Synergistic Substrate and Oxygen Activation in Salicylate Dioxygenase Revealed by QM/MM Simulations

Salicylate 1,2‐dioxygenase (SDO) is the first enzyme to be discovered to catalyze the oxidative cleavage of a monohydroxylated aromatic compound, namely salicylate, instead of the well‐known electron‐rich substrates. We have investigated the mechanism of dioxygen activation in SDO by QM/MM calculations. Our study reveals that the non‐heme Fe^II center in SDO activates salicylate and O₂ synergistically through a strong covalent interaction to facilitate the reductive cleavage of O₂. A covalent salicylate–Fe^II–O₂ complex is the reactive oxygen species in this case, and its electronic structure is best described as being between the two limiting cases, Fe^II?O₂ and Fe^II?O₂^.?, with partial electron transfer from the activated salicylate to O₂ via the Fe center. Thus SDO employs a synergistic strategy of substrate and oxygen activation to carry out the catalytic reaction, which is unprecedented in the family of iron dioxygenases. Moreover, O₂ activation in SDO happens without the assistance of a proton source. Our study essentially shows a new mechanistic possibility for O₂ activation.     

Intermediate Detection in the Casiopeina–Cysteine Interaction Ending in the Disulfide Bond Formation and Copper Reduction

A strategy to improve the cancer therapies involves agents that cause the depletion of the endogenous antioxidant glutathione (GSH), increasing its efflux out of cells and inducing apoptosis in tumoral cells due to the presence of reactive oxygen species. It has been shown that Casiopeina copper complexes caused a dramatic intracellular GSH drop, forming disulfide bonds and reducing Cu^II to Cu^I. Herein, through the determination of the [Cu^II]–SH bond before reduction, we present evidence of the adduct between cysteine and one Casiopeina as an intermediate in the cystine formation and as a model to understand the anticancer activity of copper complexes. Evidence of such an intermediate has never been presented before.     

Quercetin 2,4‐Dioxygenase Activates Dioxygen in a Side‐On O2–Ni Complex

Quercetin 2,4‐dioxygenase (quercetinase) from Streptomyces uses nickel as the active‐site cofactor to catalyze oxidative cleavage of the flavonol quercetin. How this unusual active‐site metal supports catalysis and O₂ activation is under debate. We present crystal structures of Ni‐quercetinase in three different states, thus providing direct insight into how quercetin and O₂ are activated at the Ni²⁺ ion. The Ni²⁺ ion is coordinated by three histidine residues and a glutamate residue (E⁷⁶) in all three states. Upon binding, quercetin replaces one water ligand at Ni and is stabilized by a short hydrogen bond through E⁷⁶, the carboxylate group of which rotates by 90°. This conformational change weakens the interaction between Ni and the remaining water ligand, thereby preparing a coordination site at Ni to bind O₂. O₂ binds side‐on to the Ni²⁺ ion and is perpendicular to the C2?C3 and C3?C4 bonds of quercetin, which are cleaved in the following reaction steps.     

Crystallographic Evidence for a Sterically Induced Ferryl Tilt in a Non‐Heme Oxoiron(IV) Complex that Makes it a Better Oxidant

Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non‐heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N‐methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [Fe^IV(O)(L)]²⁺ complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe–N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k₂′ values for O‐ and H‐atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N‐Fe=O arrangement by 10°.     

A Highly Sensitive and Selective Assay for Cysteine Using the Chemiluminescence Reaction of Luminol and Hydrogen Peroxide

A very simple, highly sensitive and selective chemiluminescence (CL) method was established for the determination of cysteine. This method is based on the fact that the CL reaction of luminol and hydrogen peroxide can be greatly enhanced by cysteine. The CL intensities at maximum light emission were linearly correlated with the concentration of cysteine over the range of 2.0×10^–8–6.0×10^–6molL^–1 with a detection limit of 7.5×10^–9molL^–1. The relative standard deviation was 1.7% for the determination of 1.0×10^–7molL^–1 cysteine (n=9). The feasibility of utilizing the proposed method for the determination of total concentration of cysteine in human serum was examined.     

A Myoglobin Functional Model Composed of a Ferrous Porphyrin and a Cyclodextrin Dimer with an Imidazole Linker

A 1:1 inclusion complex (Fe^IIPImCD) of 5,10,15,20‐tetrakis‐ (4‐sulfonatophenyl)porphinatoiron(II) (Fe^IIP) and an O‐methylated β‐cyclodextrin dimer with an imidazole linker (ImCD) was found to bind dioxygen in aqueous solution. The half‐saturation pressure of dioxygen (P_1/2^O2) is 1.7 torr at 25 °C, which is 10 times lower than that for a previous myoglobin functional model (hemoCD) with a pyridine linker. Meanwhile, the half‐life of oxygenated Fe^IIPImCD is 3 h, which is 10 times shorter than that of oxygenated hemoCD. The covering of the iron(II) center by a microscopic environment is essential for preventing autoxidation of oxygenated ferrous porphyrin, which is promoted by nucleophilic attack of H₂O and/or nucleophiles such as inorganic anions. Due to structural requirements, covering of the Fe^II center of Fe^IIPImCD is insufficient compared with the case of hemoCD. As a result, water molecules can penetrate more easily the cleft of the O₂–Fe^IIPImCD complex and act as an autoxidation inducer. This structure also causes poorer selectivity against carbon monoxide (M=1040). In contrast, the dioxygen affinity of Fe^IIPImCD is much higher than that of hemoCD because the imidazole moiety is a stronger electron donor than pyridine.     

Hydrating the Bispropionate Notch in Malaria Pigment: A New Structural Motif in the Iron(III)(deuteroporphyrin) Dimer

Treating deuterohemin, chloro(deuteroporphyrinato)iron(III), with a non-coordinating base in DMSO/methanol allows for the isolation of [(deuteroporphyrinato)iron(III)]₂, deuterohematin anhydride (DHA), an analogue of malaria pigment, the natural product of heme detoxification by malaria. The structure of DHA obtained from this solvent system has been solved by X-ray powder diffraction analysis and displays many similarities, yet important structural differences, to malaria pigment. Most notably, a water molecule of solvation occupies a notch created by the propionate side chains and stabilizes a markedly bent propionate ligand coordinated with a long Fe−O bond, and a carboxylate cluster associated with water molecules is generated. Together, these features account for its increased solubility and more open structure, with an increased porphyrin–porphyrin separation. The IR spectroscopic signature associated with this structure also accounts for the strong IR band at 1587 cm⁻¹ seen for many amorphous preparations of synthetic malaria pigment, and it is proposed that stabilizing these structures may be a new objective for antimalarial drugs. The important role of the vinyl substituents in this biochemistry is further demonstrated by the structure of deuterohemin obtained by single-crystal X-ray diffraction analysis.     

A Direct,Modular, and Efficient Construction of the PCP Structural Motif through Coupling of Manganese Carbyne Complexes with Phosphines

Easily available carbyne complexes of manganese were used as a source of carbyne fragments in an unconventional synthesis of backbone‐substituted diphosphinomethanes and cyclic P‐ylides upon coupling with secondary or tertiary phosphines, respectively, followed by demetalation under mild conditions.     

一种钌(Ⅱ)配合物荧光探针用于半胱氨酸和高半胱氨酸的检测

为了检测半胱氨酸和高半胱氨酸,本文合成了一种基于钌(Ⅱ)配合物的荧光探针。 结果表明,该探针可实现对半胱氨酸和高半胱氨酸的较好的灵敏性和选择性检测。 在优化的实验条件下,5~35 μmol/L浓度区间,探针的荧光强度与半胱氨酸和高半胱氨酸浓度呈良好的线性关系。 其检测限分别为0.60和0.78 μmol/L。 该研究为基于钌(Ⅱ)配合物的荧光探针定量检测生物活性分子提供了一种有用的方法。     

Opening up the Valence Shell: A T‐Shaped Iron(I) Metalloradical and Its Potential for Atom Abstraction

A thermally stable, T‐shaped, d⁷ high‐spin iron(I) complex was obtained by reduction of a PNP‐supported ferrous chloride. Paramagnetic NMR spectroscopy combined with DFT modeling was used to analyze the electronic structure of the coordinatively highly unsaturated complex. The metalloradical character of the compound was demonstrated by the formation of a benzophenone ketyl radical complex upon addition of benzophenone. Furthermore, the compound displays a rich chemistry as an oxygen‐atom abstractor from epoxides, yielding a dinuclear, diferrous [Fe₂O] complex.     

A Novel Tripodal Ligand Containing Three Different N‐Heterocyclic Donor Functions and Its Application in Catechol Dioxygenase Mimicking

Prominent donors : A pyridyl, an imidazolyl, and a pyrazolyl donor function are part of the novel tripodal ligand depicted, which thus combines three of the most prominent donors applied in ligands for bioinorganic chemistry within one coordination unit. To exploit its behaviour and potential, first investigations have been carried out in relation to catechol dioxygenase mimicry.

     

A Rotaxane‐like Cage‐in‐Ring Structural Motif for a Metallosupramolecular Pd6L12 Aggregate

A BODIPY‐based bis(3‐pyridyl) ligand undergoes self‐assembly upon coordination to tetravalent palladium(II) cations to form a Pd₆L₁₂ metallosupramolecular assembly with an unprecedented structural motif that resembles a rotaxane‐like cage‐in‐ring arrangement. In this assembly the ligand adopts two different conformations—a C‐shaped one to form a Pd₂L₄ cage which is located in the center of a Pd₄L₈ ring consisting of ligands in a W‐shaped conformation. This assembly is not mechanically interlocked in the sense of catenation but it is stabilized only by attractive π‐stacking between the peripheral BODIPY chromophores and the ligands’ skeleton as well as attractive van der Waals interactions between the long alkoxy chains. As a result, the co‐arrangement of the two components leads to a very efficient space filling. The overall structure can be described as a rotaxane‐like assembly with a metallosupramolecular cage forming the axle in a metallosupramolecular ring. This unique structural motif could be characterized via ESI mass spectrometry, NMR spectroscopy, and X‐ray crystallography.     

A Single Biosensor for Evaluating the Levels of Copper Ion and L‐Cysteine in a Live Rat Brain with Alzheimer's Disease

Copper ion (Cu²⁺) and L ‐cysteine (CySH) are closely correlated with physiological and pathological events of Alzheimer’s Disease (AD), however the detailed mechanism is still unclear, mainly owing to a lack of accurate analytical methods in live brains. Herein, we report a single biosensor for electrochemical ratiometric detection of Cu²⁺ and CySH in live rat brains with AD. N,N‐di‐(2‐picoly)ethylenediamine (DPEA) is first synthesized for specific recognition of Cu²⁺ to form a DPEA–Cu²⁺ complex. This complex shows high selectivity for CySH owing to the release of Cu²⁺ from the complex through CySH binding to Cu²⁺ center. In parallel, 5′‐MB‐GGCGCGATTTTTTTTTTTTT‐SH‐3′ (HS‐DNA‐MB, MB=Methylene Blue) is designed as an inner‐reference for providing a built‐in correction to improve the accuracy. As a result, combined with the amplified effect of Au nanoleaves, our single ratiometric biosensor can be successfully applied in real‐time detection of Cu²⁺ and CySH in the live rat brains with AD. To our knowledge, this is the first report on the accurate concentrations of Cu²⁺ and CySH in live rat brains with AD.     

比率检测高/半胱氨酸荧光染料的合成及活细胞成像研究

经一步简单反应获得了一例长波长尼罗蓝衍生物荧光染料RB-S. 该染料分子RB-S对高/半胱氨酸具有明显的荧光响应,且随着高/半胱氨酸浓度（0.03～0.33 μmol/L）逐渐增加,染料荧光强度在685 nm处减弱,相应地在755 nm处增强,在685与755 nm处的荧光变化具有良好线性关系的比率荧光变化. 通过高效液相色谱-质谱联用技术以及核磁滴定氢谱研究了染料分子检测高/半胱氨酸的机理. 试验结果表明：染料分子中的醛基与高/半胱氨酸发生成环反应生成噻唑烷. 在体外溶液测试中（pH=7.4）,染料分子RB-S对高/半胱氨酸的检出限为0.025 μmol/L,且不存在其他物质干扰,成功用于血清样品中内高/半胱氨酸荧光比率检测. 染料分子RB-S具有良好的活细胞膜通透性,使其能够对活细胞内高/半胱氨酸荧光比率成像.     

Cysteine adsorption on the Au(111) surface and the electron transfer in configuration of a scanning tunneling microscope: A quantum-chemical approach

Adsorption of two forms, molecule and radical, of amino acid L-cysteine (Cys) on the Au₁₂ cluster that simulates the (111) face of single-crystal gold is studied in the framework of the density functional theory. Effects of solvation of adsorbed Cys particles and lateral interaction in a monolayer are analyzed. The simulation predicts a commensurate adsorption energetics of the molecule and radical, with a difference between the “on-top,” “hollow,” and “bridge” positions. An analysis of lateral electrostatic interactions points to the stability of a cluster comprising six Cys particles, which conforms to the size of a fragment observed experimentally. Adsorption calculations are used to build three-dimensional isosurfaces (STM images), where the tungsten needle of the scanning tunneling microscope is simulated by a tungsten atom or by small clusters. The calculated images are sensitive to both the Cys shape and the orientation of adsorbed Cys particles. Calculation results are compared with fresh in situ submolecular-resolution STM data. Simulated images (with commensurate contributions made by sulfur atom and amino group) built for Cys radical adsorbed in the “on-top” position give best conformance to experiment.