Yan Ni

How Green is Biocatalysis? To Calculate is To Know

Green chemistry aims to minimize the environmental hazards of chemical processes and their products. Ever since its introduction by Anastas, this principle has inspired researchers to critically rethink chemistry in view of its potential impact on the environment. Especially, the 12 Principles of Green Chemistry have been an inspiring guideline for this process. Biocatalysis is widely considered as one of the key technologies fulfilling the 12 principles and thereby being green chemistry per se. The reader will recognize stereotypical boilerplate statements such as “enzymes as renewable and biodegradable catalysts”, “working under environmentally benign conditions (temperature, pH, etc.)”, and “operating in water as an environmentally benign solvent” frequently found in the introductory passages to biocatalysis publications. Indeed, these are important parameters that can make biocatalysis environmentally more acceptable than “classical” chemical methods, provided the advantages are not (over)compensated by the disadvantages. Unfortunately, the latter are discussed to a much lesser extent. We also noted a certain tendency to pick a few of the 12 principles to underline the greenness of the method published, which certainly is in contrast to the envisioned use of the 12 principles as a cohesive system. We believe that the time is now to transition from just claiming environmental benefits of (bio)catalysis to quantifying the environmental impact. Indeed, the potential of biocatalysis as a tool to make chemical processes greener has been demonstrated by a limited number of studies. These studies mostly comprise the life-cycle assessment (LCA) of the processes. Unfortunately, LCAs are still rather complex and work intensive. Therefore, LCAs are appreciated by industry to evaluate existing processes (and to use the positive result as a selling argument), whereas research-oriented academic groups generally do not possess the expertise, resources, and interest to perform LCAs. As a result, sustainability issues are often addressed in the phase of manuscript preparation as described above. We believe that on the long term, careless, qualitative use of the term green chemistry will discredit the concept. Therefore, with this contribution we wish to promote the use of simple metrics to assess the environmental footprint of a given method in a semi-quantitative way. To calculate is to know (better). Sheldon proposed the E factor (environmental factor) to assess the greenness of a given reaction. The E factor denotes the amount of waste generated per product equivalent [Eq. (1)] .

Peroxygenase-Catalyzed Oxyfunctionalization Reactions Promoted by the Complete Oxidation of Methanol

Peroxygenases catalyze a broad range of (stereo)selective oxyfunctionalization reactions. However, to access their full catalytic potential, peroxygenases need a balanced provision of hydrogen peroxide to achieve high catalytic activity while minimizing oxidative inactivation. Herein, we report an enzymatic cascade process that employs methanol as a sacrificial electron donor for the reductive activation of molecular oxygen. Full oxidation of methanol is achieved, generating three equivalents of hydrogen peroxide that can be used completely for the stereoselective hydroxylation of ethylbenzene as a model reaction. Overall we propose and demonstrate an atom-efficient and easily applicable alternative to established hydrogen peroxide generation methods, which enables the efficient use of peroxygenases for oxyfunctionalization reactions.

Specific oxyfunctionalisations catalysed by peroxygenases: opportunities, challenges and solutions

Peroxygenases enable H2O2-driven oxyfunctionalisation of organic compounds such as selective epoxidation, hydroxylation and heteroatom oxygenation. Therefore, peroxygenases are potentially very useful tools for the organic chemist. This contribution reviews the current state of the art in peroxygenases, their availability, engineering and reaction scope. Current challenges and possible solutions are critically discussed.

Chemoenzymatic Halogenation of Phenols by using the Haloperoxidase from Curvularia inaequalis

The vanadium‐dependent chloroperoxidase from Curvularia inaequalis is an efficient biocatalyst for the in situ generation of hypohalous acids and subsequent electrophilic oxidation/halogenation reactions. Especially, its superb activity and stability under operational conditions make it an attractive catalyst for organic synthesis. Herein, the efficient bromination of thymol was investigated, and turnover numbers of the enzyme were found to exceed 2 000 000. The major novelty of the work is that vanadium chloroperoxidase is more useful as a brominating enzyme than vanadium bromoperoxidase in terms of operational stability, besides being far more stable than heme‐containing peroxidases.

Selective aerobic oxidation reactions using a combination of photocatalytic water oxidation and enzymatic oxyfunctionalizations

AbstractPeroxygenases offer an attractive means to address challenges in selective oxyfunctionalization chemistry. Despite this, their application in synthetic chemistry remains challenging due to their facile inactivation by the stoichiometric oxidant H2O2. Often atom-inefficient peroxide generation systems are required, which show little potential for large-scale implementation. Here, we show that visible-light-driven, catalytic water oxidation can be used for in situ generation of H2O2 from water, rendering the peroxygenase catalytically active. In this way, the stereoselective oxyfunctionalization of hydrocarbons can be achieved by simply using the catalytic system, water and visible light.Peroxygenases can selectively functionalize organic compounds, but are sensitive to the co-substrate H2O2. Hollmann and co-workers show that water oxidation catalysts can provide a controlled supply of H2O2 to the enzyme in the presence of visible light, allowing efficient oxyfunctionalization without stoichiometric reductants.

Fueling biomass-degrading oxidative enzymes by light-driven water oxidation

Photosynthesis may be described as light-driven oxidation of water and subsequent use of the generated reducing equivalents to fix CO2 and synthesize higher energy organic compounds, such as carbohydrates. The transposition of the sustainable and atom-efficient strategy of water oxidation to in vitro controlled biocatalytic reactions is poorly studied but is of high interest for the development of photobiocatalysis, and eco-friendly catalytic tools in a wider sense. Here we demonstrate that light-driven oxidation of water catalysed by vanadium-doped TiO2 (V-TiO2), a re-usable photocatalyst, can provide the electrons that lytic polysaccharide monooxygenases (LPMOs) need to oxidatively deconstruct biomass polysaccharides. The demonstration that electrons may be generated by water oxidation alleviates the need for an externally added electron donor, which so far has been a prerequisite for LPMO activity. Importantly, photocatalytic LPMO activation was achieved in the absence of redox mediators, which represents the first demonstration of mediator-free electron transfer from V-TiO2 particles to a redox enzyme, expanding the repertoire of known and conceivable photobiocatalytic reactions. Fundamentally, this photobiocatalytic system allows activation and tight control of LPMO activity, thus offering new tools for mechanistic studies of these industrially important and ubiquitous enzymes. The latter is illustrated by real-time studies of the redox state of an LPMO, using the controllable light-V-TiO2 technology for LPMO reduction and a novel fluorescence method for monitoring re-oxidation. We also show that the light-V-TiO2 technology may be used to study pre-activation of LPMOs.

Alcohol Dehydrogenases Catalyze the Reduction of Thioesters

Alcohol dehydrogenases are well‐established catalysts for various reduction reactions. However, the reduction of carboxylic acid derivatives has not yet been reported with these enzymes. In this contribution, we demonstrated that carboxylic acid thioesters could be readily reduced by a range of alcohol dehydrogenases, albeit at significantly reduced rates relative to those observed for corresponding ketones. A molecular explanation, especially for the lower turnover rates for thioesters relative to those obtained for ketones, is presented, as is a preliminary substrate scope.

Artificial Photosynthesis: Hybrid Systems.

Oxidoreductases are promising catalysts for organic synthesis. To sustain their catalytic cycles they require efficient supply with redox equivalents. Today classical biomimetic approaches utilizing natural electron supply chains prevail but artificial regeneration approaches bear the promise of simpler and more robust reaction schemes. Utilizing visible light can accelerate such artificial electron transport chains and even enable thermodynamically unfeasible reactions such as the use of water as reductant.This contribution critically summarizes the current state of the art in photoredoxbiocatalysis (i.e. light-driven biocatalytic oxidation and reduction reactions).