Robust Light Driven Enzymatic Oxyfunctionalization via Immobilization of Unspecific Peroxygenase.

Unspecific peroxygenases have attracted interest in synthetic chemistry, especially for the oxidative activation of C-H bonds, as they only require hydrogen peroxide (H2 O2 ) instead of a cofactor. Due to their instability in even small amounts of H2 O2 , different strategies like enzyme immobilization or in situ H2 O2 production have been developed to improve the stability of these enzymes. While most strategies have been studied separately, a combination of photocatalysis with immobilized enzymes was only recently reported. To show the advantages and limiting factors of immobilized enzyme in a photobiocatalytic reaction, a comparison is made between free and immobilized enzymes. Adjustment of critical parameters such as (i) enzyme and substrate concentration, (ii) illumination wavelength and (iii) light intensity results in significantly increased enzyme stabilities of the immobilized variant. Moreover, under optimized conditions a turnover number of 334,500 was reached.

Photocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water.

Hydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.

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

Light-driven biocatalytic processes are notoriously hampered by poor penetration of light into the turbid reaction media. In this study, wirelessly powered light-emitting diodes are found to represent an efficient and scalable approach for process intensification of the photobiosynthetic production of diesel alkanes from renewable fatty acids.

Modeling and simulation-based design of electroenzymatic batch processes catalyzed by unspecific peroxygenase from A. aegerita.

Unspecific peroxygenases have attracted interest due to their ability to catalyze the oxygenation of various types of C-H bonds using only hydrogen peroxide as a cosubstrate. Due to the instability of these enzymes at even low hydrogen peroxide concentrations, careful fed-batch addition of the cosubstrate or ideally in situ production is required. While various approaches for hydrogen peroxide addition have been qualitatively assessed, only limited kinetic data concerning enzyme inactivation and peroxide accumulation has been reported so far. To obtain quantitative insights into the kinetics of such a process, a detailed data set for a peroxygenase-catalyzed benzylic hydroxylation coupled with electrochemical hydrogen peroxide production is presented. Based on this data set, we set out to model such an electroenzymatic process. For this, initial velocity data for the benzylic hydroxylation is collected and an extended Ping-Pong-Bi-Bi type rate equation is established, which sufficiently describes the enzyme kinetic. Moreover, we propose an empirical inactivation term based on the collected data set. Finally, we show that the full model does not only describe the process with sufficient accuracy, but can also be used predictively to control hydrogen peroxide feeding rates To limit the concentration of this critical cosubstrate in the system.

Artificial Light-Harvesting Complexes Enable Rieske Oxygenase Catalyzed Hydroxylations in Non-Photosynthetic cells.

In this study, we coupled a well-established whole-cell system based on E. coli via light-harvesting complexes to Rieske oxygenase (RO)-catalyzed hydroxylations in vivo. Although these enzymes represent very promising biocatalysts, their practical applicability is hampered by their dependency on NAD(P)H as well as their multicomponent nature and intrinsic instability in cell-free systems. In order to explore the boundaries of E. coli as chassis for artificial photosynthesis, and due to the reported instability of ROs, we used these challenging enzymes as a model system. The light-driven approach relies on light-harvesting complexes such as eosin Y, 5(6)-carboxyeosin, and rose bengal and sacrificial electron donors (EDTA, MOPS, and MES) that were easily taken up by the cells. The obtained product formations of up to 1.3 g L-1 and rates of up to 1.6 mm h-1 demonstrate that this is a comparable approach to typical whole-cell transformations in E. coli. The applicability of this photocatalytic synthesis has been demonstrated and represents the first example of a photoinduced RO system.

Kinetics and Optimization of the Photocatalytic Reduction of Nitrobenzene.

The photocatalytic reduction of nitrobenzene to aniline in alcoholic solutions appears as an interesting alternative to the classical hydration. However, little is known about the influence of reaction parameters on the kinetics of the reaction which were therefore studied herein. The effects of light intensity, catalyst concentration, initial concentration, and temperature were systematically investigated under more than 50 different conditions and accurately described with an appropriate kinetic model. The results show that the efficiency of the reaction is extremely high and apparent quantum yields of up to 142 % were observed under optimized conditions. Particularly interesting is the fact high efficiencies were also obtained at high reaction rates of up to 74.3 mM h-1. Overall these results demonstrate that heterogeneous photocatalytic reactions can be very efficient and productive at the same time and may therefore present a powerful tool in synthetic organic chemistry.

A Holistic Approach to Model the Kinetics of Photocatalytic Reactions.

Understanding and modeling kinetics is an essential part of the optimization and implementation of chemical reactions. In the case of photocatalytic reactions this is mostly done one-dimensionally, i.e., only considering the effect of one parameter at the same time. However, as discussed in this study, many of the relevant reaction parameters have mutual interdependencies that call for a holistic multi-dimensional approach to accurately model and understand their influence. Such an approach is described herein, and all the relevant equations given so that researchers can readily implement it to analyze and model their reactions.

Hydrocarbon Synthesis via Photoenzymatic Decarboxylation of Carboxylic Acids.

A recently discovered photodecarboxylase from Chlorella variabilis NC64A ( CvFAP) bears the promise for the efficient and selective synthesis of hydrocarbons from carboxylic acids. CvFAP, however, exhibits a clear preference for long-chain fatty acids thereby limiting its broad applicability. In this contribution, we demonstrate that the decoy molecule approach enables conversion of a broad range of carboxylic acids by filling up the vacant substrate access channel of the photodecarboxylase. These results not only demonstrate a practical application of a unique, photoactivated enzyme but also pave the way to selective production of short-chain alkanes from waste carboxylic acids under mild reaction conditions.

Grafted iron(iii) ions significantly enhance NO2 oxidation rate and selectivity of TiO2 for photocatalytic NO x abatement.

Semiconductor photocatalysis could be an effective means to combat nitrogen oxides (NO x ) based air pollution through mineralisation of NO x to nitrate. However, most of the typically TiO2-based catalysts employed show a much higher reactivity towards NO than NO2, leading to an accumulation of this unwanted and toxic intermediate. By grafting the photocatalyst with small amounts (≤0.1 at%) of isolated iron(iii) ions, the reactivity towards NO2 is increased by the factor of 9, bringing it up to par with the NO-reactivity and alleviating the problem with intermediate accumulation. Consequently, the observed selectivity of the reaction is dramatically increased from less than 40% to more than 90%. The paper also discusses possible mechanisms for this very beneficial behavior.

On the underlying mechanisms of the low observed nitrate selectivity in photocatalytic NOx abatement and the importance of the oxygen reduction reaction.

Semiconductor photocatalysis could be an effective means to combat air pollution, especially nitrogen oxides, which can be mineralized to nitrate. However, the reaction typically shows poor selectivity, releasing a number of unwanted and possibly toxic intermediates such as nitrogen dioxide. Up to now, the underlying principles that lead to this poor selectivity were not understood so a knowledge-based catalyst design for more selective materials was impossible. Herein, we present strong evidence for the slow oxygen reduction being one the causes, as the competing back-reduction of nitrate leads to the release of nitrogen dioxide. Consequently, engineering the photocatalyst for a better oxygen reduction efficiency should also increase the nitrate selectivity.

Selective Activation of C-H Bonds in a Cascade Process Combining Photochemistry and Biocatalysis.

Selective oxyfunctionalizations of inert C-H bonds can be achieved under mild conditions by using peroxygenases. This approach, however, suffers from the poor robustness of these enzymes in the presence of hydrogen peroxide as the stoichiometric oxidant. Herein, we demonstrate that inorganic photocatalysts such as gold-titanium dioxide efficiently provide H2 O2 through the methanol-driven reductive activation of ambient oxygen in amounts that ensure that the enzyme remains highly active and stable. Using this approach, the stereoselective hydroxylation of ethylbenzene to (R)-1-phenylethanol was achieved with high enantioselectivity (>98 % ee) and excellent turnover numbers for the biocatalyst (>71 000).

Properties and photochemistry of valence-induced-Ti(3+) enriched (Nb,N)-codoped anatase TiO2 semiconductors.

Nb and N codoped TiO2s are outstandingly versatile semiconductor oxides. Their high conductivity makes them valid alternatives to commercially available, but very expensive, conductive oxides. They show increased photonic efficiencies compared to the cases of solely Nb or N doped TiO2, when used as visible light sensitised photocatalysts. Furthermore, they are excellent materials for O2 sensors at very low temperature. Despite these remarkable properties, a clear picture of the electronic and optical mechanisms induced by the simultaneous presence of the dopants has just begun to be understood. Using a combination of electron paramagnetic resonance (EPR) spectroscopy, electrochemical impedance spectroscopy (EIS) and optical spectroscopy, we present here novel fundamental insights into the mechanisms responsible for the enhanced conductivity and visible light photochemistry.

Ruthenium-modified zinc oxide, a highly active vis-photocatalyst: the nature and reactivity of photoactive centres.

We recently reported a highly active photocatalyst, ruthenium-modified zinc oxide, which was found to be able to utilise the red part of the visible light spectrum for photocatalytic reactions [Bloh et al., Environ. Sci. Pollut. Res., 2012, 19, 3688-3695]. However, the origin and mechanism of the observed activity as well as the nature of the photoactive centres are still unknown. Herein, we expand on that by reporting a series of experiments specifically designed to unravel the mechanism of the visible light induced photocatalytic reactions. The absolute potentials of the valence and the conduction band edge are identified by the combined use of electrochemical impedance and UV-vis diffuse reflectance spectroscopy. The conduction band electron and the valence band hole activity are assessed through a novel approach tracing their signature oxidative species, i.e., hydrogen peroxide and hydroxyl radicals, respectively. Oxygen reduction currents are measured at different potentials to investigate the role of molecular oxygen as an electron scavenger as well as the underlying reduction pathways. Additionally, the photocatalytic activity of the samples is verified using another (ISO standard) degradation test, the gas-phase oxidation of nitric oxide. The experimental results reveal that the employed synthetic route yields a unique mixture of ruthenium(VI)-doped zinc oxide and ruthenium(VI) oxide particles with both forms of the ruthenium playing their own independent role in the enhancement of the photocatalytic activity. The ruthenium ions acting as dopants enable a better charge separation as well as the absorption of red light resulting in the direct promotion of electrons from the Ru(VI)-species to the conduction band. Both, the conduction band electrons and the thus formed Ru(VII) subsequently participate in the degradation of the pollutant molecules. The ruthenium dioxide particles, on the other hand, act as catalysts increasing the efficiency of the reaction by improving the oxygen reduction properties of the material.