report a pioneering CSP for in situ cell imaging using aldehyde-functionalized silicon nanocrystals (SiNCs) that switch their intrinsic photoluminescence from red to blue quickly when interacting with amino acids in live cells. Though conventional probes often work better in cell-free extracts than in live cells, the SiNCs display the opposite behavior and function well and fast in universal cell lines at 37 °C while requiring much higher temperature in extracts. Furthermore, the SiNCs only disperse in cytoplasm not nucleus, and their fluorescence intensity correlated linearly with the concentration of fed amino acids. We believe these nanosilicon probes will be promising tools to visualize distribution of amino acids and potentially quantify amino acid related processes in live cells.

Unlocking New Reactivities in Enzymes by Iminium Catalysis

The application of biocatalysis in conquering challenging synthesis requires the constant input of new enzymes. Developing novel biocatalysts by absorbing catalysis modes from synthetic chemistry has yielded fruitful new-to-nature enzymes. Organocatalysis was originally bio-inspired and has become the third pillar of asymmetric catalysis. Transferring organocatalytic reactions back to enzyme platforms is a promising approach for biocatalyst creation. Herein, we summarize recent developments in the design of novel biocatalysts that adopt iminium catalysis, a fundamental branch in organocatalysis. By repurposing existing enzymes or constructing artificial enzymes, various biocatalysts for iminium catalysis have been created and optimized via protein engineering to promote valuable abiological transformations. Recent advances in iminium biocatalysis illustrate the power of combining chemomimetic biocatalyst design and directed evolution to generate useful new-to-nature enzymes.

Novel Pyridinium Based Ionic Liquid Promoter for Aqueous Knoevenagel Condensation: Green and Efficient Synthesis of New Derivatives with Their Anticancer Evaluation

Herein, a distinctive dihydroxy ionic liquid ([Py-2OH]OAc) was straightforwardly assembled from the sonication of pyridine with 2-chloropropane-1,3-diol by employing sodium acetate as an ion exchanger. The efficiency of the ([Py-2OH]OAc as a promoter for the sono-synthesis of a novel library of condensed products through DABCO-catalyzed Knoevenagel condensation process of adequate active cyclic methylenes and ninhydrin was next investigated using ultimate greener conditions. All of the reactions studied went cleanly and smoothly, and the resulting Knoevenagel condensation compounds were recovered in high yields without detecting the aldol intermediates in the end products. Compared to traditional strategies, the suggested approach has numerous advantages including mild reaction conditions with no by-products, eco-friendly solvent, outstanding performance in many green metrics, and usability in gram-scale synthesis. The reusability of the ionic liquid was also studied, with an overall retrieved yield of around 97% for seven consecutive runs without any substantial reduction in the performance. The novel obtained compounds were further assessed for their in vitro antitumor potential toward three human tumor cell lines: Colo-205 (colon cancer), MCF-7 (breast cancer), and A549 (lung cancer) by employing the MTT assay, and the findings were evaluated with the reference Doxorubicin. The results demonstrated that the majority of the developed products had potent activities at very low doses. Compounds comprising rhodanine (5) or chromane (12) moieties exhibited the most promising cytotoxic effects toward three cell lines, particularly rhodanine carboxylic acid derivative (5c), showing superior cytotoxic effects against the investigated cell lines compared to the reference drug. Furthermore, automated docking simulation studies were also performed to support the results obtained.

Protein design via deep learning

Proteins with desired functions and properties are important in fields like nanotechnology and biomedicine. De novo protein design enables the production of previously unseen proteins from the ground up and is believed as a key point for handling real social challenges. Recent introduction of deep learning into design methods exhibits a transformative influence and is expected to represent a promising and exciting future direction. In this review, we retrospect the major aspects of current advances in deep-learning-based design procedures and illustrate their novelty in comparison with conventional knowledge-based approaches through noticeable cases. We not only describe deep learning developments in structure-based protein design and direct sequence design, but also highlight recent applications of deep reinforcement learning in protein design. The future perspectives on design goals, challenges and opportunities are also comprehensively discussed.

Generation of Strong Basicity in Metal-Organic Frameworks: How Do Coordination Solvents Matter?

Solid strong bases with an ordered pore structure (OPS-SSBs) have attracted much attention because of their high catalytic activity and shape selectivity as heterogeneous catalysts in various reactions. Nevertheless, high temperatures are required to fabricate OPS-SSBs by using traditional methods. Herein, we report for the first time that the coordination solvents affect basicity generation in metal-organic frameworks (MOFs) greatly and that strong basicity can be formed at comparatively low temperatures. A typical MOF, MIL-53, was employed, and three different solvents, namely, water, methanol, and N,N-dimethylformamide (DMF), were coordinated, respectively, by means of solvent exchange. Thermogravimetry-mass spectrometer analysis shows that the conversion temperature of base precursor KNO₃ is quite different on MIL-53 coordinated with different solvents. The conversion of KNO₃ to basic sites takes place at 350, 300, and 250 °C on MIL-53 coordinated with water, methanol, and DMF, respectively. It is fascinating to observe the generation temperature of strongly basic sites at 250 °C, which is noticeably lower than that on various supports, such as mesoporous silica SBA-15 (600 °C), zeolite Y (700 °C), and metal oxide ZrO₂ (730 °C). This is due to the redox interaction between coordination solvents and KNO₃, leading to a significant decrease in the temperature for KNO₃ conversion. Consequently, OPS-SSBs were prepared successfully with an ordered pore structure and strong basicity. The obtained OPS-SSBs show good shape selectivity in Knoevenagel condensation of aromatic aldehydes with different active methylene compounds. Moreover, these solid bases are highly active in the synthesis of dimethyl carbonate through transesterification reaction. This work might open up a new avenue for the fabrication of various functional materials at low temperatures through redox interactions.

Supported copper on a diamide-diacid-bridged PMO: an efficient hybrid catalyst for the cascade oxidation of benzyl alcohols/Knoevenagel condensation

In this study, a novel periodic mesoporous organosilica (PMO) containing diamide-diacid bridges was conveniently prepared using ethylenediaminetetraacetic dianhydride to support Cu(ii) species and affording supramolecular Cu@EDTAD-PMO nanoparticles efficiently. Fourier transform infrared (FT-IR) and energy dispersive X-ray (EDX) spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller (BET) analysis, and high-resolution transmission electron microscopy (HRTEM) results confirmed the successful synthesis of Cu@EDTAD-PMO. The stabilized Cu(ii) nanoparticles inside the mesochannels of the new PMO provided appropriate sites for selective oxidation of different benzyl alcohol derivatives to their corresponding benzaldehydes and subsequent Knoevenagel condensation with malononitrile. Therefore, Cu@EDTAD-PMO can be considered as a multifunctional heterogeneous catalyst, which is prepared easily through a green procedure and demonstrates appropriate stability with almost no leaching of the Cu(ii) nanoparticles into the reaction medium, and easy recovery through simple filtration. The recycled Cu@EDTAD-PMO was reused up to five times without significant loss of its catalytic activity. The stability, recoverability, and reusability of the designed heterogeneous catalyst were also studied under various reaction conditions.

Unlocking Asymmetric Michael Additions in an Archetypical Class I Aldolase by Directed Evolution

Class I aldolases catalyze asymmetric aldol addition reactions and have found extensive application in the biocatalytic synthesis of chiral β-hydroxy-carbonyl compounds. However, the usefulness of these powerful enzymes for application in other C–C bond-forming reactions remains thus far unexplored. The redesign of class I aldolases to expand their catalytic repertoire to include non-native carboligation reactions therefore continues to be a major challenge. Here, we report the successful redesign of 2-deoxy-d-ribose-5-phosphate aldolase (DERA) from Escherichia coli, an archetypical class I aldolase, to proficiently catalyze enantioselective Michael additions of nitromethane to α,β-unsaturated aldehydes to yield various pharmaceutically relevant chiral synthons. After 11 rounds of directed evolution, the redesigned DERA enzyme (DERA-MA) carried 12 amino-acid substitutions and had an impressive 190-fold enhancement in catalytic activity compared to the wildtype enzyme. The high catalytic efficiency of DERA-MA for this abiological reaction makes it a proficient “Michaelase” with potential for biocatalytic application. Crystallographic analysis provides a structural context for the evolved activity. Whereas an aldolase acts naturally by activating the enzyme-bound substrate as a nucleophile (enamine-based mechanism), DERA-MA instead acts by activating the enzyme-bound substrate as an electrophile (iminium-based mechanism). This work demonstrates the power of directed evolution to expand the reaction scope of natural aldolases to include asymmetric Michael addition reactions and presents opportunities to explore iminium catalysis with DERA-derived catalysts inspired by developments in the organocatalysis field.

Directed Evolution: Methodologies and Applications

Directed evolution aims to expedite the natural evolution process of biological molecules and systems in a test tube through iterative rounds of gene diversifications and library screening/selection. It has become one of the most powerful and widespread tools for engineering improved or novel functions in proteins, metabolic pathways, and even whole genomes. This review describes the commonly used gene diversification strategies, screening/selection methods, and recently developed continuous evolution strategies for directed evolution. Moreover, we highlight some representative applications of directed evolution in engineering nucleic acids, proteins, pathways, genetic circuits, viruses, and whole cells. Finally, we discuss the challenges and future perspectives in directed evolution.

Unlocking Iminium Catalysis in Artificial Enzymes to Create a Friedel-Crafts Alkylase

The construction and engineering of artificial enzymes consisting of abiological catalytic moieties incorporated into protein scaffolds is a promising strategy to realize non-natural mechanisms in biocatalysis. Here, we show that incorporation of the noncanonical amino acid para-aminophenylalanine (pAF) into the nonenzymatic protein scaffold LmrR creates a proficient and stereoselective artificial enzyme (LmrR_pAF) for the vinylogous Friedel-Crafts alkylation between α,β-unsaturated aldehydes and indoles. pAF acts as a catalytic residue, activating enal substrates toward conjugate addition via the formation of intermediate iminium ion species, while the protein scaffold provides rate acceleration and stereoinduction. Improved LmrR_pAF variants were identified by low-throughput directed evolution advised by alanine-scanning to obtain a triple mutant that provided higher yields and enantioselectivities for a range of aliphatic enals and substituted indoles. Analysis of Michaelis-Menten kinetics of LmrR_pAF and evolved mutants reveals that different activities emerge via evolutionary pathways that diverge from one another and specialize catalytic reactivity. Translating this iminium-based catalytic mechanism into an enzymatic context will enable many more biocatalytic transformations inspired by organocatalysis.

Chemical Evolution of Antivirals Against Enterovirus D68 through Protein-Templated Knoevenagel Reactions

The generation of bioactive molecules from inactive precursors is a crucial step in the chemical evolution of life, however, mechanistic insights into this aspect of abiogenesis are scarce. Here, we investigate the protein-catalyzed formation of antivirals by the 3C-protease of enterovirus D68. The enzyme induces aldol condensations yielding inhibitors with antiviral activity in cells. Kinetic and thermodynamic analyses reveal that the bioactivity emerges from a dynamic reaction system including inhibitor formation, alkylation of the protein target by the inhibitors, and competitive addition of non-protein nucleophiles to the inhibitors. The most active antivirals are slowly reversible inhibitors with elongated target residence times. The study reveals first examples for the chemical evolution of bio-actives through protein-catalyzed, non-enzymatic C-C couplings. The discovered mechanism works under physiological conditions and might constitute a native process of drug development.

Catalytic activity of a CuGHK peptide-based porous material

The CuGHK peptide-based porous material acts as a heterogeneous organocatalyst in the Henry reaction due to a periodic distribution of pockets decorated with lysine side chain active sites.

pH-Responsive Non-Pickering Emulsion Stabilized by Dynamic Covalent Bond Surfactants and Nano-SiO2 Particles

A novel stimulus-responsive non-Pickering emulsion stabilized by nano-SiO₂ particles was prepared in our recent study. 4-formylbenzoic acid and hexylamine through a dynamic covalent bond form a surface-active substance, which was confirmed by Fourier transform infrared (FTIR) and ¹H NMR. Through optimization experiments, it was proved that a stable emulsion can be formed by low surfactant concentration (below cmc) and low nano-SiO₂ particle concentration (0.5 wt %). In this emulsion, nano-SiO₂ particles are not located at the interface of oil-water but dispersed in the continuous phase of the emulsion, which is different from the Pickering emulsion. The negatively charged nano-SiO₂ particles and anionic surfactants repel each other, thereby synergistically stabilizing the emulsion so that the concentrations of surfactants and nanoparticles required to stabilize the emulsion are reduced. In addition, the system can also control the formation and fracture of dynamic covalent bonds by changing pH, thereby controlling the stability and demulsification of the emulsion. At the same time, this non-Pickering emulsion could be used as a microreactor for chemical synthesis and still had a high yield after three cycles. This study provides a new application direction for this environmentally friendly emulsion.

Ambient CO2/N2 Switchable Pickering Emulsion Emulsified by TETA-Functionalized Metal-Organic Frameworks

In recent years, metal-organic frameworks (MOFs) have been explored as emulsifiers for the fabrication of Pickering emulsions and then used for hybrid material synthesis and interface catalysis. Nevertheless, stimuli-responsive Pickering emulsions stabilized by MOFs have been rarely reported so far, although they are of great importance for fundamental research studies and practical applications. Herein, for the first time, triethylenetetramine (TETA)-functionalized MOFs (ZIF-90/TETA) have been designed, synthesized, and used for fabricating CO₂-/N₂-response Pickering emulsions. It is shown that even at the ZIF-90/TETA content of 0.25 wt %, the functional MOF can still efficiently emulsify n-hexane and water to form a high internal phase Pickering emulsion. Importantly, the Pickering emulsion can be easily and reversibly switched between emulsification and demulsification by bubbling of CO₂ and N₂ alternatively at atmospheric pressure. The possible mechanism of the CO₂/N₂ switchable emulsion is investigated by zeta potential, water contact angle, interfacial tension, ¹³C NMR spectroscopy, and an optical microscope. It is found that the acid-base reaction of CO₂ with TETA anchored on the surface of ZIF-90 leads to the production of hydrophilic ammonium bicarbonate and carbamate, which results in the emulsification of the Pickering emulsion. However, when N₂ is bubbled to remove CO₂, the reverse reaction takes place to cause the demulsification of the Pickering emulsion. Moreover, the CO₂/N₂ switchable Pickering emulsion has been successfully used as a microreactor for Knoevenagel reactions to demonstrate a highly efficient integration of chemical reaction, product separation, and ZIF-90/TETA recycling for a sustainable chemical process.

Enhancing promiscuous chemistries of a Schiff-base forming enzyme by divergent evolution

Directed evolution has emerged as a powerful technique for the rapid tailoring of enzymes toward particular synthetic demands, spawning a number of enzymes capable of complex chemical transformations. During random mutagenesis of a protein, changes in fitness must be assayed in order to quantify and understand the relative effect a given mutation has, and the assay employed must be carefully chosen to report directly on the transformation of interest. Here, we describe a series of medium-throughput screening techniques that have been utilized for the evolution and engineering of an artificial carboligase, RA95.5-8, resulting in improvement of catalytic efficiency of a number of promiscuous chemistries. The methods make use of common analytical chemistry equipment and low-cost materials, and may help inspire development of novel screening workflows for related transformations.

Engineered Artificial Carboligases Facilitate Regioselective Preparation of Enantioenriched Aldol Adducts

Controlling regio- and stereoselectivity of aldol additions is generally challenging. Here we show that an artificial aldolase with high specificity for acetone as the aldol donor can be reengineered via single active site mutations to accept linear and cyclic aliphatic ketones with notable efficiency, regioselectivity, and stereocontrol. Biochemical and crystallographic data show how the mutated residues modulate the binding and activation of specific aldol donors, as well as their subsequent reaction with diverse aldehyde acceptors. Broadening the substrate scope of this evolutionarily naïve catalyst proved much easier than previous attempts to redesign natural aldolases, suggesting that such proteins may be excellent starting points for the development of customized biocatalysts for diverse practical applications.

Bioaldehydes and beyond: Expanding the realm of bioderived chemicals using biogenic aldehydes as platforms

Biofuels and biochemicals derived from renewable resources are sconsidered as potential solutions for the energy crisis and associated environmental problems that human beings are facing today. However, so far the available types of bioderived chemicals are rather limited, and production efficiency is generally low. Expanding the realm of bioderived chemicals and relevant derivatives can help motivate the development of bioenergy and the general bioeconomy. Aldehydes, possessing unique reactivity, hold great promise as platform chemicals for producing a large portfolio of bioproducts. In this review, we focus on production of aldehydes from renewable bioresources and derivatization of aldehydes through chemocatalysis, biocatalysis, or de novo biosynthesis. Perspectives on combining protein engineering and cascade reactions for advanced aldehyde derivatization are also provided.

Enabling protein-hosted organocatalytic transformations

In this review, the development of organocatalytic artificial enzymes will be discussed. This area of protein engineering research has underlying importance, as it enhances the biocompatibility of organocatalysis for applications in chemical and synthetic biology research whilst expanding the catalytic repertoire of enzymes. The approaches towards the preparation of organocatalytic artificial enzymes, techniques used to improve their performance (selectivity and reactivity) as well as examples of their applications are presented. Challenges and opportunities are also discussed.

Quantitative Profiling of Protein-Derived Electrophilic Cofactors in Bacterial Cells with a Hydrazine-Derived Probe

Post-translational modification of proteins can form electrophilic cofactors that serve as a catalytic center. The derived electrophilic cofactors greatly expand protein activities and functions. However, there are few studies concerning how to profile the electrophiles in bacteria. Herein, we utilized a clickable probe called propargyl hydrazine to profile the protein-derived electrophilic cofactors in Escherichia coli (E. coli) cells. Since the cofactors are mostly carbonyl groups, the hydrazine-based probe can specifically react with the cofactors to form a Schiff base. The labeled proteins were then pulled down for mass spectrometry (MS) analysis. Fourteen proteins were shown to undergo enrichment by the probe and competitive binding by its analogue, propyl hydrazine. The identified proteins were further analyzed with targeted proteomics based on parallel reaction monitoring (PRM). Using this strategy, we obtained a global portrait of protein electrophiles in bacterial cells, among which the proteins of speD and panD were previously reported to derive pyruvoyl group as an electrophilic center while lpp can retain N-terminal formyl methionine. This quantitative chemical proteomics strategy can be used to find out protein electrophiles in bacteria and holds great potential to further characterize the protein functions.

Selective Colorimetric "Turn-On" Probe for Efficient Engineering of Iminium Biocatalysis

The efficient engineering of iminium biocatalysis has drawn considerable attention, with many applications in pharmaceutical synthesis. Here, we report a tailor-made iminium-activated colorimetric "turn-on" probe, specifically designed as a prescreening tool to facilitate engineering of iminium biocatalysis. Upon complexation of the probe with the catalytic Pro-1 residue of the model enzyme 4-oxalocrotonate tautomerase (4-OT), a brightly colored merocyanine-dye-type structure is formed. 4-OT mutants that formed this brightly colored species upon incubation with the probe proved to have a substantial activity for the iminium-based Michael-type addition of nitromethane to cinnamaldehyde, whereas mutants that showed no staining by the probe exhibited no or very low-level "Michaelase" activity. This system was exploited in a solid-phase prescreening assay termed as activated iminium colony staining (AICS) to enrich libraries for active mutants. AICS prescreening reduced the screening effort up to 20-fold. After two rounds of directed evolution, two artificial Michaelases were identified with up to 39-fold improvement in the activity for the addition of nitromethane to cinnamaldehyde, yielding the target γ-nitroaldehyde product with excellent isolated yield (up to 95%) and enantiopurity (up to >99% ee). The colorimetric activation of the turn-on probe could be extended to the class I aldolase 2-deoxy-d-ribose 5-phosphate aldolase, implicating a broader application of AICS in engineering iminium biocatalysis.

Cu(II) and magnetite nanoparticles decorated melamine-functionalized chitosan: A synergistic multifunctional catalyst for sustainable cascade oxidation of benzyl alcohols/Knoevenagel condensation

The uniform decoration of Cu(II) species and magnetic nanoparticles on the melamine-functionalized chitosan afforded a new supramolecular biopolymeric nanocomposite (Cs-Pr-Me-Cu(II)-Fe₃O₄). The morphology, structure, and catalytic activity of the Cs-Pr-Me-Cu(II)-Fe₃O₄ nanocomposite have been systematically investigated. It was found that Cs-Pr-Me-Cu(II)-Fe₃O₄ nanocomposite can smoothly promote environmentally benign oxidation of different benzyl alcohol derivatives by tert-butyl hydroperoxide (TBHP) to their corresponding benzaldehydes and subsequent Knoevenagel condensation with malononitrile, as a multifunctional catalyst. Interestingly, Fe₃O₄ nanoparticles enhance the catalytic activity of Cu(II) species. The corresponding benzylidenemalononitriles were formed in high to excellent yields at ambient pressure and temperature. The heterogeneous Cs-Pr-Me-Cu(II)-Fe₃O₄ catalyst was also very stable with almost no leaching of the Cu(II) species into the reaction medium and could be easily recovered by an external magnet. The recycled Cs-Pr-Me-Cu(II)-Fe₃O₄ was reused at least four times with slight loss of its activity. This is a successful example of the combination of chemo- and bio-drived materials catalysis for mimicing biocatalysis as well as sustainable and one pot multistep synthesis.

Temperature-Induced Reversible-Phase Transition in a Surfactant-Free Microemulsion

Microemulsion represents an important class of the colloidal system, though the development of stimuli-responsive microemulsion is still in its infancy. Here, we demonstrated the temperature responsiveness of a conventional surfactant-free microemulsion composed of n-octanol as nonpolar phase, ethanol as amphi-solvent, and water as polar phase for the first time. In the single-phase region of the phase diagram, the pre-ouzo zone was confirmed by dynamic light scattering (DLS), and the type of microemulsion was confirmed via the conductivity and polarity probe methods. The effects of temperature on the phase behavior and droplet size of the n-octanol-water-ethanol microemulsion system were systemically evaluated by the ternary phase diagram and DLS techniques. The results showed that the area of single-phase increases upon increasing temperature, but the area of pre-ouzo zone decreases accompanied by a decrease in the droplet size. Moreover, the critical point gradually draws close to the n-octanol corner with increasing temperature. When one formulation is far away from the demixing border, the droplet size can be reversibly and precisely regulated by changing temperature. When one formulation is located on the vicinity of the boundary, a minor variation in temperature can lead to a prominent phase transition between Winsor IV (high temperature) and Winsor II (low temperature). Such a temperature-responsive microemulsion can be used as a microreactor for Knoevenagel condensation. The reaction was carried out at 35 °C, and the product was collected from the water phase by simple filtration at 25 °C.

Biocatalysis with the milk protein β-lactoglobulin: promoting retroaldol cleavage of α,β-unsaturated aldehydes

Enzymes with a hydrophobic binding site and an active site lysine have been suggested to be promiscuous in their catalytic activity. β-Lactoglobulin (BLG), the principle whey protein found in milk, possesses a central calyx that binds non-polar molecules. Here, we report that BLG can catalyze the retro-aldol cleavage of α,β-unsaturated aldehydes making it a naturally occurring protein capable of catalyzing retro-aldol reactions on hydrophobic substrates. Retroaldolase activity was seen to be most effective on substrates with phenyl or naphthyl side-chains. Use of a brominated substrate analogue inhibitor increases the product yield by a factor of three. BLG's catalytic activity and its ready availability make it a prime candidate for the development of commercial biocatalysts.

Kemp Eliminases of the AlleyCat Family Possess High Substrate Promiscuity

Minimalist enzymes designed to catalyze model reactions provide useful starting points for creating catalysts for practically important chemical transformations. We have shown that Kemp eliminases of the AlleyCat family facilitate conversion of leflunomide (an immunosupressor pro-drug) to its active form teriflunomide with outstanding rate enhancement (nearly four orders of magnitude) and catalytic proficiency (more than seven orders of magnitude) without any additional optimization. This remarkable activity is achieved by properly positioning the substrate in close proximity to the catalytic glutamate with very high pKa.

Design of catalytic polypeptides and proteins

The first part of this review article lists examples of complete, empirical de novo design that made important contributions to the development of the field and initiated challenging projects. The second part of this article deals with computational design of novel enzymes in native protein scaffolds; active designs were refined through random and site-directed mutagenesis producing artificial enzymes with nearly native enzyme- like activities against a number of non-natural substrates. Combining aspects of de novo design and biological evolution of nature's enzymes has started and will accelerate the development of novel enzyme activities.

Strategies for designing non-natural enzymes and binders

The design of tailor-made enzymes is a major goal in biochemical research that can result in wide-range applications and will lead to a better understanding of how proteins fold and function. In this review we highlight recent advances in enzyme and small molecule binder design. A focus is placed on novel strategies for the design of scaffolds, developments in computational methods, and recent applications of these techniques on receptors, sensors, and enzymes. Further, the integration of computational and experimental methodologies is discussed. The outlined examples of designed enzymes and binders for various purposes highlight the importance of this topic and underline the need for tailor-made proteins.

Directed Evolution of Protein Catalysts

Directed evolution is a powerful technique for generating tailor-made enzymes for a wide range of biocatalytic applications. Following the principles of natural evolution, iterative cycles of mutagenesis and screening or selection are applied to modify protein properties, enhance catalytic activities, or develop completely new protein catalysts for non-natural chemical transformations. This review briefly surveys the experimental methods used to generate genetic diversity and screen or select for improved enzyme variants. Emphasis is placed on a key challenge, namely how to generate novel catalytic activities that expand the scope of natural reactions. Two particularly effective strategies, exploiting catalytic promiscuity and rational design, are illustrated by representative examples of successfully evolved enzymes. Opportunities for extending these approaches to more complex biocatalytic systems are also considered.

Role of conformational dynamics in the evolution of novel enzyme function

The free energy landscape concept that describes enzymes as an ensemble of differently populated conformational sub-states in dynamic equilibrium is key for evaluating enzyme activity, enantioselectivity, and specificity. Mutations introduced in the enzyme sequence can alter the populations of the pre-existing conformational states, thus strongly modifying the enzyme ability to accommodate alternative substrates, revert its enantiopreferences, and even increase the activity for some residual promiscuous reactions. In this feature article, we present an overview of the current experimental and computational strategies to explore the conformational free energy landscape of enzymes. We provide a series of recent publications that highlight the key role of conformational dynamics for the enzyme evolution towards new functions and substrates, and provide some perspectives on how conformational dynamism should be considered in future computational enzyme design protocols.

Expanding the boundary of biocatalysis: design and optimization of in vitro tandem catalytic reactions for biochemical production

Biocatalysts have been increasingly used in the synthesis of fine chemicals and medicinal compounds due to significant advances in enzyme discovery and engineering. To mimic the synergistic effects of cascade reactions catalyzed by multiple enzymes in nature, researchers have been developing artificial tandem enzymatic reactions in vivo by harnessing synthetic biology and metabolic engineering tools. There is also growing interest in the development of one-pot tandem enzymatic or chemo-enzymatic processes in vitro due to their neat and concise catalytic systems and product purification procedures. In this review, we will briefly summarize the strategies of designing and optimizing in vitro tandem catalytic reactions, highlight a few representative examples, and discuss the future trend in this field.

Speeding up enzyme discovery and engineering with ultrahigh-throughput methods

Exploring the sequence space of enzyme catalysts is ultimately a numbers game. Ultrahigh-throughput screening methods for rapid analysis of millions of variants are therefore increasingly important for investigating sequence-function relationships, searching large metagenomic libraries for interesting activities, and accelerating enzyme evolution in the laboratory. Recent applications of such technologies are reviewed here, with a particular focus on the practical benefits of droplet-based microfluidics for the directed evolution of natural and artificial enzymes. Broader implementation of such rapid, cost-effective screening technologies is likely to redefine the way enzymes are studied and engineered for academic and industrial purposes.

(E)-4-Methyl-N-((quinolin-2-yl)ethylidene)aniline as ligand for IIB supramolecular complexes: synthesis, structure, aggregation-induced emission enhancement and application in PMMA-doped hybrid material

Judicious structural design employing 2-quinolinecarboxaldehyde and 4-methylaniline was used to generate the Schiff base ligand (E)-4-methyl-N-((quinolin-2-yl)ethylidene)aniline (L). Five IIB complexes, namely, [ZnLCl₂] (1), [ZnL(NO₃)₂] (2), [ZnL(OAc)₂]₃ (3), [CdL(OAc)₂]₃ (4), and [HgLCl₂] (5) have been synthesized based on L. Single-crystal X-ray diffraction analysis indicates that complexes 1, 3 and 4 exhibit 3D networks, whereas 2 and 5 form 2D layers and 1D chains, respectively. TD-DFT calculations show a good correlation with the UV-vis absorption assigned to π → π* intraligand transitions. Furthermore, complexes 1-5 displayed strong greenish luminescent emissions (518-524 nm) in the aggregate state but weak emissions in solution (aggregation-induced emission enhancement), which may be due to the existence of C-HCl/O hydrogen bonding and ππ stacking interactions, resulting in restriction of intramolecular rotation (RIR). Variable-concentration ¹H NMR studies suggested that the aggregates undergo intramolecular changes in conformation due to intermolecular interactions. Moreover, the emission intensity and lifetime exhibited obvious increases induced by mechanical grinding and temperature reduction, which were also attributed to AIEE properties. Subsequently, complex 1 was incorporated into poly(methyl methacrylate) (PMMA), whereby 1-PMMA exhibited enhanced emission intensity (20-fold increase in comparison with that of 1), which offers opportunities for use in plastic greenhouses to increase leaf photosynthesis.