Mechanoenzymatic Transformations in the Absence of Bulk Water: A More Natural Way of Using Enzymes

Calcium Carbide (CaC2) as a C2-Synthon by Mechanochemistry

Mechanochemical reactions by ball milling have opened new avenues in chemical synthesis. Particularly, mechanochemistry has facilitated the reaction of insoluble materials to simplify stablished synthetic protocols and develop new ones. One notable application involves the use of calcium carbide (CaC2) as a C2-synthon through mechanochemistry, which has offered a more practical alternative to incorporate C2-units compared to the conventional use of highly flammable gaseous acetylene. For example, by ball milling, the acetylenic anions [C2]2- found in CaC2 have been harnessed for the synthesis of diverse functional carbon materials as well as discrete organic molecules. This Concept aims to contribute to the conceptualization of this innovative approach while highlighting both its advantages and the challenges inherent in the use of CaC2.

Alternative processes to produce chitin, chitosan, and their oligomers

Sustainable recovery of chitin and its derivatives from shellfish waste will be achieved when the industrial production of these polymers is achieved with a high control of their molecular structure, low costs, and acceptable levels of pollution. Therefore, the conventional chemical method for obtaining these biopolymers needs to be replaced or optimized. The goal of the present review is to ascertain what alternative methods are viable for the industrial-scale production of chitin, chitosan, and their oligomers. Therefore, a detailed review of recent literature was undertaken, focusing on the advantages and disadvantages of each method. The analysis of the existing data allows suggesting that combining conventional, biological, and alternative methods is the most efficient strategy to achieve sustainable production, preventing negative impacts and allowing for the recovery of high added-value compounds from shellfish waste. In conclusion, a new process for obtaining chitinous materials is suggested, with the potential of reducing the consumption of reagents, energy, and water by at least 1/10, 1/4, and 1/3 part with respect to the conventional process, respectively.

Disentangling the Effect of Pressure and Mixing on a Mechanochemical Bromination Reaction by Solid-State NMR Spectroscopy

Mechanical forces, including compressive stresses, have a significant impact on chemical reactions. Besides the preparative opportunities, mechanochemical conditions benefit from the absence of any organic solvent, the possibility of a significant synthetic acceleration and unique reaction pathways. Together with an accurate characterization of ball-milling products, the development of a deeper mechanistic understanding of the occurring transformations at a molecular level is critical for fully grasping the potential of organic mechanosynthesis. We herein studied a bromination of a cyclic sulfoximine in a mixer mill and used solid-state nuclear magnetic resonance (NMR) spectroscopy for structural characterization of the reaction products. Magic-angle spinning (MAS) was applied for elucidating the product mixtures taken from the milling jar without introducing any further post-processing on the sample. Ex situ ¹³ C-detected NMR spectra of ball-milling products showed the formation of a crystalline solid phase with the regioselective bromination of the S-aryl group of the heterocycle in position 4. Completion is reached in less than 30 minutes as deduced from the NMR spectra. The bromination can also be achieved by magnetic stirring, but then, a longer reaction time is required. Mixing the solid educts in the NMR rotor allows to get in situ insights into the reaction and enables the detection of a reaction intermediate. The pressure alone induced in the rotor by MAS is not sufficient to lead to full conversion and the reaction occurs on slower time scales than in the ball mill, which is crucial for analysing mixtures taken from the milling jar by solid-state NMR. Our data suggest that on top of centrifugal forces, an efficient mixing of the starting materials is required for reaching a complete reaction.

Solid-State Enzymatic Hydrolysis of Mixed PET/Cotton Textiles

Waste polyester textiles are not recycled due to separation challenges and partial structural degradation during use and recycling. Chemical recycling of polyethylene terephthalate (PET) textiles through depolymerization can provide a feedstock of recycled monomers to make "as-new" polymers. While enzymatic PET recycling is a more selective and more sustainable approach, methods in development, however, have thus far been limited to clean, high-quality PET feedstocks, and require an energy-intensive melt-amorphization step ahead of enzymatic treatment. Here, high-crystallinity PET in mixed PET/cotton textiles could be directly and selectively depolymerized to terephthalic acid (TPA) by using a commercial cutinase from Humicola insolens under moist-solid reaction conditions, affording up to 30±2 % yield of TPA. The process was readily combined with cotton depolymerization through simultaneous or sequential application of the cellulase enzymes CTec2®, providing up to 83±4 % yield of glucose without any negative influence on the TPA yield.

Mechanoenzymatic reactions for the hydrolysis of PET†

Recent advances in the enzymatic degradation of poly(ethylene terphthalate) (PET) have led to a number of PET hydrolytic enzymes and mutants being developed. With the amount of PET building up in the natural world, there is a pressing need to develop scalable methods of breaking down the polymer into its monomers for recycling or other uses. Mechanoenzymatic reactions have gained traction recently as a green and efficient alternative to traditional biocatalytic reactions. For the first time we report increased yields of PET degradation by whole cell PETase enzymes by up to 27-fold by utilising ball milling cycles of reactive aging, when compared with typical solution-based reactions. This methodology leads to up to a 2600-fold decrease in the solvent required when compared with other leading degradation reactions in the field and a 30-fold decrease in comparison to reported industrial scale PET hydrolysis reactions.

Mechanoenzymatic reactions are described for the degradation of different PET materials using whole cell PETases.

Semisynthetic transformation of banana peel to enhance the conversion of sugars to 5-hydroxymethylfurfural

This study aimed to efficiently convert banana peels (BP) into 5-hydroxymethylfurfural (HMF) by using an integrated mechanoenzymatic/catalytic approach. There is no report on HMF production using mechanoenzymatic hydrolysis. Moreover, this method enables saccharification of lignocellulose without bulk solvents or pretreatment. The effects of the reaction volume, milling time, and reactive aging (RAging) on the mechanoenzymatic hydrolysis of BP were studied. The solvent-free enzymatic hydrolysis of BP under RAging conditions was found to provide higher glucose (40.5 wt%) and fructose (17.2 wt%) yields than chemical hydrolysis. Next, the conversion of the resulting monosaccharides into HMF in the presence of the AlCl₃·H₂O/HCl-DMSO/H₂O system resulted in 71.9 mol% yield, which is so far the highest HMF yield obtained from cellulosic food wastes. Under identical reaction conditions, direct conversion of untreated BP to HMF yielded 22.7 mol% HMF, suggesting that mechanoenzymatic hydrolysis greatly promotes the release of sugars from BP to improve HMF yield.

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

Lipase-Mediated Mechanoenzymatic Synthesis of Sugar Esters in Dissolved Unconventional and Neat Reaction Systems

Mechanochemical and biocatalytic approaches in modern research are two major assets to develop greener processes. In the present study, these modular tools of sustainability are pointed toward the production of versatile and daily employed compounds such as surfactants. Toward this aim, glycolipids, a class of nonionic surfactants composed of ubiquitous and primary metabolites such as sugar and fatty acid moieties, represent a promising alternative to petroleum-derived surface-active agents. Therefore, the combination of biocatalysis with mechanochemistry aiming at glycolipid synthesis seemed a logical step that was taken in this study for the first time. The monoacylated model compound glucose-6-O-decanoate was synthesized with the help of a bead mill apparatus using two different unconventional dissolved reaction systems, namely, menthol-based hydrophobic deep eutectic solvents and 2-methyl-2-butanol, thus reaching up to 12% yield in the latter based on the conversion of vinyl decanoate, after only 90 min of reaction. In addition, a neat reaction system using an excess of vinylated fatty ester as an adjuvant allowed a 27 mM/h space-time yield. The overall significant increase in productivities, up to 6 times, compared to standard heating and shaking methods, shows the tremendous potential of mechanoenzymatic synthesis.

Mechano-Enzymatic Degradation of the Chitin from Crustacea Shells for Efficient Production of N-acetylglucosamine (GlcNAc)

Chitin, the second richest polymer in nature, is composed of the monomer N-acetylglucosamine (GlcNAc), which has numerous functions and is widely applied in the medical, food, and chemical industries. However, due to the highly crystalline configuration and low accessibility in water of the chitin resources, such as shrimp and crab shells, the chitin is difficult utilize, and the traditional chemical method causes serious environment pollution and a waste of resources. In the present study, three genes encoding chitinolytic enzymes, including the N-acetylglucosaminidase from Ostrinia furnacalis (OfHex1), endo-chitinase from Trichoderma viride (TvChi1), and multifunctional chitinase from Chitinolyticbacter meiyuanensis (CmChi1), were expressed in the Pichia pastoris system, and the positive transformants with multiple copies were isolated by the PTVA (post-transformational vector amplification) method, respectively. The three recombinants OfHex1, TvChi1, and CmChi1 were induced by methanol and purified by the chitin affinity adsorption method. The purified recombinants OfHex1 and TvChi1 were characterized, and they were further used together for degrading chitin from shrimp and crab shells to produce GlcNAc through liquid-assisted grinding (LAG) under a water-less condition. The substrate chitin concentration reached up to 300 g/L, and the highest yield of the product GlcNAc reached up to 61.3 g/L using the mechano-enzymatic method. A yield rate of up to 102.2 g GlcNAc per 1 g enzyme was obtained.

Mechanoenzymatic Reactions Involving Polymeric Substrates or Products

Mechanoenzymology is an emerging field in which mechanical mixing is used to sustain enzymatic reactions in low-solvent or solvent-free mixtures. Many enzymes have been reported that thrive under such conditions. Considering the central role of biopolymers and synthetic polymers in our society, this minireview highlights the use of mechanoenzymology for the synthesis or depolymerization of oligomeric or polymeric materials. In contrast to traditional in-solution reactions, solvent-free mechanoenzymology has the advantages of avoiding solubility issues, proceeding in a minimal volume, and reducing solvent waste while potentially improving the reaction efficiency and accessing new reactivity. It is expected that this strategy will continue to gain popularity and find more applications.

Mechanoenzymology in the Kinetic Resolution of β-Blockers: Propranolol as a Case Study

Recent advances in biotechnology, protein engineering, and enzymatic immobilization have made it possible to carry out biocatalytic transformations through alternative non-conventional activation strategies. In particular, mechanoenzymology (i.e., the use of the mechanical force produced by milling or grinding to activate a biotransformation) has become a new area in so-called "green chemistry", reshaping key fundaments of biocatalysis and leading to the exploration of enzymatic transformations under more sustainable conditions. Significantly, numerous chiral active pharmaceutical ingredients have been synthesized via mechanoenzymatic methods, boosting the use of biocatalysis in the synthesis of chiral drugs. In this regard and aiming to widen the scope of the young field of mechanoenzymology, a dual kinetic resolution of propranolol precursors was explored. The biocatalytic methodology mediated by Candida antarctica Lipase B (CALB) and activated by mechanical force allowed the isolation of both enantiomeric precursors of propranolol with high enantiomeric excess (up to 99% ee), complete conversion (c = 50%), and excellent enantiodifferentiation (E > 300). Moreover, the enantiomerically pure products were used to synthesize both enantiomers of the β-blocker propranolol with high enantiopurity.

Enzymatic depolymerization of highly crystalline polyethylene terephthalate enabled in moist-solid reaction mixtures

Less than 9% of the plastic produced is recycled after use, contributing to the global plastic pollution problem. While polyethylene terephthalate (PET) is one of the most common plastics, its thermomechanical recycling generates a material of lesser quality. Enzymes are highly selective, renewable catalysts active at mild temperatures; however, they lack activity toward the more crystalline forms of PET commonly found in consumer plastics, requiring the energy-expensive melt-amorphization step of PET before enzymatic depolymerization. We report here that, when used in moist-solid reaction mixtures instead of the typical dilute aqueous solutions or slurries, the cutinase from Humicola insolens can directly depolymerize amorphous and crystalline regions of PET equally, without any pretreatment, with a 13-fold higher space-time yield and a 15-fold higher enzyme efficiency than reported in prior studies with high-crystallinity material. Further, this process shows a 26-fold selectivity for terephthalic acid over other hydrolysis products.

Mechanoenzymology: State of the Art and Challenges towards Highly Sustainable Biocatalysis

Global awareness of the importance of developing environmentally friendlier and more sustainable methods for the synthesis of valuable chemical compounds has led to the design of novel synthetic strategies, involving bio- and organocatalysis as well as the application of novel efficient and ground-breaking technologies such as present-day solvent-free mechanochemistry. In this regard, the evaluation of biocatalytic protocols mediated by the combination of mechanical activation and enzymatic catalysis has recently attracted the attention of the chemical community. Such mechanoenzymatic strategy represents an innovative and promising "green" approach in chemical synthesis that poses nevertheless new paradigms regarding the relative resilience of biomolecules to the mechanochemical stress and to the apparent high energy, at least in so-called hot-spots, during the milling process. Herein, relevant comments on the conceptualization of such mechanoenzymatic approach as a sustainable option in chemical synthesis, recent progress in the area, and associated challenges are discussed.

High yielding, one-step mechano-enzymatic hydrolysis of cellulose to cellulose nanocrystals without bulk solvent

Traditional methods of enzymatic hydrolysis of cellulose to cellulose nanocrystals (CNCs) are limited due to the low enzymatic efficiency and large amount of waste liquid. The purpose of this study is to improve the yield and production efficiency of CNCs by enzymatic hydrolysis. A one-step mechano-enzymatic hydrolysis method was developed by utilizing the synergy of wet grinding and enzymatic hydrolysis reaction to efficiently prepare CNCs. Under the optimal reaction conditions, the maximum CNCs yield of 49.3% was achieved with higher thermal stability and crystallinity index of 76.7%. Mechano-enzymatic hydrolysis followed the first order pseudo-kinetics, and fractal kinetics demonstrated that mechanical force of rotation speed affected the fractal dimensions and binding ability between substrate and enzyme. This study provides an alternative method to prepare CNCs, which can significantly avoid the use of bulk water, improve the production efficiency of CNCs and thus lower the production cost.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.

Solvent-free esterifications mediated by immobilized lipases: a review from thermodynamic and kinetic perspectives

Esters are a highly relevant class of compounds in the industrial context, and biocatalysis applied to ester syntheses is already a reality for some chemical companies.

Mechanochemical Synthesis of Short DNA Fragments

We demonstrate the first mechanochemical synthesis of DNA fragments by ball milling, enabling the synthesis of oligomers of controllable sequence and length using multi-step, one-pot reactions, without bulk solvent or the need to isolate intermediates. Mechanochemistry allowed for coupling of phosphoramidite monomers to the 5'-hydroxyl group of nucleosides, iodine/water oxidation of the resulting phosphite triester linkage, and removal of the 5'-dimethoxytrityl (DMTr) protecting group in situ in good yields (up to 60 % over three steps) to produce DNA dimers in a one-pot manner. H-Phosphonate chemistry under milling conditions enabled coupling and protection of the H-phosphonate linkage, as well as removal of the 5'-DMTr protecting group in situ, enabling a one-pot process with good yields (up to 65 % over three steps, or ca. 87 % per step). Sulfurization of the internucleotide linkage was possible using elemental sulfur (S8) or sulfur transfer reagents, yielding the target DNA phosphorothioate dimers in good yield (up to 80 % over two steps). This work opens the door to creation of solvent-free synthesis methodologies for DNA and RNA therapeutics.

Efficient Enzymatic Hydrolysis of Biomass Hemicellulose in the Absence of Bulk Water

Current enzymatic methods for hemicellulosic biomass depolymerization are solution-based, typically require a harsh chemical pre-treatment of the material and large volumes of water, yet lack in efficiency. In our study, xylanase (E.C. 3.2.1.8) from Thermomyces lanuginosus is used to hydrolyze xylans from different sources. We report an innovative enzymatic process which avoids the use of bulk aqueous, organic or inorganic solvent, and enables hydrolysis of hemicellulose directly from chemically untreated biomass, to low-weight, soluble oligoxylosaccharides in >70% yields.