Intensification of Photobiocatalytic Decarboxylation of Fatty Acids for the Production of Biodiesel

Synthesis of 1H-isoindolin-1-ones via a simple photodecarboxylative addition of carboxylates to phthalimides and evaluation of their antibiotic activity

A variety of 3-hydroxy-isoindolin-1-one derivatives were synthesized using the photodecarboxylative addition of carboxylates to phthalimide derivatives in aqueous media. Subsequent acid-catalyzed dehydration furnished 3-(alkyl and aryl)methyleneisoindolin-1-ones with variable E-diastereoselectivity in good to excellent overall yields. Noteworthy, the parent 3-phenylmethyleneisoindolin-1-one underwent isomerization and oxidative decomposition when exposed to light and air. Selected 3-hydroxy-isoindolin-1-one and 3-(alkyl and aryl)methyleneisoindolin-1-one derivatives showed moderate antibacterial activity that justifies future elaboration and study of these important bioactive scaffolds.

Photoenzymatic Decarboxylation to Produce Hydrocarbon Fuels: A Critical Review

Photoenzymatic decarboxylation shows great promise as a pathway for the generation of hydrocarbon fuels. CvFAP, which is derived from Chlorella variabilis NC64A, is a photodecarboxylase capable of converting fatty acids into hydrocarbons. CvFAP is an example of coupling biocatalysis and photocatalysis to produce alkanes. The catalytic process is mild, and it does not yield toxic substances or excess by-products. However, the activity of CvFAP can be readily inhibited by several factors, and further enhancement is required to improve the enzyme yield and stability. In this article, we will examine the latest advancements in CvFAP research, with a particular focus on the enzyme's structural and catalytic mechanism, summarized some limitations in the application of CvFAP, and laboratory-level methods for enhancing enzyme activity and stability. This review can serve as a reference for future large-scale industrial production of hydrocarbon fuels.

Photocatalytic Transformation of Biomass and Biomass Derived Compounds-Application to Organic Synthesis

Biomass and biomass-derived compounds have become an important alternative feedstock for chemical industry. They may replace fossil feedstocks such as mineral oil and related platform chemicals. These compounds may also be transformed conveniently into new innovative products for the medicinal or the agrochemical domain. The production of cosmetics or surfactants as well as materials for different applications are examples for other domains where new platform chemicals obtained from biomass can be used. Photochemical and especially photocatalytic reactions have recently been recognized as being important tools of organic chemistry as they make compounds or compound families available that cannot be or are difficultly synthesized with conventional methods of organic synthesis. The present review gives a short overview with selected examples on photocatalytic reactions of biopolymers, carbohydrates, fatty acids and some biomass-derived platform chemicals such as furans or levoglucosenone. In this article, the focus is on application to organic synthesis.

Towards the rate limit of heterologous biotechnological reactions in recombinant cyanobacteria

BACKGROUND

Cyanobacteria have emerged as highly efficient organisms for the production of chemicals and biofuels. Yet, the productivity of the cell has been low for commercial application. Cyanobacterial photobiotransformations utilize photosynthetic electrons to form reducing equivalents, such as NADPH-to-fuel biocatalytic reactions. These photobiotransformations are a measure to which extent photosynthetic electrons can be deviated toward heterologous biotechnological processes, such as the production of biofuels. By expressing oxidoreductases, such as YqjM from Bacillus subtilis in Synechocystis sp. PCC 6803, a high specific activity was obtained in the reduction of maleimides. Here, we investigated the possibility to accelerate the NAD(P)H-consuming redox reactions by addition of carbohydrates as exogenous carbon sources such as D-Glucose under light and darkness.

RESULTS

A 1.7-fold increase of activity (150 µmol min^-1 g_DCW^-1) was observed upon addition of D-Glucose at an OD₇₅₀ = 2.5 (DCW = 0.6 g L^-1) in the biotransformation of 2-methylmaleimide. The stimulating effect of D-Glucose was also observed at higher cell densities in light and dark conditions as well as in the reduction of other substrates. No increase in both effective photosynthetic yields of Photosystem II and Photosystem I was found upon D-Glucose addition. However, we observed higher NAD(P)H fluorescence when D-Glucose was supplemented, suggesting increased glycolytic activity. Moreover, the system was scaled-up (working volume of 200 mL) in an internally illuminated Bubble Column Reactor exhibiting a 2.4-fold increase of specific activity under light-limited conditions.

CONCLUSIONS

Results show that under photoautotrophic conditions at a specific activity of 90 µmol min^-1 g_DCW^-1, the ene-reductase YqjM in Synechocystis sp. PCC 6803 is not NAD(P)H saturated, which is an indicator that an increase of the rates of heterologous electron consuming processes for catalysis and biofuel production will require funnelling further reducing power from the photosynthetic chain toward heterologous processes.

Photoenzymatic decarboxylation: A promising way to produce sustainable aviation fuels and fine chemicals

As one of the fastest-growing carbon emission sources, the aviation sector is severely restricted by carbon emission reduction targets. Sustainable aviation fuel (SAF) has emerged as the most potential alternative to traditional aviation fuel, but harsh production technologies limit its commercialization. Fatty acids photodecarboxylase from Chlorella variabilis NC64A (CvFAP), the latest discovered photoenzyme, provides promising approaches to produce various carbon-neutral biofuels and fine chemicals. This review highlights the state-of-the-art strategies to enhance the application of CvFAP in carbon-neutral biofuel and fine chemicals production, including supplementing alkane as decoy molecular, screening efficient CvFAP variants with directed evolution, constructing genetic strains, employing biphasic catalytic system, and immobilizing CvFAP in an efficient photobioreactor. Furthermore, future opportunities are suggested to enhance photoenzymatic decarboxylation and explore the catalytic mechanism of CvFAP. This review provides a broad context to improve CvFAP catalysis and advance its potential applications.

Immobilization and Application of Fatty Acid Photodecarboxylase in Deep Eutectic Solvents

Since its discovery in 2017, the fatty acid decarboxylase (FAP) photoenzyme has been the focus of extensive research, given its ability to convert fatty acids into alka(e)nes using merely visible blue light. Unfortunately, there are still some drawbacks that limit the applicability of this biocatalyst, such as poor solubility of the substrates in aqueous media, poor photostability, and the impossibility of reusing the catalyst for several cycles. In this work, we demonstrate the use of FAP in non-conventional media as a free enzyme and an immobilized preparation. Namely, its applicability in deep eutectic solvents (DESs) and a proof-of-concept immobilization using a commercial His-tag selective carrier, a thorough study of reaction and immobilization conditions in each case, as well as reusability studies are shown. We observed an almost complete selectivity of the enzyme towards C18 decarboxylation over C16 when used in a DES, with a product analytical yield up to 81 % when using whole cells. Furthermore, when applying the immobilized enzyme in DES, we obtained yields >10-fold higher than the ones obtained in aqueous media.

Photobioreactors for cultivation and synthesis: Specifications, challenges, and perspectives

Due to their versatility and the high biomass yield produced, cultivation of phototrophic organisms is an increasingly important field. In general, open ponds are chosen to do it because of economic reasons; however, this strategy has several drawbacks such as poor control of culture conditions and a considerable risk of contamination. On the other hand, photobioreactors are an attractive choice to perform cultivation of phototrophic organisms, many times in a large scale and an efficient way. Furthermore, photobioreactors are being increasingly used in bioprocesses to obtain valuable chemical products. In this review, we briefly describe different photobioreactor set-ups, including some of the recent designs, and their characteristics. Additionally, we discuss the current challenges and advantages that each different type of photobioreactor presents, their applicability in biocatalysis and some modern modeling tools that can be applied to further enhance a certain process.

Strategies for Transferring Photobiocatalysis to Continuous Flow Exemplified by Photodecarboxylation of Fatty Acids

The challenges of light-dependent biocatalytic transformations of lipophilic substrates in aqueous media are manifold. For instance, photolability of the catalyst as well as insufficient light penetration into the reaction vessel may be further exacerbated by a heterogeneously dispersed substrate. Light penetration may be addressed by performing the reaction in continuous flow, which allows two modes of applying the catalyst: (i) heterogeneously, immobilized on a carrier, which requires light-permeable supports, or (ii) homogeneously, dissolved in the reaction mixture. Taking the light-dependent photodecarboxylation of palmitic acid catalyzed by fatty-acid photodecarboxylase from Chlorella variabilis (CvFAP) as a showcase, strategies for the transfer of a photoenzyme-catalyzed reaction into continuous flow were identified. A range of different supports were evaluated for the immobilization of CvFAP, whereby Eupergit C250 L was the carrier of choice. As the photostability of the catalyst was a limiting factor, a homogeneous system was preferred instead of employing the heterogenized enzyme. This implied that photolabile enzymes may preferably be applied in solution if repair mechanisms cannot be provided. Furthermore, when comparing different wavelengths and light intensities, extinction coefficients may be considered to ensure comparable absorption at each wavelength. Employing homogeneous conditions in the CvFAP-catalyzed photodecarboxylation of palmitic acid afforded a space-time yield unsurpassed by any reported batch process (5.7 g·L-1·h-1, 26.9 mmol·L-1·h-1) for this reaction, demonstrating the advantage of continuous flow in attaining higher productivity of photobiocatalytic processes.

Triplet-triplet annihilation-based photon-upconversion to broaden the wavelength spectrum for photobiocatalysis

Photobiocatalysis is a growing field of biocatalysis. Especially light-driven enzyme catalysis has contributed significantly to expanding the scope of synthetic organic chemistry. However, photoenzymes usually utilise a rather narrow wavelength range of visible (sun)light. Triplet-triplet annihilation-based upconversion (TTA-UC) of long wavelength light to shorter wavelength light may broaden the wavelength range. To demonstrate the feasibility of light upconversion we prepared TTA-UC poly(styrene) (PS) nanoparticles doped with platinum(II) octaethylporphyrin (PtOEP) photosensitizer and 9,10-diphenylanthracene (DPA) annihilator (PtOEP:DPA@PS) for application in aqueous solutions. Photoexcitation of PtOEP:DPA@PS nanoparticles with 550 nm light led to upconverted emission of DPA 418 nm. The TTA-UC emission could photoactivate flavin-dependent photodecarboxylases with a high energy transfer efficiency. This allowed the photodecarboxylase from Chlorella variabilis NC64A to catalyse the decarboxylation of fatty acids into long chain secondary alcohols under green light (λ = 550 nm).

Green Chemistry, Biocatalysis, and the Chemical Industry of the Future

In the movement to decarbonize our economy and move away from fossil fuels we will need to harness the waste products of our activities, such as waste lignocellulose, methane, and carbon dioxide. Our wastes need to be integrated into a circular economy where used products are recycled into a manufacturing carbon cycle. Key to this will be the recycling of plastics at the resin and monomer levels. Biotechnology is well suited to a future chemical industry that must adapt to widely distributed and diverse biological chemical feedstocks. Our increasing mastery of biotechnology is allowing us to develop enzymes and organisms that can synthesize a widening selection of desirable bulk chemicals, including plastics, at commercially viable productivities. Integration of bioreactors with electrochemical systems will permit new production opportunities with enhanced productivities and the advantage of using a low-carbon electricity from renewable and sustainable sources.

Synthetic Fuels from Biomass: Photocatalytic Hydrodecarboxylation of Octanoic Acid by Ni Nanoparticles Deposited on TiO2

Decarboxylation of low-value fatty acids from biomass is a simple process to produce synthetic fuels suitable to be blended with gasoline or diesel. The present study reports the photocatalytic decarboxylation of octanoic acid in the presence of H₂ by a series of modified TiO₂ to form mixtures of n-heptane and tetradecane as major products in variable proportions, depending on the photocatalyst and the reaction conditions. It was found that the photocatalytic activity increases upon an optimal reductive NaBH₄ treatment, presumably by generation of surface oxygen vacancies and by the deposition of Ni nanoparticles in the appropriate loading. Under the optimized conditions, an almost complete octanoic acid conversion and a combined selectivity to n-heptane and tetradecane over 80 % were reached at 10 h of UV/Vis light irradiation with a 300 W Xe lamp. No changes in the photocatalytic performance were observed for six consecutive runs. The present results illustrate the possibility that photocatalytic decarboxylation offers for the transformation of biomass into synthetic fuels under mild conditions.

Photobiocatalytic Oxyfunctionalization with High Reaction Rate using a Baeyer-Villiger Monooxygenase from Burkholderia xenovorans in Metabolically Engineered Cyanobacteria

Baeyer–Villiger monooxygenases (BVMOs) catalyze the oxidation of ketones to lactones under very mild reaction conditions. This enzymatic route is hindered by the requirement of a stoichiometric supply of auxiliary substrates for cofactor recycling and difficulties with supplying the necessary oxygen. The recombinant production of BVMO in cyanobacteria allows the substitution of auxiliary organic cosubstrates with water as an electron donor and the utilization of oxygen generated by photosynthetic water splitting. Herein, we report the identification of a BVMO from Burkholderia xenovorans (BVMO_Xeno) that exhibits higher reaction rates in comparison to currently identified BVMOs. We report a 10-fold increase in specific activity in comparison to cyclohexanone monooxygenase (CHMO_Acineto) in Synechocystis sp. PCC 6803 (25 vs 2.3 U g_DCW^–1 at an optical density of OD₇₅₀ = 10) and an initial rate of 3.7 ± 0.2 mM h^–1. While the cells containing CHMO_Acineto showed a considerable reduction of cyclohexanone to cyclohexanol, this unwanted side reaction was almost completely suppressed for BVMO_Xeno, which was attributed to the much faster lactone formation and a 10-fold lower K_M value of BVMO_Xeno toward cyclohexanone. Furthermore, the whole-cell catalyst showed outstanding stereoselectivity. These results show that, despite the self-shading of the cells, high specific activities can be obtained at elevated cell densities and even further increased through manipulation of the photosynthetic electron transport chain (PETC). The obtained rates of up to 3.7 mM h^–1 underline the usefulness of oxygenic cyanobacteria as a chassis for enzymatic oxidation reactions. The photosynthetic oxygen evolution can contribute to alleviating the highly problematic oxygen mass-transfer limitation of oxygen-dependent enzymatic processes.

Internal Illumination to Overcome the Cell Density Limitation in the Scale-up of Whole-Cell Photobiocatalysis

Cyanobacteria have the capacity to use photosynthesis to fuel their metabolism, which makes them highly promising production systems for the sustainable production of chemicals. Yet, their dependency on visible light limits the cell-density, which is a challenge for the scale-up. Here, it was shown with the example of a light-dependent biotransformation that internal illumination in a bubble column reactor equipped with wireless light emitters (WLEs) could overcome this limitation. Cells of the cyanobacterium Synechocystis sp. PCC 6803 expressing the gene of the ene-reductase YqjM were used for the reduction of 2-methylmaleimide to (R)-2-methylsuccinimide with high optical purity (>99 % ee). Compared to external source of light, illumination by floating wireless light emitters allowed a more than two-fold rate increase. Under optimized conditions, product formation rates up to 3.7 mm h-1 and specific activities of up to 65.5 U gDCW -1 were obtained, allowing the reduction of 40 mm 2-methylmaleimide with 650 mg isolated enantiopure product (73 % yield). The results demonstrate the principle of internal illumination as a means to overcome the intrinsic cell density limitation of cyanobacterial biotransformations, obtaining high reaction rates in a scalable photobioreactor.

Stabilisation of the Fatty Acid Decarboxylase from Chlorella variabilis by Caprylic Acid

The fatty acid photodecarboxylase from Chlorella variabilis NC64 A (CvFAP) catalyses the light-dependent decarboxylation of fatty acids. Photoinactivation of CvFAP still represents one of the major limitations of this interesting enzyme en route to practical application. In this study we demonstrate that the photostability of CvFAP can easily be improved by the administration of medium-chain length carboxylic acids such as caprylic acid indicating that the best way of maintaining CvFAP stability is 'to keep the enzyme busy'.

Inductive Tracking Methodology for Wireless Sensors in Photoreactors

In this paper, we present a methodology for locating wireless sensors for the use in photoreactors. Photoreactors are, e.g., used to cultivate photosynthetic active microorganisms. For measuring important parameters like, e.g., the temperature inside the reactor, sensors are needed. Wireless locatable floating sensors would enable it to measure the data anywhere inside the reactor and to get a spatial resolution of the registered data. Due to the well defined propagation properties of magnetic fields and the fact that they are not significantly influenced in underwater environments when using low frequencies, a magnetic induction (MI) system is chosen for the data transmission as well as for the localization task. We designed an inductive transmitter and a receiver capable of measuring the magnetic field in every three spatial directions. The transmitting frequency is set at approx. 300kHz. This results in a wavelength of approx. 1km which clearly exceeds the dimensions of our measurement setup where the transmitter–receiver distances in general are lower than one meter. Due to this fact, only the quasi-static field component has to be considered and the location of the transmitter is calculated by measuring its magnetic field at defined positions and in using the magnetic dipole field equation in order to model its magnetic field geometry. The used measurement setup consists of a transmitter and two receivers. The first measurements were performed without a water filled photoreactor since no differences in the propagation criteria of magnetic fields are expected due to the negligibly low differences in the relative magnetic permeability of water and air. The system is calibrated and validated by using a LIDAR depth camera that is also used to locate the transmitter. The transmitter positions measured with the camera are therefore compared with the inductively measured ones.