Effective intermediate-spin iron in O2-transporting heme proteins

Chloride- and Hydrosulfide-Bound 2Fe Complexes as Models of the Oxygen-Stable State of [FeFe] Hydrogenase

[FeFe] hydrogenases demonstrate remarkable catalytic efficiency in hydrogen evolution and oxidation processes. However, susceptibility of these enzymes to oxygen-induced degradation impedes their practical deployment in hydrogen-production devices and fuel cells. Recent investigations into the oxygen-stable (Hinact) state of the H-cluster revealed its inherent capacity to resist oxygen degradation. Herein, we present findings on Cl- and SH-bound [2Fe-2S] complexes, bearing relevance to the oxygen-stable state within a biological context. A characteristic attribute of these complexes is the terminal Cl-/SH- ligation to the iron center bearing the CO bridge. Structural analysis of the t-Cl demonstrates a striking resemblance to the Hinact state of DdHydAB and CbA5H. The t-Cl/t-SH exhibit reversible oxidation, with both redox species, electronically, being the first biomimetic analogs to the Htrans and Hinact states. These complexes exhibit notable resistance against oxygen-induced decomposition, supporting the potential oxygen-resistant nature of the Htrans and Hinact states. The swift reductive release of the Cl-/SH-group demonstrates its labile and kinetically controlled binding. The findings garnered from these investigations offer valuable insights into properties of the enzymatic O2-stable state, and key factors governing deactivation and reactivation conversion. This work contributes to the advancement of bio-inspired molecular catalysts and the integration of enzymes and artificial catalysts into H2-evolution devices and fuel-cell applications.

Theoretical Investigation of Transient Species Following Photodissociation of Ironpentacarbonyl in Ethanol Solution

Photodissociation of ironpentacarbonyl [1Fe(CO)5] in solution generates transient species in different electronic states, which we studied theoretically. From ab initio molecular dynamics simulations in ethanol solution, the closed-shell parent compound 1Fe(CO)5 is found to interact weakly with the solvent, whereas the irontetracarbonyl [Fe(CO)4] species, formed after photodissociation, has a strongly spin-dependent behavior. It coordinates a solvent molecule tightly in the singlet state [1Fe(CO)4] and weakly in the triplet state [3Fe(CO)4]. From the simulations, we have gained insights into intersystem crossing in solvated irontetracarbonyl based on the distinct structural differences induced by the change in multiplicity. Alternative forms of coordination between 1Fe(CO)4 and functional groups of the ethanol molecule are simulated, and a quantum chemical investigation of the energy landscape for the coordinated irontetracarbonyl gives information about the interconversion of different transient species in solution. Furthermore, insights from the simulations, in which we find evidence of a solvent exchange mechanism, challenge the previously proposed mechanism of chain walking for under-coordinated metal carbonyls in solution.

Iron(I) Complex Bearing an Open-Shell Diazenido Ligand

Low-valent transition-metal diazenido species are important intermediates in transition-metal-mediated dinitrogen reduction reactions. Isolable complexes of the type unanimously feature closed-shell diazenido ligands. Those bearing open-shell diazenido ligands have remained elusive. Herein, we report the synthesis, characterization, and reactivity of a d7 iron(I) complex featuring an open-shell silyldiazenido ligand, [(ICy)Fe(NNSiiPr3)(η2:η2-dvtms)] (1, ICy = 1,3-dicyclohexylimidazole-2-ylidene, dvtms = divinyltetramethyldisiloxane). Complex 1 is prepared in good yield by silylation of the iron(-I)-N2 complex [K(18-crown-6)][(ICy)Fe(N2)(η2:η2-dvtms)] with iPr3SiOTf and has been fully characterized by various spectroscopic methods. Theoretical studies, in combination with characterization data, established an S = 1/2 ground spin-state for 1 that can best be described as a quartet iron(I) center featuring an antiferromagnetically coupled triplet silyldiazenido ligand. The diazenido and alkene ligands in 1 are labile, as indicated by the facile disproportionation reaction of 1 at ambient temperature to transform into the iron(II) bis(diazenido) species [(ICy)(NNSiiPr3)2Fe(dvtms)Fe(NNSiiPr3)2(ICy)] (2) and the iron(0) species [(ICy)Fe(η2:η2-dvtms)] and also the alkene-exchange reaction of 1 with PhCH═CHBC8H14 to form [(ICy)Fe(NNSiiPr3)(η2-trans-PhCH═CHBC8H14)] (3). Complex 1 is light-sensitive. Upon photolysis, it undergoes a SiiPr3 radical-transfer reaction to yield [(ICy)Fe(σ:η2-MeCHSiMe2OSiMe2CH═CHSiiPr3)] (4) and N2. The reactions of 1 with the trityl radical and organic bromides yield iron(II) complexes, which indicates its reducing nature. Moreover, 1 is a weak hydrogen-atom abstractor, as indicated by its inertness toward HSi(SiMe3)3 and cyclohexa-1,4-diene and the low calculated N-H bond dissociation energy (48 kcal/mol) of its corresponding iron(II) iso-hydrazenido species.

Molecular Dynamics and Structural Studies of Zinc Chloroquine Complexes

Chloroquine (CQ) is a first-choice drug against malaria and autoimmune diseases. It has been co-administered with zinc against SARS-CoV-2 and soon dismissed because of safety issues. The structural features of Zn-CQ complexes and the effect of CQ on zinc distribution in cells are poorly known. In this study, state-of-the-art computations combined with experiments were leveraged to solve the structural determinants of zinc-CQ interactions in solution and the solid state. NMR, ESI-MS, and X-ray absorption and diffraction methods were combined with ab initio molecular dynamics calculations to address the kinetic lability of this complex. Within the physiological pH range, CQ binds Zn²⁺ through the quinoline ring nitrogen, forming [Zn(CQH)Cl_x(H₂O)_3-x]^(3+)-x (x = 0, 1, 2, and 3) tetrahedral complexes. The Zn(CQH)Cl₃ species is stable at neutral pH and at high chloride concentrations typical of the extracellular medium, but metal coordination is lost at a moderately low pH as in the lysosomal lumen. The pentacoordinate complex [Zn(CQH)(H₂O)₄]³⁺ may exist in the absence of chloride. This in vitro/in silico approach can be extended to other metal-targeting drugs and bioinorganic systems.

Correlation coefficient-directed label-free characterization of native proteins by surface-enhanced Raman spectroscopy

Investigation of proteins in their native state is the core of proteomics towards better understanding of their structures and functions. Surface-enhanced Raman spectroscopy (SERS) has shown its unique advantages in protein characterization with fingerprint information and high sensitivity, which makes it a promising tool for proteomics. It is still challenging to obtain SERS spectra of proteins in the native state and evaluate the native degree. Here, we constructed 3D physiological hotspots for a label-free dynamic SERS characterization of a native protein with iodide-modified 140 nm Au nanoparticles. We further introduced the correlation coefficient to quantitatively evaluate the variation of the native degree, whose quantitative nature allows us to explicitly investigate the Hofmeister effect on the protein structure. We realized the classification of a protein of SARS-CoV-2 variants in 15 min, which has not been achieved before. This study offers an effective tool for tracking the dynamic structure of proteins and biomedical research.

Determination of the iron(IV) local spin states of the Q intermediate of soluble methane monooxygenase by Kβ X-ray emission spectroscopy

Soluble methane monooxygenase (sMMO) facilitates the conversion of methane to methanol at a non-heme Fe^IV₂ intermediate MMOH_Q, which is formed in the active site of the sMMO hydroxylase component (MMOH) during the catalytic cycle. Other biological systems also employ high-valent Fe^IV sites in catalysis; however, MMOH_Q is unique as Nature’s only identified Fe^IV₂ intermediate. Previous ⁵⁷Fe Mössbauer spectroscopic studies have shown that MMOH_Q employs antiferromagnetic coupling of the two Fe^IV sites to yield a diamagnetic cluster. Unfortunately, this lack of net spin prevents the determination of the local spin state (S_loc) of each of the irons by most spectroscopic techniques. Here, we use Fe Kβ X-ray emission spectroscopy (XES) to characterize the local spin states of the key intermediates of the sMMO catalytic cycle, including MMOH_Q trapped by rapid-freeze-quench techniques. A pure XES spectrum of MMOH_Q is obtained by subtraction of the contributions from other reaction cycle intermediates with the aid of Mössbauer quantification. Comparisons of the MMOH_Q spectrum with those of known S_loc = 1 and S_loc = 2 Fe^IV sites in chemical and biological models reveal that MMOH_Q possesses S_loc = 2 iron sites. This experimental determination of the local spin state will help guide future computational and mechanistic studies of sMMO catalysis.

Iron accumulation typifies renal cell carcinoma tumorigenesis but abates with pathological progression, sarcomatoid dedifferentiation, and metastasis

Iron is a potent catalyst of oxidative stress and cellular proliferation implicated in renal cell carcinoma (RCC) tumorigenesis, yet it also drives ferroptosis that suppresses cancer progression and represents a novel therapeutic target for advanced RCC. The von Hippel Lindau (VHL)/hypoxia-inducible factor-α (HIF-α) axis is a major regulator of cellular iron, and its inactivation underlying most clear cell (cc) RCC tumors introduces both iron dependency and ferroptosis susceptibility. Despite the central role for iron in VHL/HIF-α signaling and ferroptosis, RCC iron levels and their dynamics during RCC initiation/progression are poorly defined. Here, we conducted a large-scale investigation into the incidence and prognostic significance of total tissue iron in ccRCC and non-ccRCC patient primary tumor cancer cells, tumor microenvironment (TME), metastases and non-neoplastic kidneys. Prussian Blue staining was performed to detect non-heme iron accumulation in over 1600 needle-core sections across multiple tissue microarrays. We found that RCC had significantly higher iron staining scores compared with other solid cancers and, on average, >40 times higher than adjacent renal epithelium. RCC cell iron levels correlated positively with TME iron levels and inversely with RCC levels of the main iron uptake protein, transferrin receptor 1 (TfR1/TFRC/CD71). Intriguingly, RCC iron levels, including in the TME, decreased significantly with pathologic (size/stage/grade) progression, sarcomatoid dedifferentiation, and metastasis, particularly among patients with ccRCC, despite increasing TfR1 levels, consistent with an increasingly iron-deficient tumor state. Opposite to tumor iron changes, adjacent renal epithelial iron increased significantly with RCC/ccRCC progression, sarcomatoid dedifferentiation, and metastasis. Lower tumor iron and higher renal epithelial iron each predicted significantly shorter ccRCC patient metastasis-free survival. In conclusion, iron accumulation typifies RCC tumors but declines toward a relative iron-deficient tumor state during progression to metastasis, despite precisely opposite dynamics in adjacent renal epithelium. These findings raise questions regarding the historically presumed selective advantage for high iron during all phases of cancer evolution, suggesting instead distinct tissue-specific roles during RCC carcinogenesis and early tumorigenesis versus later progression. Future study is warranted to determine how the relative iron deficiency of advanced RCC contributes to ferroptosis resistance and/or introduces a heightened susceptibility to iron deprivation that might be therapeutically exploitable.

Isolating Fe-O2 Intermediates in Dioxygen Activation by Iron Porphyrin Complexes

Dioxygen (O₂) is an environmentally benign and abundant oxidant whose utilization is of great interest in the design of bioinspired synthetic catalytic oxidation systems to reduce energy consumption. However, it is unfortunate that utilization of O₂ is a significant challenge because of the thermodynamic stability of O₂ in its triplet ground state. Nevertheless, nature is able to overcome the spin state barrier using enzymes, which contain transition metals with unpaired d-electrons facilitating the activation of O₂ by metal coordination. This inspires bioinorganic chemists to synthesize biomimetic small-molecule iron porphyrin complexes to carry out the O₂ activation, wherein Fe-O₂ species have been implicated as the key reactive intermediates. In recent years, a number of Fe-O₂ intermediates have been synthesized by activating O₂ at iron centers supported on porphyrin ligands. In this review, we focus on a few examples of these advances with emphasis in each case on the particular design of iron porphyrin complexes and particular reaction environments to stabilize and isolate metal-O₂ intermediates in dioxygen activation, which will provide clues to elucidate structures of reactive intermediates and mechanistic insights in biological processes.

Tryptophan regulates Drosophila zinc stores

SignificanceZinc deficiency in the human population, a major public health concern, can also be a consequence of nutritional deficiency in protein uptake. The discovery that tryptophan metabolites 3-hydroxykynurenine and xanthurenic acid are major zinc-binding ligands in insect cells establishes the kynurenine pathway as a regulator of systemic zinc homeostasis. Many biological processes influenced by zinc and the kynurenine pathway, including the regulation of innate and acquired immune responses to viral infections, have not been studied in light of the direct molecular links revealed in this study.

Lewis acid protection turns cyanide containing [FeFe]-hydrogenase mimics into proton reduction catalysts

Sustainable sources of hydrogen are a vital component of the envisioned energy transition. Understanding and mimicking the [FeFe]-hydrogenase provides a route to achieving this goal. In this study we re-visit a molecular mimic of the hydrogenase, the propyl dithiolate bridged complex [Fe₂(μ-pdt)(CO)₄(CN)₂]²⁻, in which the cyanide ligands are tuned via Lewis acid interactions. This system provides a rare example of a cyanide containing [FeFe]-hydrogenase mimic capable of catalytic proton reduction, as demonstrated by cyclic voltammetry. EPR, FTIR, UV-vis and X-ray absorption spectroscopy are employed to characterize the species produced by protonation, and reduction or oxidation of the complex. The results reveal that biologically relevant iron-oxidation states can be generated, potentially including short-lived mixed valent Fe(i)Fe(ii) species. We propose that catalysis is initiated by protonation of the diiron complex and the resulting di-ferrous bridging hydride species can subsequently follow two different pathways to promote H₂ gas formation depending on the applied reduction potential.

Mimicking the hydrogen-bonding interactions of the [FeFe]-hydrogenase active-site using Lewis acids transforms an otherwise unstable cyanide containing hydrogenase mimic into a proton reduction catalyst.

The influence of secondary interactions on the [Ni(O2)]+ mediated aldehyde oxidation reactions

A rate enhancement of one to two orders of magnitude can be obtained in the aldehyde deformylation reactions by replacing the -N(CH₃) groups of [Ni^III(O₂)(Me₄[12]aneN₄)]⁺ (Me₄[12]aneN₄ = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) and [Ni^III(O₂)(Me₄[13]aneN₄)]⁺ (Me₄[13]aneN₄ = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane) complexes by -NH in [Ni^III(O₂)([12]aneN₄)]⁺ (2; [12]aneN₄ = 1,4,7,10-tetraazacyclododecane) and [Ni^III(O₂)([13]aneN₄)]⁺ (4; [13]aneN₄ = 1,4,7,10-tetraazacyclotridecane). Based on detailed spectroscopic, reaction-kinetics and theoretical investigations, the higher reactivities of 2 and 4 are attributed to the changes in the secondary-sphere interactions between the [Ni^III(O₂)]⁺ and [12]aneN₄ or [13]aneN₄ moieties, which open up an alternative electrophilic pathway for the aldehyde oxidation reaction. Identification of primary kinetic isotope effects on the reactivity and stability of 2 when the -NH groups of the [12]aneN₄ ligand are deuterated may also suggest the presence of secondary interaction between the -NH groups of [12]aneN₄ and [Ni^III(O₂)]⁺ moieties, although, such interactions are not obvious in the DFT calculated optimized structure at the employed level of theory.

Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer

Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. In this study, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50-1.5 ml min^-1) sheath stream with a low-flow-rate (5-90 µl min^-1) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in 3D and sample widths as small as 5 µm. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 µm are demonstrated. Further, convection-diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.

Structure-Redox Response Correlation in a Few Select Heme Systems Using X-ray Absorption Spectroelectrochemistry

Heme based biomolecules control some of the most crucial life processes, such as oxygen and electron transport during respiration and energy metabolism, respectively. The active site of the heme, viz., the metal center, plays a key role and attributes functionality to these biomolecules. During the oxygen binding and debinding processes, it is important to note that the oxidation state of iron in hemoglobin (+II in the native form) does not undergo any change. However, the spin states of the metal center change. We present here a comprehensive study of the redox response of such molecules, based on the electronic structure of the active site. The local electronic structure of heme in a few selective molecular systems is studied in operando via synchrotron X-ray absorption spectroscopy (Fe K-edge) and cyclic voltammetry. Our objective is to identify the electronic structural parameters that can effectively be correlated with the redox reversibility. Evolution in these parameters can be followed to trace the overall changes in redox state of the system. Our data indicate that axial coordination and spin state of the iron center are two such parameters that are intimately connected with the redox response.

Potential-Induced Spin Changes in Fe/N/C Electrocatalysts Assessed by In Situ X-ray Emission Spectroscopy

The commercial success of the electrochemical energy conversion technologies required for the decarbonization of the energy sector requires the replacement of the noble metal-based electrocatalysts currently used in (co-)electrolyzers and fuel cells with inexpensive, platinum-group metal-free analogs. Among these, Fe/N/C-type catalysts display promising performances for the reduction of O₂ or CO₂ , but their insufficient activity and stability jeopardize their implementation in such devices. To circumvent these issues, a better understanding of the local geometric and electronic structure of their catalytic active sites under reaction conditions is needed. Herein we shed light on the electronic structure of the molecular sites in two Fe/N/C catalysts by probing their average spin state with X-ray emission spectroscopy (XES). Chiefly, our in situ XES measurements reveal for the first time the existence of reversible, potential-induced spin state changes in these materials.

Iron Metabolism: An Under Investigated Driver of Renal Pathology in Lupus Nephritis

Nephritis is a common manifestation of systemic lupus erythematosus, a condition associated with inflammation and iron imbalance. Renal tubules are the work horse of the nephron. They contain a large number of mitochondria that require iron for oxidative phosphorylation, and a tight control of intracellular iron prevents excessive generation of reactive oxygen species. Iron supply to the kidney is dependent on systemic iron availability, which is regulated by the hepcidin-ferroportin axis. Most of the filtered plasma iron is reabsorbed in proximal tubules, a process that is controlled in part by iron regulatory proteins. This review summarizes tubulointerstitial injury in lupus nephritis and current understanding of how renal tubular cells regulate intracellular iron levels, highlighting the role of iron imbalance in the proximal tubules as a driver of tubulointerstitial injury in lupus nephritis. We propose a model based on the dynamic ability of iron to catalyze reactive oxygen species, which can lead to an accumulation of lipid hydroperoxides in proximal tubular epithelial cells. These iron-catalyzed oxidative species can also accentuate protein and autoantibody-induced inflammatory transcription factors leading to matrix, cytokine/chemokine production and immune cell infiltration. This could potentially explain the interplay between increased glomerular permeability and the ensuing tubular injury, tubulointerstitial inflammation and progression to renal failure in LN, and open new avenues of research to develop novel therapies targeting iron metabolism.

Temperature Dependence of Structural Dynamics at the Catalytic Cofactor of [FeFe]-hydrogenase

[FeFe]-hydrogenases are nature's blueprint for efficient hydrogen turnover. Understanding their enzymatic mechanism may improve technological H₂ fuel generation. The active-site cofactor (H-cluster) consists of a [4Fe-4S] cluster ([4Fe]_H), cysteine-linked to a diiron site ([2Fe]_H) carrying an azadithiolate (adt) group, terminal cyanide and carbon monoxide ligands, and a bridging carbon monoxide (μCO) in the oxidized protein (Hox). Recently, the debate on the structure of reduced H-cluster states was intensified by the assignment of new species under cryogenic conditions. We investigated temperature effects (4-280 K) in infrared (IR) and X-ray absorption spectroscopy (XAS) data of [FeFe]-hydrogenases using fit analyses and quantum-chemical calculations. IR data from our laboratory and literature sources were evaluated. At ambient temperatures, reduced H-cluster states with a bridging hydride (μH^-, in Hred and Hsred) or with an additional proton at [4Fe]_H (Hred') or at the distal iron of [2Fe]_H (Hhyd) prevail. At cryogenic temperatures, these species are largely replaced by states that hold a μCO, lack [4Fe]_H protonation, and bind an additional proton at the adt nitrogen (HredH⁺ and HsredH⁺). XAS revealed the atomic coordinate dispersion (i.e., the Debye-Waller parameter, 2σ²) of the iron-ligand bonds and Fe-Fe distances in the oxidized and reduced H-cluster. 2σ² showed a temperature dependence typical for the so-called protein-glass transition, with small changes below ∼200 K and a pronounced increase above this "breakpoint". This behavior is attributed to the freezing-out of larger-scale anharmonic motions of amino acid side chains and water species. We propose that protonation at [4Fe]_H as well as ligand rearrangement and μH^- binding at [2Fe]_H are impaired because of restricted molecular mobility at cryogenic temperatures so that protonation can be biased toward adt. We conclude that a H-cluster with a μCO, selective [4Fe]_H or [2Fe]_H protonation, and catalytic proton transfer via adt facilitates efficient H₂ conversion in [FeFe]-hydrogenase.

Spin cascade and doming in ferric hemes: Femtosecond X-ray absorption and X-ray emission studies

The structure-function relationship is at the heart of biology, and major protein deformations are correlated to specific functions. For ferrous heme proteins, doming is associated with the respiratory function in hemoglobin and myoglobins. Cytochrome c (Cyt c) has evolved to become an important electron-transfer protein in humans. In its ferrous form, it undergoes ligand release and doming upon photoexcitation, but its ferric form does not release the distal ligand, while the return to the ground state has been attributed to thermal relaxation. Here, by combining femtosecond Fe Kα and Kβ X-ray emission spectroscopy (XES) with Fe K-edge X-ray absorption near-edge structure (XANES), we demonstrate that the photocycle of ferric Cyt c is entirely due to a cascade among excited spin states of the iron ion, causing the ferric heme to undergo doming, which we identify. We also argue that this pattern is common to a wide diversity of ferric heme proteins, raising the question of the biological relevance of doming in such proteins.

Femtosecond X-ray emission study of the spin cross-over dynamics in haem proteins

In haemoglobin the change from the low-spin (LS) hexacoordinated haem to the high spin (HS, S = 2) pentacoordinated domed deoxy-myoglobin (deoxyMb) form upon ligand detachment from the haem and the reverse process upon ligand binding are what ultimately drives the respiratory function. Here we probe them in the case of Myoglobin-NO (MbNO) using element- and spin-sensitive femtosecond Fe K_α and K_β X-ray emission spectroscopy at an X-ray free-electron laser (FEL). We find that the change from the LS (S = 1/2) MbNO to the HS haem occurs in ~800 fs, and that it proceeds via an intermediate (S = 1) spin state. We also show that upon NO recombination, the return to the planar MbNO ground state is an electronic relaxation from HS to LS taking place in ~30 ps. Thus, the entire ligand dissociation-recombination cycle in MbNO is a spin cross-over followed by a reverse spin cross-over process.

The change from low-spin hexacoordinated to high-spin pentacoordinated domed form in heam upon ligand detachment and the reverse process underlie the respiratory function. The authors, using femtosecond time-resolved X-ray emission spectroscopy, capture the transient states connecting the two forms in myoglobin-NO upon NO photoinduced detachment.

[FeFe]-hydrogenase maturation: H-cluster assembly intermediates tracked by electron paramagnetic resonance, infrared, and X-ray absorption spectroscopy

[FeFe]-hydrogenase enzymes employ a unique organometallic cofactor for efficient and reversible hydrogen conversion. This so-called H-cluster consists of a [4Fe-4S] cubane cysteine linked to a diiron complex coordinated by carbon monoxide and cyanide ligands and an azadithiolate ligand (adt = NH(CH2S)2)·[FeFe]-hydrogenase apo-protein binding only the [4Fe-4S] sub-complex can be fully activated in vitro by the addition of a synthetic diiron site precursor complex ([2Fe]adt). Elucidation of the mechanism of cofactor assembly will aid in the design of improved hydrogen processing synthetic catalysts. We combined electron paramagnetic resonance, Fourier-transform infrared, and X-ray absorption spectroscopy to characterize intermediates of H-cluster assembly as initiated by mixing of the apo-protein (HydA1) from the green alga Chlamydomonas reinhardtii with [2Fe]adt. The three methods consistently show rapid formation of a complete H-cluster in the oxidized, CO-inhibited state (Hox-CO) already within seconds after the mixing. Moreover, FTIR spectroscopy support a model in which Hox-CO formation is preceded by a short-lived Hred'-CO-like intermediate. Accumulation of Hox-CO was followed by CO release resulting in the slower conversion to the catalytically active state (Hox) as well as formation of reduced states of the H-cluster.

The hazardous effects of the environmental toxic gases on amyloid beta-peptide aggregation: A theoretical perspective

Alzheimer's disease (AD) is one of the leading causes of dementia in elderly people. It has been well documented that the exposure to environmental toxins such as CO, CO₂, SO₂ and NO₂ that are present in the air is considered as a hallmark for the progression of Alzheimer's disease. However, their actual mechanism by which environmental toxin triggers the aggregation of Aβ42 peptide at the molecular and atomic levels remain unknown. In this study, molecular dynamics simulation was carried out to study the aggregation mechanism of the Aβ₄₂ peptide due to its interaction of toxic gas (CO, CO₂, SO₂ and NO₂). During the 400 ns simulation, all the Aβ₄₂ interacted toxic gas (CO, CO₂, SO_2, and NO₂) complexes have smaller Root Mean Square Deviation values when compared to the Aβ₄₂ peptide, which shows that the interaction of toxic gases (CO, CO₂, SO_2, and NO₂) would increase the Aβ₄₂ peptide structural stability. The radius of gyration analysis also supports that Aβ₄₂ interacted CO₂ and SO₂ complexes have the minimum value in the range of 0.95 nm and 1.5 nm. It is accounted that the Aβ₄₂ interacted CO₂ and SO₂ complexes have a greater compact structure in comparison to Aβ₄₂ interacted CO and NO₂ complexes. Furthermore, all the Aβ₄₂ interacted toxic gas (CO, CO₂, SO_2, and NO₂) complexes exhibited an enhanced secondary structural probability for coil and turn regions with a reduced α-helix probability, which indicates that the interaction of toxic gases may enhance the toxicity and aggregation of Aβ₄₂.

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

The relationship between ligand design and spin state in base metal compounds is surveyed. Implications and applications of these principles for light-harvesting dyes, catalysis and materials chemistry are summarised.

A reaction pathway to compound 0 intermediates in oxy-myoglobin through interactions with hydrogen sulfide and His64

Myoglobin (Mb) binds oxygen with high affinity as a low spin singlet complex and thus functions as an oxygen storage protein. Yet, hybrid Density Functional Theory/Molecular Mechanical (DFT/MM) calculations of oxy-Mb models predict that the O₂ bond is much less resistant to breaking in the presence of hydrogen sulfide (H₂S) compared with water. Specifically, a hydrogen atom from H₂S can be transferred to the distal oxygen atom through homolytic cleavage of the S-H bond to form the intermediate Compound (Cpd) 0 structure and a thiyl radical. In the presence of a neutral His64 (Nε protonation, His64-ε) and H₂S, only a metastable Cpd 0 would be formed as the active site is devoid of any additional proton donor to fully break the O₂ bond. In contrast, the calculations predict that the triplet state is significantly favored over the open shell singlet diradical state throughout the entire reaction coordinate in the presence of H₂S and a positively charged His64. Furthermore, a positively charged His64 can readily donate a proton to Cpd 0 to fully break the O₂ bond resulting in a configuration analogous to reported reaction models of a hemoglobin mutant bound to H₂O₂ with H₂S present. Typically, exotic techniques are required to generate Cpd 0 but under the conditions just described the intermediate is readily detected in UV-Vis spectra at room temperature. The effect is observed as a 2 nm red shift of the Soret band from 414 nm to 416 nm (pH 5.0, His64-εδ) and from 416 nm to 418 nm (pH 6.6, His64-ε).

The multifaceted role of iron in renal health and disease

Iron is an essential element that is indispensable for life. The delicate physiological body iron balance is maintained by both systemic and cellular regulatory mechanisms. The iron-regulatory hormone hepcidin assures maintenance of adequate systemic iron levels and is regulated by circulating and stored iron levels, inflammation and erythropoiesis. The kidney has an important role in preventing iron loss from the body by means of reabsorption. Cellular iron levels are dependent on iron import, storage, utilization and export, which are mainly regulated by the iron response element-iron regulatory protein (IRE-IRP) system. In the kidney, iron transport mechanisms independent of the IRE-IRP system have been identified, suggesting additional mechanisms for iron handling in this organ. Yet, knowledge gaps on renal iron handling remain in terms of redundancy in transport mechanisms, the roles of the different tubular segments and related regulatory processes. Disturbances in cellular and systemic iron balance are recognized as causes and consequences of kidney injury. Consequently, iron metabolism has become a focus for novel therapeutic interventions for acute kidney injury and chronic kidney disease, which has fuelled interest in the molecular mechanisms of renal iron handling and renal injury, as well as the complex dynamics between systemic and local cellular iron regulation.

Suppressive effects of iron chelation in clear cell renal cell carcinoma and their dependency on VHL inactivation

Increasing data implicate iron accumulation in tumorigenesis of the kidney, particularly the clear cell renal cell carcinoma (ccRCC) subtype. The von Hippel Lindau (VHL)/hypoxia inducible factor-α (HIF-α) axis is uniquely dysregulated in ccRCC and is a major regulator and regulatory target of iron metabolism, yet the role of iron in ccRCC tumorigenesis and its potential interplay with VHL inactivation remains unclear. We investigated whether ccRCC iron accumulation occurs due to increased cell dependency on iron for growth and survival as a result of VHL inactivation. Free iron levels were compared between four VHL-mutant ccRCC cell lines (786-0, A704, 769-P, RCC4) and two benign renal tubule epithelial cell lines (RPTEC, HRCEp) using the Phen Green SK fluorescent iron stain. Intracellular iron deprivation was achieved using two clinical iron chelator drugs, deferasirox (DFX) and deferoxamine (DFO), and chelator effects were measured on cell line growth, cell cycle phase, apoptosis, HIF-1α and HIF-2α protein levels and HIF-α transcriptional activity based on expression of target genes CA9, OCT4/POU5F1 and PDGFβ/PDGFB. Similar assays were performed in VHL-mutant ccRCC cells with and without ectopic wild-type VHL expression. Baseline free iron levels were significantly higher in ccRCC cell lines than benign renal cell lines. DFX depleted cellular free iron more rapidly than DFO and led to greater growth suppression of ccRCC cell lines (>90% at ~30-150 µM) than benign renal cell lines (~10-50% at up to 250 µM). Similar growth responses were observed using DFO, with the exception that a prolonged treatment duration was necessary to deplete cellular iron adequately for differential growth suppression of the less susceptible A704 ccRCC cell line relative to benign renal cell lines. Apoptosis and G1-phase cell cycle arrest were identified as potential mechanisms of chelator growth suppression based on their induction in ccRCC cell lines but not benign renal cell lines. Iron chelation in ccRCC cells but not benign renal cells suppressed HIF-1α and HIF-2α protein levels and transcriptional activity, and the degree and timing of HIF-2α suppression correlated with the onset of apoptosis. Restoration of wild-type VHL function in ccRCC cells was sufficient to prevent chelator-induced apoptosis and G1 cell cycle arrest, indicating that ccRCC susceptibility to iron deprivation is VHL inactivation-dependent. In conclusion, ccRCC cells are characterized by high free iron levels and a cancer-specific dependency on iron for HIF-α overexpression, cell cycle progression and apoptotic escape. This iron dependency is introduced by VHL inactivation, revealing a novel interplay between VHL/HIF-α dysregulation and ccRCC iron metabolism. Future study is warranted to determine if iron deprivation using chelator drugs provides an effective therapeutic strategy for targeting HIF-2α and suppressing tumor progression in ccRCC patients.

Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex

Hemoglobin and myoglobin are oxygen-binding proteins with S = 0 heme {FeO₂}⁸ active sites. The electronic structure of these sites has been the subject of much debate. This study utilizes Fe K-edge X-ray absorption spectroscopy (XAS) and 1s2p resonant inelastic X-ray scattering (RIXS) to study oxyhemoglobin and a related heme {FeO₂}⁸ model compound, [(pfp)Fe(1-MeIm)(O₂)] (pfp = meso-tetra(α,α,α,α-o-pivalamido-phenyl)porphyrin, or TpivPP, 1-MeIm = 1-methylimidazole) (pfpO₂), which was previously analyzed using L-edge XAS. The K-edge XAS and RIXS data of pfpO₂ and oxyhemoglobin are compared with the data for low-spin Fe^II and Fe^III [Fe(tpp)(Im)₂]^0/+ (tpp = tetra-phenyl porphyrin) compounds, which serve as heme references. The X-ray data show that pfpO₂ is similar to Fe^II, while oxyhemoglobin is qualitatively similar to Fe^III, but with significant quantitative differences. Density-functional theory (DFT) calculations show that the difference between pfpO₂ and oxyhemoglobin is due to a distal histidine H bond to O₂ and the less hydrophobic environment in the protein, which lead to more backbonding into the O₂ A valence bond configuration interaction multiplet model is used to analyze the RIXS data and show that pfpO₂ is dominantly Fe^II with 6-8% Fe^III character, while oxyhemoglobin has a very mixed wave function that has 50-77% Fe^III character and a partially polarized Fe-O₂ π-bond.

Recent development in biodegradable nanovehicle delivery system-assisted immunotherapy

A schematic illustration of BNDS biodegradation and release antigen delivery for assisting immunotherapy.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

The Role of Reactive Oxygen Species and Ferroptosis in Heme-Mediated Activation of Human Platelets

Hemolysis, a process by which the destruction of red blood cells leads to the release of hemoglobin, is a critical event observed during hemolytic disorders. Under oxidative stress conditions, hemoglobin can release its heme prosthetic group, which is highly cytotoxic and can catalyze the generation of reactive oxygen species (ROS), leading to several undesired redox reactions in the cells. Herein, we demonstrate for the first time that heme can mediate the activation and death of human platelets through ferroptosis, which is an iron-dependent form of nonapoptotic cell death. This study also suggests that the heme-mediated lipid peroxidation and ferroptosis in platelets may play an important role in hemolytic disorders.

The dioxygen adducts of iron and manganese porphyrins: electronic structure and binding energy

The electronic structures of adducts of O₂ and metal porphyrins were thoroughly investigated by highly accurate DMRG-CASPT2.