Visible Light-Driven O2 Reduction by a Porphyrin–Laccase System

Constructing cuprous oxide-modified zinc tetraphenylporphyrin ultrathin nanosheets heterojunction for enhanced photocatalytic carbon dioxide reduction to methane

The application of supermolecular naonostructures in the photocatalytic carbon dioxide reduction reaction (CO2RR) has attracted increasing attentions. However, it still faces significant challenges, such as low selectivity for multi-electron products and poor stability. Here, the cuprous oxide (Cu2O)-modified zinc tetraphenylporphyrin ultrathin nanosheets (ZnTPP NSs) are successfully constructed through the aqueous chemical reaction. Comprehensive characterizations confirm the formation of type-II heterojunction between Cu2O and ZnTPP in Cu2O@ZnTPP, and the electron transfer from Cu2O to ZnTPP through the Zn-O-Cu bond under the static contact. Under the visible-light irradiation (λ > 420 nm), the optimized Cu2O@ZnTPP sample as catalyst for photocatalytic CO2RR exhibits the methane (CH4) evolution rate of 120.9 μmol/g/h, which is ∼ 4 and ∼ 10 times those of individual ZnTPP NSs (28.0 μmol/g/h) and Cu2O (12.8 μmol/g/h), respectively. Meanwhile, the CH4 selectivity of ∼ 98.7 % and excellent stability can be achieved. Further experiments reveal that Cu2O@ZnTPP has higher photocatalytic conversion efficiency than Cu2O and ZnTPP NSs, and the photoinduced electron transfer from ZnTPP to Cu2O can be identified via the path of ZnTPP→ (ZnTPP•ZnTPP)*→ ZnTPP-→ Zn-O-Cu → Cu2O. Consequently, Cu2O@ZnTPP exhibits a shorter electron-hole separation lifetime (3.3 vs. 9.3 ps) and a longer recombination lifetime (23.1 vs. 13.4 ps) than individual ZnTPP NSs. This work provides a strategy to construct the organic nanostructures for photocatalytic CO2RR to multi-electron products.

Enhanced Interfacial Electron Transfer in Photocatalyst-Natural Enzyme Coupled Artificial Photosynthesis System: Tuning Strategies and Molecular Simulations

Laccase is capable of catalyzing a vast array of reactions, but its low redox potential limits its potential applications. The use of photocatalytic materials offers a solution to this problem by converting absorbed visible light into electrons to facilitate enzyme catalysis. Herein, MIL-53(Fe) and NH2-MIL-53(Fe) serve as both light absorbers and enzyme immobilization carriers, and laccase is employed for solar-driven chemical conversion. Electron spin resonance spectroscopy results confirm that visible light irradiation causes rapid transfer of photogenerated electrons from MOF excitation to T1 Cu(II) of laccase, significantly increasing the degradation rate constant of tetracycline (TC) from 0.0062 to 0.0127 min-1. Conversely, there is only minimal or no electron transfer between MOF and laccase in the physical mixture state. Theoretical calculations demonstrate that the immobilization of laccase's active site and its covalent binding to the metal-organic framework surface augment the coupled system's activity, reducing the active site accessible from 27.8 to 18.1 Å. The constructed photo-enzyme coupled system successfully combines enzyme catalysis' selectivity with photocatalysis's high reactivity, providing a promising solution for solar energy use.

Engineering a Dual-Functionalized PolyHIPE Resin for Photobiocatalytic Flow Chemistry

The use of a dual resin for photobiocatalysis, encompassing both a photocatalyst and an immobilized enzyme, brings several challenges, including effective immobilization, maintaining photocatalyst and enzyme activity and ensuring sufficient light penetration. However, the benefits, such as integrated processes, reusability, easier product separation, and potential for scalability, can outweigh these challenges, making dual resin systems promising for efficient and sustainable photobiocatalytic applications. In this study, we employed a photosensitizer-containing porous emulsion-templated polymer as a functional support that is used to covalently anchor a chloroperoxidase from Curvularia inaequalis (CiVCPO). We demonstrate the versatility of this heterogeneous photobiocatalytic material, which enables the bromination of four aromatic substrates, including rutin-a natural occurring flavonol-under blue light (456 nm) irradiation and continuous flow conditions.

An artificial metalloenzyme that can oxidize water photocatalytically: design, synthesis, and characterization

In nature, light-driven water oxidation (WO) catalysis is performed by photosystem II via the delicate interplay of different cofactors positioned in its protein scaffold. Artificial systems for homogeneous photocatalytic WO are based on small molecules that often have limited solubility in aqueous solutions. In this work, we alleviated this issue and present a cobalt-based WO-catalyst containing artificial metalloenzyme (ArM) that is active in light-driven, homogeneous WO catalysis in neutral-pH aqueous solutions. A haem-containing electron transfer protein, cytochrome B5 (CB5), served to host a first-row transition-metal-based WO catalyst, CoSalen (CoIISalen, where H2Salen = N,N'-bis(salicylidene)ethylenediamine), thus producing an ArM capable of driving photocatalytic WO. The CoSalen ArM formed a water-soluble pre-catalyst in the presence of [Ru(bpy)3](ClO4)2 as photosensitizer and Na2S2O8 as the sacrificial electron acceptor, with photocatalytic activity similar to that of free CoSalen. During photocatalysis, the CoSalen-protein interactions were destabilized, and the protein partially unfolded. Rather than forming tens of nanometer sized CoOx nanoparticles as free CoSalen does under photocatalytic WO conditions, the CB5 : CoSalen ArM showed limited protein cross-linking and remained soluble. We conclude that a weak, dynamic interaction between a soluble cobalt species and apoCB5 was formed, which generated a catalytically active adduct during photocatalysis. A detailed analysis was performed on protein stability and decomposition processes during the harsh oxidizing reaction conditions of WO, which will serve for the future design of WO ArMs with improved activity and stability.

Synthesis and Photocatalytic CO2 Reduction of a Cyclic Zinc(II) Porphyrin Trimer with an Encapsulated Rhenium(I) Bipyridine Tricarbonyl Complex

We previously reported a cyclic Zn(II) porphyrin trimer in which three Zn porphyrins are alternately bridged by three 2,2'-bipyridine (bpy) moieties, enabling the encapsulation of metal complexes within the nanopore formed by the Zn porphyrins. In this study, we introduced a [Re(CO)3 Br] fragment into one of the bpy moieties of the cyclic trimer to form the catalytic Re(4,4'-R2 -bpy)(CO)3 Br center (R=methyl ester). The ester groups (R) play an important role in the synthesis of the cyclic structure. However, it was observed that these ester groups significantly deactivated the photocatalytic CO2 reduction reaction. Therefore, we converted the ester groups with a suitable reducing reagent into hydroxymethyl groups, followed by acetylation to form acetoxymethyl groups. This modification remarkably enhanced the photocatalytic activity of the cyclic trimer=Re complex system for CO2 reduction. Moreover, in the modified system, the presence of the Re complex induced room-temperature phosphorescence of the Zn porphyrin. The phosphorescence was significantly quenched by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole, indicating that efficient electron transfer mediated by the excited triplet state of the Zn porphyrin occurs during the photocatalytic CO2 reduction.

Artificial photosynthesis systems for solar energy conversion and storage: platforms and their realities

In natural photosynthesis, photosynthetic organisms such as green plants realize efficient solar energy conversion and storage by integrating photosynthetic components on the thylakoid membrane of chloroplasts. Inspired by natural photosynthesis, researchers have developed many artificial photosynthesis systems (APS's) that integrate various photocatalysts and biocatalysts to convert and store solar energy in the fields of resource, environment, food, and energy. To improve the system efficiency and reduce the operation cost, reaction platforms are introduced in APS's since they allow for great stability and continuous processing. A systematic understanding of how a reaction platform affects the performance of artificial photosynthesis is conducive for designing an APS with superb solar energy utilization. In this review, we discuss the recent APS's researches, especially those confined on/in platforms. The importance of different platforms and their influences on APS's performance are emphasized. Generally, confined platforms can enhance the stability and repeatability of both photocatalysts and biocatalysts in APS's as well as improve the photosynthetic performance due to the proximity effect. For functional platforms that can participate in the artificial photosynthesis reactions as active parts, a high integration of APS's components on/in these platforms can lead to efficient electron transfer, enhanced light-harvesting, or synergistic catalysis, resulting in superior photosynthesis performance. Therefore, the integration of APS's components is beneficial for the transfer of substrates and photoexcited electrons in artificial photosynthesis. We finally summarize the current challenges of APS's development and further efforts on the improvement of APS's.

Acceleration of oxidation promoted by laccase irradiation with red light

Irradiation with red light is able to improve yields and shorten the reaction time in enzymatic reactions.

Photocatalytic CO2 reduction sensitized by a special-pair mimic porphyrin connected with a rhenium(i) tricarbonyl complex

Zn porphyrins with an imidazolyl group at the meso position generate a highly stable porphyrin dimer by complementary coordination from the imidazolyl to the Zn ion in noncoordinating solvents such as chloroform, which mimics the natural special pair in photosynthesis. In this work, we have synthesized an imidazolyl-substituted Zn porphyrin connected with a Re 2,2-bipyridine tricarbonyl complex as a CO₂ reduction catalyst via a p-phenylene linker, affording a homodimer with two Re complexes on both sides (ReDRe). The dimeric structure is easily dissociated into the corresponding monomers in coordinating solvents. Therefore, we prepared a mixture containing a heterodimer with the Re carbonyl complex on one side (ReD) by simple mixing with an imidazolyl Zn porphyrin and evaporating the solvent. Using the Grubbs catalyst, the subsequent olefin metathesis reaction of the mixture gave covalently linked porphyrin dimers through the allyloxy side chains, enabling the isolation of the stable hetero- (ReD′) and homo-dimers (ReD′Re) with gel permeation chromatography. The Zn porphyrin dimers have intense absorption bands in the visible light region and acted as good photosensitizers in photocatalytic CO₂ reduction in a mixture of N,N-dimethylacetamide and triethanolamine (5 : 1 v/v) containing 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the electron donor, giving CO with high selectivity and durability. Under irradiation with strong light intensity, the reaction rate in ReD′ exceeded that of the previous porphyrin Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Re complex dyad, ZnP-phen=Re. For instance, after irradiation at 560 nm for 18 h, the turnover number (TON_CO) of ReD′ reached 2800, whereas the TON_CO of ZnP-phen=Re was 170. The high activity in the system using the porphyrin dimer originates from no accumulation of the one-electron reduced species of the porphyrin that inhibit light absorption due to the inner-filter effect.

An artificial special pair was connected with a Re 2,2-bipyridine tricarbonyl complex. The special pair derivative acted as a good photosensitizer in photocatalytic CO₂ reduction, giving CO with high selectivity and durability.

Photocatalyst-enzyme hybrid systems for light-driven biotransformation

Enzymes catalyse target reactions under mild conditions with high efficiency, as well as excellent regional-, stereo-, and enantiomeric selectivity. Photocatalysis utilises sustainable and environment-friendly light power to realise efficient chemical conversion. By combining the interdisciplinary advantages of photo- and enzymatic catalysis, the photocatalyst-enzyme hybrid systems have proceeded various light-driven biotransformation with high efficiency under environmentally benign conditions, thus, attracting unparalleled focus during the last decades. It has also been regarded as a promising pathway towards green chemistry utilising ubiquitous solar energy. This systematic review gives insight into this research field by classifying the existing photocatalyst-enzyme hybrid systems into three sections based on different hybridizing modes between photo- and enzymatic catalysis. Furthermore, existing challenges and proposed strategies are discussed within this context. The first system summarised is the cofactor-mediated hybrid system, in which natural/artificial cofactors act as reducing equivalents that connect photocatalysts with enzymes for light-driven enzymatic biotransformation. Second, the direct contact-based photocatalyst-enzyme hybrid systems are described, including two different kinds of electron exchange sites on the enzyme molecules. Third, some cases where photocatalysts and enzymes are integrated into a reaction cascade with specific intermediates will be discussed in the following chapter. Finally, we provide perspective concerning the future of this field.

Metal-templated synthesis of rigid and conformationally restricted cyclic bisporphyrins: specific retention times on a cyanopropyl-modified silica gel column

A series of rigid and conformationally restricted cyclic bis(zinc porphyrin)s connected via 2,2'-bipyridine and phthalamide, isophthalamide, or terephthalamide moieties were prepared by metal-templated synthesis. The yields were significantly improved when compared with those obtained under metal-free conditions. In particular, phthalamide and terephthalamide derivatives were obtained only by metal-templated synthesis. Structural analyses and dynamics of the exchange between the conformers in each cyclic porphyrin were examined by NMR spectroscopy. Although the distances between the two zinc porphyrins were extended in the order of phthalamide, isophthalamide, and terephthalamide derivatives, the order of the specific retention of the cyclic porphyrins on cyanopropyl-modified silica gel (CN-MS) chromatography columns varied. Thus, this order was reversed in the isophthalamide and terephthalamide derivatives. Based on the rigid structure of the terephthalamide derivative, the origin of the specific retention on the CN-MS chromatography column was attributed to both the distance and rigidity of the cyclic porphyrins.

Tracking light-induced electron transfer toward O2 in a hybrid photoredox-laccase system

Photobiocatalysis uses light to perform specific chemical transformations in a selective and efficient way. The intention is to couple a photoredox cycle with an enzyme performing multielectronic catalytic activities. Laccase, a robust multicopper oxidase, can be envisioned to use dioxygen as a clean electron sink when coupled to an oxidation photocatalyst. Here, we provide a detailed study of the coupling of a [Ru(bpy)₃]²⁺ photosensitizer to laccase. We demonstrate that efficient laccase reduction requires an electron relay like methyl viologen. In the presence of dioxygen, electrons transiently stored in superoxide ions are scavenged by laccase to form water instead of H₂O₂. The net result is the photo accumulation of highly oxidizing [Ru(bpy)₃]³⁺. This study provides ground for the use of laccase in tandem with a light-driven oxidative process and O₂ as one-electron transfer relay and as four-electron substrate to be a sustainable final electron acceptor in a photocatalytic process.

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An electron relay boosts photoreduction of laccase

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Superoxide is efficiently captured by laccase preventing formation of H₂O₂

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Light activation reveals information on elementary steps inside the enzyme

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Laccase enables O₂ as terminal electron acceptor for oxidative photocatalysis

Clicked Bifunctional Dendrimeric and Cyclopeptidic Addressable Redox Scaffolds for the Functionalization of Carbon Nanotubes with Redox Molecules and Enzymes

Carbon nanotube electrodes were modified with ferrocene and laccase using two different click reactions strategies and taking advantage of bifunctional dendrimers and cyclopeptides. Using diazonium functionalization and the efficiency of oxime ligation, the combination of both multiwalled carbon nanotube surfaces and modified dendrimers or cyclopeptides allows the access to a high surface coverage of ferrocene in the order of 50 nmol cm^-2, a 50-fold increase compared to a classic click reaction without oxime ligation of these highly branched macromolecules. Furthermore, this original immobilization strategy allows the immobilization of mono- and bi-functionalized active multicopper enzymes, laccases, via copper(I)-catalyzed azide-alkyne cycloaddition. Electrochemical studies underline the high efficiency of the oxime-ligated dendrimers or cyclopeptides for the immobilization of redox entities on surfaces while being detrimental to electron tunneling with enzyme active sites despite controlled orientation.

Photocatalytic CO2 Reductions Catalyzed by meso-(1,10-Phenanthrolin-2-yl)-Porphyrins Having a Rhenium(I) Tricarbonyl Complex

We have prepared Zn and free-base porphyrins appended with a fac-Re(phen)(CO)3 Br (where phen is 1,10-phenanthroline) at the meso position of the porphyrin, and performed photocatalytic CO2 reduction using porphyrin-Re dyads in the presence of either triethylamine (TEA) or 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. The Zn porphyrin dyad showed a high turnover number for CO production compared with the free-base porphyrin dyad, suggesting that the central Zn ion of porphyrin plays an important role in suppressing electron accumulation on the porphyrin part and achieving high durability of the photocatalytic CO2 reduction using both TEA and BIH. The effect of acids on the CO2 reduction was investigated using the Zn porphyrin-Re dyad and BIH. Acetic acid, a relatively strong Brønsted acid, rapidly causes the porphyrin's color to fade upon irradiation and dramatically decreases CO production, whereas proper weak Brønsted acids such as 2,2,2-trifluoroethanol and phenol enhance the CO2 reduction.

Photodynamic activity attained through the ruptured π-conjugation of pyridyl groups with a porphyrin macrocycle: synthesis and the photophysical and photobiological evaluation of 5-mono-(4-nitrophenyl)-10,15,20-tris-[4-(phenoxymethyl)pyridine]-porphyrin and its Zn(ii) complex

This article compares a reported hydrophobic and photobiologically inert porphyrin synthon, (NPh)TPyP, bearing a single meso-4-nitrophenyl group and three meso-pyridyl groups (A₃B type) with a new photobiologically active metal-free porphyrin, P₃N, and its zinc-complex, P₃NZn, which bear a meso-4-nitrophenyl group along with three distal pyridyl groups. Both P₃N and P₃NZn experience ruptured π-conjugation with the porphyrin macrocycle and attain hydrophilicity, as indicated via density functional theory (DFT) calculations, becoming photobiologically active under in vitro conditions. The non-invasive photodynamic activity (PDA) predominantly shown by the zinc-complex P₃NZn (with higher hydrophilicity) towards KRAS-mutated human lung-cancer cells (A549) was studied. The results indicate the existence of intracellular singlet oxygen inflicted anticancer PDA, which is apparent through the upregulation of intracellular reactive oxygen species (ROS) and the downregulation of both intracellular superoxide dismutase (SOD) and intracellular reduced glutathione (GSH) levels. The trends obtained from both SOD and GSH assays were indicators of therapeutic defence against oxidative stress via neutralizing superoxide anions (SOA).

Efficiency of Site-Specific Clicked Laccase-Carbon Nanotubes Biocathodes towards O2 Reduction

A maximization of a direct electron transfer (DET) between redox enzymes and electrodes can be obtained through the oriented immobilization of enzymes onto an electroactive surface. Here, a strategy for obtaining carbon nanotube (CNTs) based electrodes covalently modified with perfectly control-oriented fungal laccases is presented. Modelizations of the laccase-CNT interaction and of electron conduction pathways serve as a guide in choosing grafting positions. Homogeneous populations of alkyne-modified laccases are obtained through the reductive amination of a unique surface-accessible lysine residue selectively engineered near either one or the other of the two copper centers in enzyme variants. Immobilization of the site-specific alkynated enzymes is achieved by copper-catalyzed click reaction on azido-modified CNTs. A highly efficient reduction of O₂ at low overpotential and catalytic current densities over -3 mA cm^-2 are obtained by minimizing the distance from the electrode surface to the trinuclear cluster.

Efficient photocatalytic proton-coupled electron-transfer reduction of O2 using a saddle-distorted porphyrin as a photocatalyst

Photocatalytic O2 reduction reactions proceeded to produce H2O2 using a diprotonated saddle-distorted dodecaphenylporphyrin as a photocatalyst. The quantum yield (12%), the turnover number (3000 for 6 h), and the turnover frequency (500 h-1) are achieved in photocatalytic systems based on free-base porphyrins for the first time. The photocatalytic reaction mechanism has been revealed by ns-laser flash photolysis and kinetic analysis.

Catalytic promiscuity enabled by photoredox catalysis in nicotinamide-dependent oxidoreductases

Strategies that provide enzymes with the ability to catalyse non-natural reactions are of considerable synthetic value. Photoredox catalysis has proved adept at expanding the synthetic repertoire of existing catalytic platforms, yet, in the realm of biocatalysis it has primarily been used for cofactor regeneration. Here we show that photoredox catalysts can be used to enable new catalytic function in nicotinamide-dependent enzymes. Under visible-light irradiation, xanthene-based photocatalysts enable a double-bond reductase to catalyse an enantioselective deacetoxylation. Mechanistic experiments support the intermediacy of an α-acyl radical, formed after the elimination of acetate. Isotopic labelling experiments support nicotinamide as the source of the hydrogen atom. Preliminary calculations and mechanistic experiments suggest that binding to the protein attenuates the reduction potential of the starting material, an important feature for localizing radical formation to the enzyme active site. The generality of this approach is highlighted with the radical dehalogenation of α-bromoamides catalysed by ketoreductases with Eosin Y as a photocatalyst.

Photobiocatalysis: Activating Redox Enzymes by Direct or Indirect Transfer of Photoinduced Electrons

Biocatalytic transformation has received increasing attention in the green synthesis of chemicals because of the diversity of enzymes, their high catalytic activities and specificities, and mild reaction conditions. The idea of solar energy utilization in chemical synthesis through the combination of photocatalysis and biocatalysis provides an opportunity to make the "green" process greener. Oxidoreductases catalyze redox transformation of substrates by exchanging electrons at the enzyme's active site, often with the aid of electron mediator(s) as a counterpart. Recent progress indicates that photoinduced electron transfer using organic (or inorganic) photosensitizers can activate a wide spectrum of redox enzymes to catalyze fuel-forming reactions (e.g., H₂ evolution, CO₂ reduction) and synthetically useful reductions (e.g., asymmetric reduction, oxygenation, hydroxylation, epoxidation, Baeyer-Villiger oxidation). This Review provides an overview of recent advances in light-driven activation of redox enzymes through direct or indirect transfer of photoinduced electrons.

Visualization of nonemissive triplet species of Zn(ii) porphyrins through Cu(ii) porphyrin emission via the reservoir mechanism in a porphyrin macroring

A macroring shows long-lived near-IR emission from Cu(ii) porphyrin via the reservoir mechanism. The significant emission quenching by O₂ suggests that the T₁ state of Zn(ii) porphyrin can be monitored by the near-IR emission.

Synthesis and Photophysical Properties of Porphyrin Macrorings Composed of Free-Base Porphyrins and Slipped-Cofacial Zinc Porphyrin Dimers

The self-assembled macroring N-(Zn-Fb-Zn)₃ has been constructed by intermolecular complementary coordination among three trisporphyrin Zn-Fb-Zn molecules, each of which consists of a central free-base porphyrin and two imidazolyl-zinc-porphyrin ends. Thus, N-(Zn-Fb-Zn)₃ has three slipped-cofacial zinc porphyrin dimers ("special pair model") and three free-base porphyrins, alternately. The zinc porphyrin dimers in N-(Zn-Fb-Zn)₃ are covalently connected by a ring-closing olefin metathesis reaction between the allyl ether groups substituted on the zinc porphyrin dimers, giving a covalently linked macroring C-(Zn-Fb-Zn)₃. The fluorescence spectra of C-(Zn-Fb-Zn)₃ in several solvents show that the photoinduced energy transfer from one of the zinc porphyrin dimers to a free-base porphyrin occurs intramolecularly in toluene, whereas the photoinduced electron transfer predominantly occurs intramolecularly in N,N-dimethylformamide. Treatment of C-(Zn-Fb-Zn)₃ with copper(II) acetate gives a Cu-containing heteromultinuclear porphyrin macroring C-(Zn-Cu-Zn)₃, demonstrating that C-(Zn-Fb-Zn)₃ could be a good precursor to construct various heteromultinuclear porphyrin macrorings.

Porphyrins as Photoredox Catalysts: Experimental and Theoretical Studies

Metalloporphyrins not only are vital in biological systems but also are valuable catalysts in organic synthesis. On the other hand, catalytic properties of free base porphyrins have been less explored. They are mostly known as efficient photosensitizers for the generation of singlet oxygen via photoinduced energy transfer processes, but under light irradiation, they can also participate in electron transfer processes. Indeed, we have found that free base tetraphenylporphyrin (H₂TPP) is an efficient photoredox catalyst for the reaction of aldehydes with diazo compounds leading to α-alkylated derivatives. The performance of a porphyrin catalyst can be optimized by tailoring various substituents at the periphery of the macrocycle at both the β and meso positions. This allows for the fine tuning of their optical and electrochemical properties and hence their catalytic activity.

Oxidation catalysis via visible-light water activation of a [Ru(bpy)3]2+ chromophore BSA–metallocorrole couple

Light induced enantioselective oxidation of thioanisole with water as the oxygen atom source is catalyzed by a Mn-corrole–BSA artificial metalloenzyme in the presence of a photoactivable ruthenium complex.

Ru(II)-diimine functionalized metalloproteins: From electron transfer studies to light-driven biocatalysis

The unique photochemical properties of Ru(II)-diimine complexes have helped initiate a series of seminal electron transfer studies in metalloenzymes. It has thus been possible to experimentally determine rate constants for long-range electron transfers. These studies have laid the foundation for the investigation of reactive intermediates in heme proteins and for the design of light-activated biocatalysts. Various metalloenzymes such as hydrogenase, carbon monoxide dehydrogenase, nitrogenase, laccase and cytochrome P450 BM3 have been functionalized with Ru(II)-diimine complexes. Upon visible light-excitation, these photosensitized metalloproteins are capable of sustaining photocatalytic activity to reduce small molecules such as protons, acetylene, hydrogen cyanide and carbon monoxide or activate molecular dioxygen to produce hydroxylated products. The Ru(II)-diimine photosensitizers are hence able to deliver multiple electrons to metalloenzymes buried active sites, circumventing the need for the natural redox partners. In this review, we will highlight the key achievements of the light-driven biocatalysts, which stem from the extensive electron transfer investigations. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

Visible-Light-Driven Oxidation of Organic Substrates with Dioxygen Mediated by a [Ru(bpy)3 ](2+) /Laccase System

Oxidation reactions are highly important chemical transformations that still require harsh reaction conditions and stoichiometric amounts of chemical oxidants that are often toxic. To circumvent these issues, olefins oxidation is achieved in mild conditions upon irradiation of an aqueous solution of the complex [Ru(bpy)3 ](2+) and the enzyme laccase. Epoxide formation is coupled to the light-driven reduction of O2 by [Ru(bpy)3 ](2+) /laccase system. The reactivity can be explained by dioxygen acting both as an oxidative agent and as renewable electron acceptor, avoiding the use of a sacrificial electron acceptor.

Detection of zinc finger protein (EGR1) based on electrogenerated chemiluminescence from singlet oxygen produced in a nanoclay-supported porphyrin environment

Early growth response protein 1 (EGR1), as a characteristic example of zinc finger proteins, acts as a transcription factor in eukaryotic cells, mediating protein-protein interactions. Here, a novel electrochemiluminescence (ECL)-based protocol for EGR1 assay was developed with a new eco-friendly emitter: singlet oxygen produced in the vicinity of nanoclay-supported zinc proto-porphyrin IX (ZnPPIX). Its electrochemical reduction stimulates an intense monochromic CL irradiation at 644 nm from the dissolved oxygen as endogenous coreactant in the aqueous solution. This ECL derivation was rationalized via hyphenated spectroscopy and theoretical calculation. To promote hydrophilicity and solid-state immobilization of porphyrins, the lamellar artificial laponite was employed as a nanocarrier owning to its large specific area without the blackbody effect. The facile exfoliation of laponite produced quality monolayered nanosheets and facilitated the adsorption and flattening of PPIX upon the surface, resulting in a highly efficient ECL emission. Based on the release of Zn(2+) in zinc finger domains of EGR1 upon contact with the ECL-inactive PPIX, which was monitored by circular dichroism and UV-absorption, a sensitive Zn(2+)-selective electrode for the "signal-on" detection of EGR1 was prepared with a detection limit down to 0.48 pg mL(-1) and a linearity over 6 orders of magnitude. The proposed porphyrin-based ECL system thus infused fresh blood into the traditional ECL family, showing great promise in bioassays of structural Zn(II) proteins and zinc finger-binding nucleotides.

Laccases as palladium oxidases

The first example of a coupled catalytic system involving an enzyme and a palladium(ii) catalyst competent for the aerobic oxidation of alcohol in mild conditions is described. In the absence of dioxygen, the fungal laccase LAC3 is reduced by a palladium(0) species as evidenced by the UV/VIS and ESR spectra of the enzyme. During the oxidation of veratryl alcohol performed in water, at room temperature and atmospheric pressure, LAC3 regenerates the palladium catalyst, is reduced and catalyzes the four-electron reduction of dioxygen into water with no loss of enzyme activity. The association of a laccase with a water-soluble palladium complex results in a 7-fold increase in the catalytic efficiency of the complex. This is the first step in the design of a family of renewable palladium catalysts for aerobic oxidation.

Detection of subsurface trace impurity in polished fused silica with biological method

Subsurface damage (SSD), especially photoactive impurities, degrades the performance of high energy optics by reduction in the laser induced damage threshold. As the polishing defects are trace content and lie beneath the surface, they are difficult to detect. We herein present a biological method to measure impurities on polished fused silica, based on the intense inhibiting ability about trace level of ceria on enzyme activity. And the enzyme activity is measured in the individual etching solutions of a sequential etching process. Results show that detectability of the biological method satisfies the needs of trace impurity detection with low cost and simple apparatus. Furthermore ceria can be used to tag SSD in lapped and polished optics.

Gram-scale production of a basidiomycetous laccase in Aspergillus niger

We report on the expression in Aspergillus niger of a laccase gene we used to produce variants in Saccharomyces cerevisiae. Grams of recombinant enzyme can be easily obtained. This highlights the potential of combining this generic laccase sequence to the yeast and fungal expression systems for large-scale productions of variants.