Two-Electron Reduction versus One-Electron Oxidation of the Type 3 Pair in the Multicopper Oxidases

A bioinspired model for copper monooxygenase: direct aromatic hydroxylation using O2

A novel copper(I) complex, [CuI(L)(CH3CN)]CF3SO3 (1) (L = 1,1,2-tri(pyridin-2-yl)propan-1-ol), has been synthesized, characterized, and investigated as a bioinspired model for copper monooxygenases. Under aerobic conditions in CH3CN, complex 1 undergoes conversion to a dicopper complex, [(CuIIL)(CuIIL H)(SO3CF3)2]·CF3SO3·H2O (2), whose molecular structure reveals a Cu-Cu distance of 2.96 Å. A dicopper(II) complex, [(LCuII)2(SO3CF3)2] (3), has been synthesized for comparison, which exhibits a similar Cu-Cu distance of 2.97 Å. EPR spectroscopy has ascertained the solution-state geometries of complexes 2 and 3, which displayed g∥ > g⊥ values, indicative of distorted square pyramidal geometries consistent with their solid-state structures. Complex 1 selectively hydroxylates benzene in the presence of O2 and Et3N, affording 7% phenol based on the substrate, without any side products. However, the use of H2O2 as the oxygen source under identical conditions significantly increases the phenol yield to 19%. The catalytically active intermediates generated by the reaction of complex 1 with dioxygen showed an O (π*σ) → Cu ligand-to-metal charge transfer (LMCT) transition at 360 nm and a d-d transition at 650 nm. These spectral features are more pronounced with H2O2, showing a new LMCT transition at 360 nm and a very weak d-d transition at 689 nm. This is supported by solution FT-IR spectroscopy, which showed an O-O stretching frequency at 890 cm-1 (DFT spectra at 829 cm-1), corresponding to a Cu-OOH intermediate. The structure of the [(L)CuII-OOH]+ species was optimized by DFT calculations. Kinetic isotope effect (KIE) studies using C6H6/C6D6 (1 : 1) (kH/kD = 1.03) and isotopic labeling experiments using H218O2 support our proposed mechanism of benzene hydroxylation. In contrast, dinuclear complexes 2 and 3 exhibited poor benzene hydroxylation activity even with H2O2 and yielded only 4% and 6% phenol, respectively, along with by-products such as biphenyl and quinone under identical conditions.

Photoinduced Electron Transfer in Ruthenium-Polypyridyl-Modified Laccases: Probing Pathways From the Surface to the Metal Centers

This study explores the grafting of a ruthenium-polypyridyl photosensitizer onto the surface of a multi-copper oxidase to investigate photoinduced electron transfer (PIET) processes within the enzyme. Four Ru-laccase covalent hybrids unique in the position of the ruthenium complex relative to the Type 1 copper (T1) and trinuclear copper (TNC) centers of the enzyme are compared. The strategic positioning of the photosensitizer highlights the roles of distance and intervening medium on PIET pathways. Our results reveal photoreduction scenarios where the TNC center is reduced directly without first passing through the T1 center. We further illustrate the efficiency and robustness of these hybrid systems through continuous irradiation experiments and laser flash photolysis, shedding light on mechanistic pathways and potential applications in sustainable photocatalysis.

Bioinspired copper(II) complexes catalyzed oxidative coupling of aminophenols with broader substrate scope

The strategic selection of ligand systems in metal complexes has demonstrated a profound impact on the efficiency and specificity of biomimetic reactions. In this work, we introduce a series of aminoquinoline-based copper(II) complexes (1-4) distinguished by systematic variation in terminal amine substituents: di-n-methyl (L1(H)), di-n-ethyl (L2(H)), di-n-propyl (L3(H)), and di-n-butyl (L4(H)). These complexes are synthesized, characterized, and evaluated as the catalyst for the oxidative coupling of different aminophenol derivatives. Remarkably, complex 1, featuring a methyl substituent, exhibited unparalleled catalytic performance, achieving an 86 % (Kcat - 9.7 × 104 h-1) conversion of o-aminophenol to the desired product, 2-amino-phenoxazin-3-one, alongside water and hydrogen peroxide as byproducts. Notably, complex 1 demonstrated exceptional versatility, extending its catalytic activity to other substrates with remarkable activity. Mechanistic investigations, supported by mass-spectrometric analysis, revealed the formation of a complex-substrate adduct with all substrates, enabling us to propose a detailed reaction pathway. The work highlights the benefits of ligand design in improving catalytic performance and sets a new standard for aminoquinoline-based copper(II) complexes in oxidative coupling reactions. To the best of our knowledge, this work is the first to report a wider substrate scope for PHS activity with copper(II) complexes.

Deciphering the effect of amine versus imine ligands of copper(II) complexes in 2-aminophenol oxidation

A series of amine (1-6) and imine (5',6') based copper(II) complexes with tridentate (NNO) ligand donors were synthesized and characterized using modern analytical techniques. All the complexes were subjected to 2-aminophenol (OAP) oxidation to form 2-aminophenoxazin-3-one, as a functional analogue of an enzyme, phenoxazinone synthase. In addition, a critical comparison of the reactivity using the amine-based complexes with their respective imine counterparts was achieved in both experimental as well as theoretical studies. For instance, the kinetic measurement revealed that the imine-based copper(II) complexes (kcat, 2.4 × 105-6.2 × 106 h-1) are better than amine-based (kcat, 6.3 × 104-3.9 × 105 h-1) complexes. The complex-substrate adducts [Cu(L3)(OAP)] (7) and [Cu(L3')(OAP)] (7') were characterized for both systems by mass spectrometry. Further, the DFT study was performed with amine- (3) and imine- (3') based copper(II) complexes, to compare their efficacy in the oxidation of OAP. The mechanistic investigations reveal that the key elementary step to determine the reactivity of 3 and 3' is the proton-coupled electron transfer (PCET) step occurring from the intermediates 7/7'. Further, the computed HOMO-LUMO energy gap of 7' was smaller than 7 by 0.8 eV, which indicates the facile PCET compared to that of 7. Moreover, the coupling of the OAP moiety using imine-complexes (ΔGR.E = -5.8 kcal/mol) was found to be thermodynamically more favorable than amine complexes (ΔGR.E = +3.3 kcal/mol). Overall, the theoretical findings are in good agreement with the experimental results.

General Synergistic Hybrid Catalyst Synthesis Method Using a Natural Enzyme Scaffold-Confined Metal Nanocluster

Due to differences in the chemical properties or optimal reaction conditions of the catalysts, the challenge in the design of bio-chemical hybrid catalysts is that the bio-catalysts or chemical catalysts usually cannot maintain the initial catalytic performance. Herein, we report a general bio-chemical hybrid catalyst synthesis method using a natural enzyme scaffold-confined metal nanocluster. A redox-active enzyme is a nanoreactor that allows access to and reduces metal ions into metal nanoclusters in situ, resulting in the enzyme-confined metal nanocluster hybrid catalyst with a synergistic effect to boost catalytic performance. Specifically, bilirubin oxidase-Ir nanoclusters (BOD-Ir NCs) with catalytic properties for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are designed. The BOD-Ir NCs exhibit an approximately 2-fold ORR activity compared with pure BOD and a 4-fold OER activity compared with pure Ir NCs. BOD-Ir NCs exhibit stability for over 50,000 s, exceeding that of pure Ir NCs (22,000 s). The synergistic catalytic performance is attributed to the following: the mild preparation condition and matched sizes of BOD and the Ir NCs maintain the natural activity of BOD; the highly conductive Ir NCs improve the ORR activity of BOD; and the confining effect of BOD, which improves the stability and activity of the Ir NCs during the OER. In particular, BOD-Ir NCs exhibit a high half-wave potential of 0.97 V for the ORR and a low overpotential of 319 mV at 10 mA cm^-2 for the OER, surpassing most of reported catalysts under neutral conditions. Furthermore, laccase-Ir NCs and glucose oxidase-Pd NCs with synergistic catalytic performances are fabricated, proving the universality of this synthetic method. This facile strategy for designing synergistic hybrid catalysts is expected to be applied to more complex chemical transformations.

Challenges in Elucidating the Free Energy Scheme of the Laccase Catalyzed Reduction of Oxygen

Artificial redox catalysts are typically limited by unfavorable scaling relations of reaction intermediates leading to a significant overpotential in multi-electron redox reactions such as for example the oxygen reduction reaction (ORR). The multicopper oxidase laccase is able to catalyze the ORR in nature. In particular the high-potential variants show a remarkably low overpotential for the ORR and apparently do not suffer from such unfavorable scaling relations. Although laccases are intensively studied, it is presently unknown why the overpotential for ORR is so low and a clear description regarding the thermodynamics of the catalytic cycle and the underlying design principles is lacking. In order to understand the laccase catalyzed ORR from an electrochemical perspective, elucidation of the free energy scheme would be of high value. This article reviews the energetics of the proposed laccase catalyzed ORR mechanisms based on experimental and computational studies. However, there are still remaining challenges to overcome to elucidate the free energy scheme of laccase. Obtaining thermodynamic data on intermediates is hard or even impossible with analytical techniques. On the other hand, several computational studies have been performed with significantly different parameters and conditions, thus making a direct comparison difficult. For these reasons, a consensus on a clear free energy scheme is still lacking. We anticipate that ultimately conquering these challenges will result in a better understanding of laccase catalyzed ORR and will allow for the design of low overpotential redox catalysts.

Mononuclear copper(ii) Schiff base complexes as effective models for phenoxazinone synthase

Copper(ii) complexes of tridentate (N2O) Schiff base ligands as efficient catalysts for 2-aminophenol oxidation to 2-aminophenoxazin-3-one with excellent reaction rates.

Multidimensional redox potential/pKa coupling in multicopper oxidases from molecular dynamics: implications for the proton transfer mechanism

Bilirubin oxidases (BOD) are metalloenzymes that catalyze the conversion of O₂ and bilirubin to biliverdin and water in the metabolism of chlorophyll and porphyrin. In this work we have used the CpHMD method to analyze the effects of the different oxidation states on the BOD trinuclear cluster (TNC). Our results demonstrate that there is a link between the different oxidation states of copper ions and the protonation capacity of nearby titratable residues. Each configuration affects pK_a differently, creating proton gradients within the enzyme that act in an extremely orderly manner. This order is closely linked to the catalytic mechanism and leads us to the conclusion of the entry of the O₂ molecule and its reduction in water molecules is associated with the probability of the release of protons from nearby acid groups. With this information, we deduce that under the initial reaction conditions the acidic side chains of nearby residues can be protonated; this allows the enzyme to reduce the activation energy of the reaction by coupling the proton transfer to oxidation state changes in the metallic center.

Electron Transfer to the Trinuclear Copper Cluster in Electrocatalysis by the Multicopper Oxidases

High-potential multicopper oxidases (MCOs) are excellent catalysts able to perform the oxygen reduction reaction (ORR) at remarkably low overpotentials. Moreover, MCOs are able to interact directly with the electrode surfaces via direct electron transfer (DET), that makes them the most commonly used electrocatalysts for oxygen reduction in biofuel cells. The central question in MCO electrocatalysis is whether the type 1 (T1) Cu is the primary electron acceptor site from the electrode, or whether electrons can be transferred directly to the trinuclear copper cluster (TNC), bypassing the rate-limiting intramolecular electron transfer step from the T1 site. Here, using site-directed mutagenesis and electrochemical methods combined with data modeling of electrode kinetics, we have found that there is no preferential superexchange pathway for DET to the T1 site. However, due to the high reorganization energy of the fully oxidized TNC, electron transfer from the electrode to the TNC does occur primarily through the T1 site. We have further demonstrated that the lower reorganization energy of the TNC in its two-electron reduced, alternative resting, form enables DET to the TNC, but this only occurs in the first turnover. This study provides insight into the factors that control the kinetics of electrocatalysis by the MCOs and a guide for the design of more efficient biocathodes for the ORR.

Encapsulation of tricopper cluster in a synthetic cryptand enables facile redox processes from CuICuICuI to CuIICuIICuII states

One-pot reaction of tris(2-aminoethyl)amine (TREN), [Cu^I(MeCN)₄]PF₆, and paraformaldehyde affords a mixed-valent [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ complex. The macrocyclic azacryptand TREN4 contains four TREN motifs, three of which provide a bowl-shape binding pocket for the [Cu₃(μ₃-OH)]³⁺ core. The fourth TREN caps on top of the tricopper cluster to form a cryptand, imposing conformational constraints and preventing solvent interaction. Contrasting the limited redox capability of synthetic tricopper complexes reported so far, [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ exhibits several reversible single-electron redox events. The distinct electrochemical behaviors of [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ and its solvent-exposed analog [TREN3Cu^IICu^IICu^II(μ₃-O)](PF₆)₄ suggest that isolation of tricopper core in a cryptand enables facile electron transfer, allowing potential application of synthetic tricopper complexes as redox catalysts. Indeed, the fully reduced [TREN4Cu^ICu^ICu^I(μ₃-OH)](PF₆)₂ can reduce O₂ under acidic conditions. The geometric constraints provided by the cryptand are reminiscent of Nature's multicopper oxidases (MCOs). For the first time, a synthetic tricopper cluster was isolated and fully characterized at Cu^ICu^ICu^I (4a), Cu^IICu^ICu^I (4b), and Cu^IICu^IICu^I (4c) states, providing structural and spectroscopic models for many intermediates in MCOs. Fast electron transfer rates (10⁵ to 10⁶ M^-1 s^-1) were observed for both Cu^ICu^ICu^I/Cu^IICu^ICu^I and Cu^IICu^ICu^I/Cu^IICu^IICu^I redox couples, approaching the rapid electron transfer rates of copper sites in MCO.

Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase

The copper(II) complexes [Cu(L)NO₃] (1-9) of newer N₃O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04-0.11). The Cu-O_Phenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1-9 appeared around -0.102 to -0.428 V versus Ag/Ag⁺ in water. The electronic spectra of the complexes showed the d-d transitions around 643-735 nm and axial EPR parameter (g_||, 2.243-2.270; A_||, 164-179 × 10^-4 cm^-1) that corresponds to square pyramidal geometry. The bonding parameters α², 0.760-0.825; β², 0.761-0.994; γ², 0.504-0.856 and K_||, 0.698-0.954 and K_⊥, 0.383-0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41-61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98-15.65 × 10^-3 s^-1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (k_cat) 9.11 × 10⁵ h^-1 for 1 and 4.66 × 10⁵ h^-1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.

Using NMR spectroscopy to investigate the role played by copper in prion diseases

Prion diseases are a group of rare neurodegenerative disorders that develop as a result of the conformational conversion of normal prion protein (PrPC) to the disease-associated isoform (PrPSc). The mechanism that actually causes disease remains unclear. However, the mechanism underlying the conformational transformation of prion protein is partially understood-in particular, there is strong evidence that copper ions play a significant functional role in prion proteins and in their conformational conversion. Various models of the interaction of copper ions with prion proteins have been proposed for the Cu (II)-binding, cell-surface glycoprotein known as prion protein (PrP). Changes in the concentration of copper ions in the brain have been associated with prion diseases and there is strong evidence that copper plays a significant functional role in the conformational conversion of PrP. Nevertheless, because copper ions have been shown to have both a positive and negative effect on prion disease onset, the role played by Cu (II) ions in these diseases remains a topic of debate. Because of the unique properties of paramagnetic Cu (II) ions in the magnetic field, their interactions with PrP can be tracked even at single atom resolution using nuclear magnetic resonance (NMR) spectroscopy. Various NMR approaches have been utilized to study the kinetic, thermodynamic, and structural properties of Cu (II)-PrP interactions. Here, we highlight the different models of copper interactions with PrP with particular focus on studies that use NMR spectroscopy to investigate the role played by copper ions in prion diseases.

Rapid Decay of the Native Intermediate in the Metallooxidase Fet3p Enables Controlled FeII Oxidation for Efficient Metabolism

The multicopper oxidases (MCOs) couple four 1e^- oxidations of substrate to the 4e^- reduction of O₂ to H₂O. These divide into two groups: those that oxidize organic substrates with high turnover frequencies (TOFs) up to 560 s^-1 and those that oxidize metal ions with low TOFs, ∼1 s^-1 or less. The catalytic mechanism of the organic oxidases has been elucidated, and the high TOF is achieved through rapid intramolecular electron transfer (IET) to the native intermediate (NI), which only slowly decays to the resting form. Here, we uncover the factors that govern the low TOF in Fet3p, a prototypical metallooxidase, in the context of the MCO mechanism. We determine that the NI decays rapidly under optimal turnover conditions, and the mechanism thereby becomes rate-limited by slow IET to the resting enzyme. Development of a catalytic model leads to the important conclusions that proton delivery to the NI controls the mechanism and enables the slow turnover in Fet3p that is functionally significant in Fe metabolism enabling efficient ferroxidase activity while avoiding ROS generation.

Inhibition in multicopper oxidases: a critical review

This review critiques the literature on inhibition of O₂-reduction catalysis in multicopper oxidases like laccase and bilirubin oxidase and provide recommendations for best practice when carrying out experiments and interpreting published data.

Triazole appending ruthenium(ii) polypyridine complex for selective sensing of phosphate anions through C–H–anion interaction and copper(ii) ions via cancer cells

Selective fluorescence enhancement by H₂PO₄⁻/H₂P₂O₇²⁻ anions and maximum fluorescence quenching by Cu²⁺ ions were attained upon treatment with different types of anions and cations, respectively.

Comparison of Direct and Mediated Electron Transfer for Bilirubin Oxidase from Myrothecium Verrucaria. Effects of Inhibitors and Temperature on the Oxygen Reduction Reaction

One of the processes most studied in bioenergetic systems in recent years is the oxygen reduction reaction (ORR). An important challenge in bioelectrochemistry is to achieve this reaction under physiological conditions. In this study, we used bilirubin oxidase (BOD) from Myrothecium verrucaria, a subclass of multicopper oxidases (MCOs), to catalyse the ORR to water via four electrons in physiological conditions. The active site of BOD, the T2/T3 cluster, contains three Cu atoms classified as T2, T3α, and T3β depending on their spectroscopic characteristics. A fourth Cu atom; the T1 cluster acts as a relay of electrons to the T2/T3 cluster. Graphite electrodes were modified with BOD and the direct electron transfer (DET) to the enzyme, and the mediated electron transfer (MET) using an osmium polymer (OsP) as a redox mediator, were compared. As a result, an alternative resting (AR) form was observed in the catalytic cycle of BOD. In the absence and presence of the redox mediator, the AR direct reduction occurs through the trinuclear site (TNC) via T1, specifically activated at low potentials in which T2 and T3α of the TNC are reduced and T3β is oxidized. A comparative study between the DET and MET was conducted at various pH and temperatures, considering the influence of inhibitors like H2O2, F−, and Cl−. In the presence of H2O2 and F−, these bind to the TNC in a non-competitive reversible inhibition of O2. Instead; Cl− acts as a competitive inhibitor for the electron donor substrate and binds to the T1 site.

Laccases: structure, function, and potential application in water bioremediation

The global rise in urbanization and industrial activity has led to the production and incorporation of foreign contaminant molecules into ecosystems, distorting them and impacting human and animal health. Physical, chemical, and biological strategies have been adopted to eliminate these contaminants from water bodies under anthropogenic stress. Biotechnological processes involving microorganisms and enzymes have been used for this purpose; specifically, laccases, which are broad spectrum biocatalysts, have been used to degrade several compounds, such as those that can be found in the effluents from industries and hospitals. Laccases have shown high potential in the biotransformation of diverse pollutants using crude enzyme extracts or free enzymes. However, their application in bioremediation and water treatment at a large scale is limited by the complex composition and high salt concentration and pH values of contaminated media that affect protein stability, recovery and recycling. These issues are also associated with operational problems and the necessity of large-scale production of laccase. Hence, more knowledge on the molecular characteristics of water bodies is required to identify and develop new laccases that can be used under complex conditions and to develop novel strategies and processes to achieve their efficient application in treating contaminated water. Recently, stability, efficiency, separation and reuse issues have been overcome by the immobilization of enzymes and development of novel biocatalytic materials. This review provides recent information on laccases from different sources, their structures and biochemical properties, mechanisms of action, and application in the bioremediation and biotransformation of contaminant molecules in water. Moreover, we discuss a series of improvements that have been attempted for better organic solvent tolerance, thermo-tolerance, and operational stability of laccases, as per process requirements.

O2 Reduction to Water by High Potential Multicopper Oxidases: Contributions of the T1 Copper Site Potential and the Local Environment of the Trinuclear Copper Cluster

High potential multicopper oxidases (MCOs) have T1 reduction potentials >600 mV (vs normal hydrogen electrode), making them important catalysts for O₂ reduction in various biotechnological applications. The oxygen reduction mechanism for the low potential MCOs is well-characterized; however, O₂ reactivity of high potential MCOs is not well understood. In this study, we have shown that laccase from Trametes versicolor, where the T1 redox potential is increased by ∼350 mV over that of the low potential MCOs corresponding to an 8 kcal/mol decrease in the driving force, exhibits a slower intramolecular electron transfer (IET) rate to the trinuclear Cu cluster (TNC) in the native intermediate (NI), relative to the low potential MCO from Rhus vernicifera laccase. This IET rate is, however, >10² times faster than the decay rate of the NI, demonstrating that this intermediate form of the enzyme is catalytically relevant enabling fast turnover. However, in contrast to the low potential MCOs where T1 reduction by substrate is rate limiting, the rate limiting step in turnover of high potential MCOs is the first IET to NI. Part of the reduction potential difference of the T1 sites in high vs low potential MCOs is balanced by an ∼100 mV higher reduction potential of NI due to the more positive protein environment in the vicinity of the TNC.

Coordination geometry-induced optical imaging of l-cysteine in cancer cells using imidazopyridine-based copper(ii) complexes

Overexpression of cysteine cathepsins proteases has been documented in a wide variety of cancers, and enhances the l-cysteine concentration in tumor cells. We report the synthesis and characterization of copper(ii) complexes [Cu(L1)₂(H₂O)](SO₃CF₃)₂, 1, L1 = 3-phenyl-1-(pyridin-2-yl)imidazo[1,5-a]pyridine, [Cu(L2)₂(SO₃CF₃)]SO₃CF₃, 2, L2 = 3-(4-methoxyphenyl)-1-pyridin-2-yl-imidazo[1,5-a]pyridine, [Cu(L3)₂(H₂O)](SO₃CF₃)₂, 3, L3 = 3-(3,4-dimethoxy-phenyl)-1-pyridin-2-yl-imidazo[1,5-a]pyridine and [Cu(L4)₂(H₂O)](SO₃CF₃)₂, 4, L4 = dimethyl-[4-(1-pyridin-2-yl-imidazo[1,5-a]pyridin-3-yl)phenyl]amine as 'turn-on' optical imaging probes for l-cysteine in cancer cells. The molecular structure of complexes adopted distorted trigonal pyramidal geometry (τ, 0.68-0.87). Cu-N_py bonds (1.964-1.989 Å) were shorter than Cu-N_imi bonds (2.024-2.074 Å) for all complexes. Geometrical distortion was strongly revealed in EPR spectra, showing g_‖ (2.26-2.28) and A_‖ values (139-163 × 10^-4 cm^-1) at 70 K. The d-d transitions appeared around 680-741 and 882-932 nm in HEPES, which supported the existence of five-coordinate geometry in solution. The Cu(ii)/Cu(i) redox potential of 1 (0.221 V vs. NHE) was almost identical to that of 2 and 3 but lower than that of 4 (0.525 V vs. NHE) in HEPES buffer. The complexes were almost non-emissive in nature, but became emissive by the interaction of l-cysteine in 100% HEPES at pH 7.34 via reduction of Cu(ii) to Cu(i). Among the probes, probe 2 showed selective and efficient turn-on fluorescence behavior towards l-cysteine over natural amino acids with a limit of detection of 9.9 × 10^-8 M and binding constant of 2.3 × 10⁵ M^-1. The selectivity of 2 may have originated from a nearly perfect trigonal plane adopted around a copper(ii) center (∼120.70°), which required minimum structural change during the reduction of Cu(ii) to Cu(i) while imaging Cys. The other complexes, with their distorted trigonal planes, required more reorganizational energy, which resulted in poor selectivity. Probe 2 was employed for optical imaging of l-cysteine in HeLa cells and macrophages. It exhibited brighter fluorescent images by visualizing Cys at pH 7.34 and 37 °C. It showed relatively less toxicity for these cell lines as ascertained by the MTT assay.

Mechanism of chloride inhibition of bilirubin oxidases and its dependence on potential and pH

Bilirubin oxidases (BODs) belong to the multi-copper oxidase (MCO) family and efficiently reduce O2 at neutral pH and in physiological conditions where chloride concentrations are over 100 mM. BODs were consequently considered to be Cl- resistant contrary to laccases. However, there has not been a detailed study on the related effect of chloride and pH on the redox state of immobilized BODs. Here, we investigate by electrochemistry the catalytic mechanism of O2 reduction by the thermostable Bacillus pumilus BOD immobilized on carbon nanofibers in the presence of NaCl. The addition of chloride results in the formation of a redox state of the enzyme, previously observed for different BODs and laccases, which is only active after a reductive step. This behavior has not been previously investigated. We show for the first time that the kinetics of formation of this state is strongly dependent on pH, temperature, Cl- concentration and on the applied redox potential. UV-visible spectroscopy allows us to correlate the inhibition process by chloride with the formation of the alternative resting form of the enzyme. We demonstrate that O2 is not required for its formation and show that the application of an oxidative potential is sufficient. In addition, our results suggest that the reactivation may proceed thought the T3 β.

The Rise of Radicals in Bioinorganic Chemistry

Prior to 1950, the consensus was that biological transformations occurred in two-electron steps, thereby avoiding the generation of free radicals. Dramatic advances in spectroscopy, biochemistry, and molecular biology have led to the realization that protein-based radicals participate in a vast array of vital biological mechanisms. Redox processes involving high-potential intermediates formed in reactions with O₂ are particularly susceptible to radical formation. Clusters of tyrosine (Tyr) and tryptophan (Trp) residues have been found in many O₂-reactive enzymes, raising the possibility that they play an antioxidant protective role. In blue copper proteins with plastocyanin-like domains, Tyr/Trp clusters are uncommon in the low-potential single-domain electron-transfer proteins and in the two-domain copper nitrite reductases. The two-domain muticopper oxidases, however, exhibit clusters of Tyr and Trp residues near the trinuclear copper active site where O₂ is reduced. These clusters may play a protective role to ensure that reactive oxygen species are not liberated during O₂ reduction.

Aryl Group Transfer from Tetraarylborato Anions to an Electrophilic Dicopper(I) Center and Mixed-Valence μ-Aryl Dicopper(I,II) Complexes

The synthesis of discrete, cationic binuclear μ-aryl dicopper complexes [Cu2(μ-η(1):η(1)-Ar)DPFN]X (Ar = C6H5, 3,5-(CF3)2C6H3, and C6F5; DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine; X = BAr4(-) and NTf2(-); Tf = SO2CF3) was achieved by treatment of a dicopper complex [Cu2(μ-η(1):η(1)-NCCH3)DPFN]X2 (X = PF6(-) and NTf2(-)) with tetraarylborates. Structural characterization revealed symmetrically bridging aryl groups, and (1)H NMR spectroscopy evidenced the same structure in solution at 24 °C. Electrochemical investigation of the resulting arylcopper complexes uncovered reversible redox events that led to the synthesis and isolation of a rare mixed-valence organocopper complex [Cu2(μ-η(1):η(1)-Ph)DPFN](NTf2)2 in high yield. The solid-state structure of the mixed-valence μ-phenyl complex exhibits inequivalent copper centers, despite a short Cu···Cu distance. Electronic and variable-temperature electron paramagnetic resonance spectroscopy of the mixed-valence μ-phenyl complex suggest that the degree of spin localization is temperature-dependent, with a high degree of spin localization observed at lower temperatures. Electronic structure calculations agree with the experimental results and suggest that the spin is localized almost entirely on one metal center.