Plasma-Driven in Situ Production of Hydrogen Peroxide for Biocatalysis

Production and transport of plasma-generated hydrogen peroxide from gas to liquid

In this work, the transport of hydroxyl radicals and hydrogen peroxide from a humid atmospheric pressure plasma jet into plasma-treated liquids is analysed. The concentration of H2O2 was measured by a spectrophotometric approach using the reagent ammonium metavanadate. OH was measured by the terephthalic acid dosimeter and the chemiluminescence of luminol. The plasma jet used is based on the design of the well-investigated COST reference jet and is extended by a capillary between the two electrodes. In addition to the experiments, the 0-dimensional plasma-chemical kinetics code GlobalKin was used to analyse the plasma chemistry in the gas phase in more detail. After 5 min plasma treatment, a maximum H2O2 concentration of 1 mM was found in the liquid, while the OH concentration was a factor 50 lower. The concentrations of both species in the liquid increased with plasma power, and the H2O2 concentration also increased with the humidity concentration of the feed gas, while the OH concentration first increased with humidity admixture and then decreased. The transport of both species could be controlled by the treatment distance, the gas flow rate and low frequency pulsing of the RF jet in such a way that the selectivity towards the long-lived species H2O2 was increased. Qualitative trends in the simulated number densities of gas phase H2O2 and OH at the location of the gas-liquid interface fit relatively well to the experimental measurements in the liquid.

Gas plasmas technology: from biomolecule redox research to medical therapy

Physical plasma is one consequence of gas ionization, i.e. its dissociation of electrons and ions. If operated in ambient air containing oxygen and nitrogen, its high reactivity produces various reactive oxygen and nitrogen species (RONS) simultaneously. Technology leap innovations in the early 2010s facilitated the generation of gas plasmas aimed at clinics and operated at body temperature, enabling their potential use in medicine. In parallel, their high potency as antimicrobial agents was systematically discovered. In combination with first successful clinical trials, this led in 2013 to the clinical approval of first medical gas plasma devices in Europe for promoting the healing of chronic and infected wounds and ulcers in dermatology. While since then, thousands of patients have benefited from medical gas plasma therapy, only the appreciation of the critical role of gas plasma-derived RONS led to unraveling first fragments of the mechanistic basics of gas plasma-mediated biomedical effects. However, drawing the complete picture of effectors and effects is still challenging. This is because gas plasma-produced RONS not only show a great variety of dozens of types but also each of them having distinct spatio-temporal concentration profiles due to their specific half-lives and reactivity with other types of RONS as well as different types of (bio) molecules they can react with. However, this makes gas plasmas fascinating and highly versatile tools for biomolecular redox research, especially considering that the technical capacity of increasing and decreasing individual RONS types holds excellent potential for tailoring gas plasmas toward specific applications and disease therapies.

Immobilization protects enzymes from plasma-mediated inactivation

Non-thermal plasmas are used in various applications to inactivate biological agents or biomolecules. A complex cocktail of reactive species, (vacuum) UV radiation and in some cases exposure to an electric field together cause the detrimental effects. In contrast to this disruptive property of technical plasmas, we have shown previously that it is possible to use non-thermal plasma-generated species such as H2O2 as cosubstrates in biocatalytic reactions. One of the main limitations in plasma-driven biocatalysis is the relatively short enzyme lifetime under plasma-operating conditions. This challenge could be overcome by immobilizing the enzymes on inert carrier materials. Here, we tested whether immobilization is suited to protect proteins from inactivation by plasma. To this end, using a dielectric barrier discharge device (PlasmaDerm), plasma stability was tested for five enzymes immobilized on ten different carrier materials. A comparative analysis of the treatment times needed to reduce enzyme activity of immobilized and free enzyme by 30% showed a maximum increase by a factor of 44. Covalent immobilization on a partly hydrophobic carrier surface proved most effective. We conclude from the study, that immobilization universally protects enzymes under plasma-operating conditions, paving the way for new emerging applications.

Non-Thermal Plasma Application in Medicine-Focus on Reactive Species Involvement

Non-thermal plasma (NTP) application in medicine is a dynamically developing interdisciplinary field. Despite the fact that basics of the plasma phenomenon have been known since the 19th century, growing scientific attention has been paid in recent years to the use of plasma in medicine. Three most important plasma-based effects are pivotal for medical applications: (i) inactivation of a broad spectrum of microorganisms, (ii) stimulation of cell proliferation and angiogenesis with lower plasma treatment intensity, and (iii) inactivation of cells by initialization of cell death with higher plasma intensity. In this review, we explain the underlying chemical processes and reactive species involvement during NTP in human (or animal) tissues, as well as in bacteria inactivation, which leads to sterilization and indirectly supports wound healing. In addition, plasma-mediated modifications of medical surfaces, such as surgical instruments or implants, are described. This review focuses on the existing knowledge on NTP-based in vitro and in vivo studies and highlights potential opportunities for the development of novel therapeutic methods. A full understanding of the NTP mechanisms of action is urgently needed for the further development of modern plasma-based medicine.

Enabling Peroxygenase Activity in Cytochrome P450 Monooxygenases by Engineering Hydrogen Peroxide Tunnels

Given prominent physicochemical similarities between H₂O₂ and water, we report a new strategy for promoting the peroxygenase activity of P450 enzymes by engineering their water tunnels to facilitate H₂O₂ access to the heme center buried therein. Specifically, the H₂O₂-driven activities of two native NADH-dependent P450 enzymes (CYP199A4 and CYP153A_M.aq) increase significantly (by >183-fold and >15-fold, respectively). Additionally, the amount of H₂O₂ required for an artificial P450 peroxygenase facilitated by a dual-functional small molecule to obtain the desired product is reduced by 95%-97.5% (with ∼95% coupling efficiency). Structural analysis suggests that mutating the residue at the bottleneck of the water tunnel may open a second pathway for H₂O₂ to flow to the heme center (in addition to the natural substrate tunnel). This study highlights a promising, generalizable strategy whereby P450 monooxygenases can be modified to adopt peroxygenase activity through H₂O₂ tunnel engineering, thus broadening the application scope of P450s in synthetic chemistry and synthetic biology.

Degradation of sulfamonomethoxine in solution using pulsed plasma discharge - identification of by-products and toxicity of treated solution to green algae

This study investigated the degradation of sulfamonomethoxine (SMM) by pulsed plasma discharge. SMM was successfully degraded following the first-order kinetics model. The percentage removal of SMM was estimated by the total input energy of plasma discharge, which was dependent on the initial SMM concentration. In addition, three types of by-products were observed at an early reaction time, which were then degraded. In contrast, the ecotoxicity of the treated solution by plasma discharge was assessed by an acute toxicity test using the green alga Raphidocelis subcapitata. The plasma discharge in water generated hydrogen peroxide with a concentration higher than the EC₅₀ for R. subcapitata. It is therefore necessary to remove H₂O₂ or prevent the generation of H₂O₂ for the degradation of antibiotics in solutions using plasma discharge.

A Unique P450 Peroxygenase System Facilitated by a Dual-Functional Small Molecule: Concept, Application, and Perspective

Cytochrome P450 monooxygenases (P450s) are promising versatile oxidative biocatalysts. However, the practical use of P450s in vitro is limited by their dependence on the co-enzyme NAD(P)H and the complex electron transport system. Using H₂O₂ simplifies the catalytic cycle of P450s; however, most P450s are inactive in the presence of H₂O₂. By mimicking the molecular structure and catalytic mechanism of natural peroxygenases and peroxidases, an artificial P450 peroxygenase system has been designed with the assistance of a dual-functional small molecule (DFSM). DFSMs, such as N-(ω-imidazolyl fatty acyl)-l-amino acids, use an acyl amino acid as an anchoring group to bind the enzyme, and the imidazolyl group at the other end functions as a general acid-base catalyst in the activation of H₂O₂. In combination with protein engineering, the DFSM-facilitated P450 peroxygenase system has been used in various oxidation reactions of non-native substrates, such as alkene epoxidation, thioanisole sulfoxidation, and alkanes and aromatic hydroxylation, which showed unique activities and selectivity. Moreover, the DFSM-facilitated P450 peroxygenase system can switch to the peroxidase mode by mechanism-guided protein engineering. In this short review, the design, mechanism, evolution, application, and perspective of these novel non-natural P450 peroxygenases for the oxidation of non-native substrates are discussed.

In situ H2O2 generation methods in the context of enzyme biocatalysis

Hydrogen peroxide is a versatile oxidant that has use in medical and biotechnology industries. Many enzymes require this oxidant as a reaction mediator in order to undergo their oxygenation chemistries. While there is a reliable method for generating hydrogen peroxide via an anthraquinone cycle, there are several advantages for generating hydrogen in situ. As highlighted in this review, this is particularly beneficial in the case of biocatalysts that require hydrogen peroxide as a reaction mediator because the exogenous addition of hydrogen peroxide can damage their reactive heme centers and render them inactive. In addition, generation of hydrogen peroxide in situ does not dilute the reaction mixture and cause solution parameters to change. The environment would also benefit from a hydrogen peroxide synthesis cycle that does not rely on nonrenewable chemicals obtained from fossil fuels. Generation of hydrogen peroxide in situ for biocatalysis using enzymes, bioelectrocatalyis, photocatalysis, and cold temperature plasmas are addressed. Particular emphasis is given to reaction processes that support high total turnover numbers (TTNs) of the hydrogen peroxide-requiring enzymes. Discussion of innovations in the use of hydrogen peroxide-producing enzyme cascades for antimicrobial activity, wastewater effluent treatment, and biosensors are also included.

Nuclear Waste and Biocatalysis: A Sustainable Liaison?

It is well-known that energy-rich radiation induces water splitting, eventually yielding hydrogen peroxide. Synthetic applications, however, are scarce and to the best of our knowledge, the combination of radioactivity with enzyme-catalysis has not been considered yet. Peroxygenases utilize H₂O₂ as an oxidant to promote highly selective oxyfunctionalization reactions but are also irreversibly inactivated in the presence of too high H₂O₂ concentrations. Therefore, there is a need for efficient in situ H₂O₂ generation methods. Here, we show that radiolytic water splitting can be used to promote specific biocatalytic oxyfunctionalization reactions. Parameters influencing the efficiency of the reaction and current limitations are shown. Particularly, oxidative inactivation of the biocatalyst by hydroxyl radicals influences the robustness of the overall reaction. Radical scavengers can alleviate this issue, but eventually, physical separation of the enzymes from the ionizing radiation will be necessary to achieve robust reaction schemes. We demonstrate that nuclear waste can also be used to drive selective, peroxygenase-catalyzed oxyfunctionalization reactions, challenging our view on nuclear waste in terms of sustainability.

Mass Production of Plasma Activated Water: Case Studies of Its Biocidal Effect on Algae and Cyanobacteria

Efficient treatment of contaminated water in industrially viable volumes is still a challenging task. The hydrodynamic cavitation plasma jet (HCPJ) is a promising plasma source for industrial-scale generation of biologically active environments at high flow rates of several m3/h. The combined effect of a hydro-mechanical phenomenon consisting of hydrodynamic cavitation and electrical discharge in cavitation voids was found to be highly efficient for large-volume generation of reactive oxygen species, ultraviolet (UV) radiation, and electro-mechanical stress in a liquid environment. Here, the persistence of biocidal properties of HCPJ-activated water (i.e., plasma-activated water (PAW)) was tested by the study of algae and cyanobacteria inactivation. Algae and cyanobacteria cultivated in media containing PAW (1:1) were completely inactivated after 72 h from first exposure. The test was performed at a total power input of up to 0.5 kWh/m3 at the treated liquid flow rate of 1 m3/h. A beneficial modification of our previous HCPJ design is described and thoroughly characterized with respect to the changes of hydrodynamic flow conditions as well as discharge performance and its optical characteristics. The modification proved to provide high biocidal activity of the resulting PAW, which confirms a strong potential for further design optimization of this promising water (liquid) plasma source.