Ferric Species Research Articles

Regulating the dominant reactive oxygen species from Fe(IV)-oxo to 1O2 by deprotonation of Fe(IV)-oxo in electro-Fe(II)/periodate system

The ferrous species (Fe(II))-periodate (PI) system employs efficient decontamination by generating ferryl species (Fe(IV)-oxo) under acidic pH conditions. However, the mechanisms by which the Fe(II)/PI system operates across a broad pH range remain elusive. In light of the rapid oxidation and consumption of Fe(II) at neutral pH, we developed an electro-Fe(II) system utilizing an iron plate anode to activate PI (E(Fe/C)/PI). Efficient sulfamethoxazole (SMX) degradation was achieved under the E(Fe/C)/PI system across a range of pH values. Quenching and chemical probe experiments revealed a transition from Fe(IV)-oxo to singlet oxygen (1O2) as the predominant reactive species from acidic to near-neutral pH, respectively. Density functional theory (DFT) calculations indicated that the differences in one-electron transfer capacity and hydrolysis pathways for various protonated Fe(IV)-oxo species underlie the variations in catalytic mechanisms. This study not only enhances the efficiency of the Fe(II)/PI system but also provides a comprehensive understanding of the activation mechanisms at different pH levels.

Two-Dimensional MoS2 nanosheets as cocatalyst to augment the nanocatalytic tumor therapy

As an appealing modality of tumor nanocatalytic therapeutics, Fenton chemistry has been extensively employed to combat cancer through the generation of highly cytotoxic reactive oxygen species (ROS). For iron-triggered catalytic Fenton chemistry, the therapeutic consequence was largely dependent on the available catalytically active ferrous species. In this work, hydrothermally synthesized molybdenum disulfide (MoS2-PEG) nanosheets were served as Fenton cocatalyst to enhance the Fenton kinetics as contributed by ultrasmall gallic acid (GA)-coordinated Fe nanoparticles (GA-Fe NPs) in tumor destruction. The tetravalent Mo4+ species of MoS2-PEG cocatalysts could efficiently transfer Fe3+ species into Fe2+ species by the reduction process, regenerating the catalytically active Fe2+ species and restoring the catalytic activity of GA-Fe nanocatalysts during the Fenton-based tumor therapeutics. Based on systematic in vitro and in vivo investigations of cocatalyst-augmented Fenton nanotherapeutics against 4-T1 tumor cells/xenografts, we have demonstrated a proof-of-concept enhanced nanocatalytic tumor-therapeutic modality by MoS2-PEG/GA-Fe nanocatalyst/cocatalyst hybridized system, which offers an explicit cocatalytic strategy to improve the therapeutic efficacy during nanocatalytic tumor therapy.

Efficacy of Chitosan-Carboxylic Acid Hydrogels in Reducing and Chelating Iron for the Removal of Rust from Stone Surface.

In the field of stone conservation, the removal of iron stains is one of the most challenging issues due to the stability and low solubility of the ferrous species. In the present paper, three different chitosan-based hydrogels added with acetic, oxalic or citric acids are applied on different lithotypes, i.e., granite, travertine and marble, widely diffused in monumental heritages, and artificially stained by deposition of a rust dispersion. The reducing power of carboxylic acids is combined with the good chelating properties of chitosan to effectively remove rust from stone surfaces. As evidenced by colorimetry on three samples of each lithotype and confirmed by 1H-NMR relaxometry and SEM/EDS analyses, the chitosan-oxalic acid hydrogel shows the best performance and a single application of 24 h is enough to get a good restoration of the stone original features. Lastly, the chitosan-oxalic acid hydrogel performs well when a rusted iron grid is placed directly on the lithic surfaces to simulate a more realistic pollution. Current work in progress is devoted to finding better formulations for marble, which is the most challenging to clean or, with a different approach, to developing protective agents to prevent rust deposition.

Efficient removal of Cr(VI) by mechanochemically modified pyrites: Self-regulated neutralization behaviors

The pH of the aqueous environment commonly occurs during the application of natural/modified pyrite in water/soil remediation. In this study, mechanochemical modification of natural pyrite (M−pyrite) was employed to remove Cr(VI) over a wide pH range. Self-regulated neutralization was observed during Cr(VI) removal by M−pyrite in the initial pH range of 2.0–12.3. The reductive immobilization of total Cr occurred effectively when the solution self-regulated to a circumneutral pH (pH 5.0–9.0), but only efficient Cr(VI) reduction occurred under strongly acidic or alkaline conditions. Three pathways, i.e., Fe(II) removal, surface sulfide reduction and both combinations, could be revealed for Cr(VI) removal by M−pyrite at circumneutral pH and under strongly alkaline and acidic conditions. As Cr(VI) is reduced by ferrous and sulfide species, the protons generated from pyrite oxidation by O2 are consumed, causing neutral restoration to resist the initial sharp decrease in pH. Effective immobilization of the reduced Cr(VI) was confirmed at an initial pH of 5.0–9.0, but this immobilization did not occur at more acidic and alkaline pH values due to strong Fe(II) dissolution or surface precipitation. The M−pyrite was successfully applied in treating four types of saline industrial Cr(VI) wastewaters, demonstrating the advantage of self-regulated neutralization resistance in complex wastewater matrices.

Verification of passivity condition of carbon steel under cathodic protection through in-situ spectroscopy

Cathodic protection (CP) is a consolidated technique involving the circulation of a cathodic current (iprot) from an anode to a structure. This causes the electrode alkalisation depending on iprot and on the kinetics of the diffusive motion of ions, influencing the surface state of the metal. In this paper, the surface condition of carbon steel under CP has been studied through in-situ Raman spectroscopy and atomic force microscopy (AFM), combined with electrochemical impedance spectroscopy (EIS), for a protection potential (Eprot) of − 1.1 V/CSE (Cu/CuSO4). Results show that depending on alkalisation, Fe3O4 converts to other ferric species like γ-Fe2O3 and α-FeOOH.

Aliquat 336@SiO2 ionogels: Synthesis of, and insight into, iron(III) extraction mechanisms

The immobilisation of ionic liquids (ILs) in porous solid matrices enables the design of ionogels, which are now regarded as a promising material in extraction science. Here, by the co-gelation of TMOS and MTMS in a commercially available ionic liquid, Aliquat 336 (A336Cl), a series of ionogels were synthesised with various levels of IL content and matrix hydrophobicity. Both of these factors were shown to have a small effect on Fe(III) extraction efficiency (57–70 %), while they strongly influenced the re-extraction efficiency (15–45 %) of the materials. The ionogels with the highest IL content (80 %) and a highly hydrophilic silica matrix showed the best extraction and re-extraction performance. A thorough characterisation of the ionogels confirmed the confinement of the IL in silica and revealed Fe(III) extraction mechanisms. It was shown that iron was extracted from the aqueous solutions by A336Cl@SiO2 ionogels in the form of FeCl4– ions typical of the extraction by pure A336Cl. Unexpectedly, the iron extraction by the ionogels resulted in the formation of Fe2Cl7– species that had not been observed earlier in the A336Cl-based extraction systems. Moreover, iron(III) directly bound to hydrophilic silica through Si–O–Fe bridges, and it was also found that, in the ionogels, the admixtures of alcohols could even reduce ferric ions to ferrous species. For the ionogels, both iron extraction and re-extraction followed pseudo-second order kinetics. Iron re-extraction from the ionogels with aqueous sulfuric acid solution resulted in the loss of recyclability, most probably due to the formation of FeSO4⋅H2O in the ionogels. The cycling performance of the ionogels can be improved by their conditioning in chloride-rich media after re-extraction stages.

Self-endowed magnetic photocatalysts derived from iron-rich sludge and its recycling in photocatalytic process for tetracycline degradation

The disposal of iron-rich sludge by landfill or incineration poses environmental risks and wastes resources. The utilization of iron-rich sludge for magnetic material preparation offers a sustainable and resource-efficient solution for its disposal. Herein, self-endowed magnetic photocatalysts were initially prepared by pyrolysis using iron-rich sludge without any additives. The photocatalysts performance were evaluated for tetracycline degradation, with the highest degradation rate of 95.3 % at a concentration of 10 mg·L-1 (pH = 7) within 5 h being achieved for the photocatalyst prepared at 800 °C. The reactive radical species in the photocatalysis process were confirmed to be •OH and O2•- activated by ferrous oxygen species under light irradiation. Furthermore, quinone-like structures induced bound persistent free radicals, which emerged as the predominant factors influencing 1O2 formation. The employed photocatalyst can be efficiently separated and recovered owing to its magnetism. This work presents an economic solution for antibiotic removal using waste iron-rich sludge.

Myoglobin-Catalyzed Azide Reduction Proceeds via an Anionic Metal Amide Intermediate.

Nitrene transfer reactions catalyzed by heme proteins have broad potential for the stereoselective formation of carbon-nitrogen bonds. However, competition between productive nitrene transfer and the undesirable reduction of nitrene precursors limits the broad implementation of such biocatalytic methods. Here, we investigated the reduction of azides by the model heme protein myoglobin to gain mechanistic insights into the factors that control the fate of key reaction intermediates. In this system, the reaction proceeds via a proposed nitrene intermediate that is rapidly reduced and protonated to give a reactive ferrous amide species, which we characterized by UV/vis and Mössbauer spectroscopies, quantum mechanical calculations, and X-ray crystallography. Rate-limiting protonation of the ferrous amide to produce the corresponding amine is the final step in the catalytic cycle. These findings contribute to our understanding of the heme protein-catalyzed reduction of azides and provide a guide for future enzyme engineering campaigns to create more efficient nitrene transferases. Moreover, harnessing the reduction reaction in a chemoenzymatic cascade provided a potentially practical route to substituted pyrroles.

Fe@Fe2O3/etched carbon felt as a cathode for efficient bisphenol a removal in a flow-through electro-Fenton system: Electron transfer pathway and underlying mechanism

The advanced oxidation processes based on the electro-Fenton reactions show great potentials as a wastewater treatment approach for removing emerging contaminants. In the electro-Fenton system, some factors still exist affecting the treatment efficiency, e.g., the formation of iron sludge, a narrow pH working range, and slow conversion rates between Fe2+ and Fe3+. In this research, a novel heterogeneous flow-through electro-Fenton system was constructed, and carbon felt was used as the template material for the preparation of an etched carbon felt cathode loaded with zero-valent iron and iron oxide (Fe@Fe2O3/ECF) for the degradation of bisphenol A, which addressed the aforementioned issues. The Fe@Fe2O3/ECF electrode showed excellent performance across a pH range between 3 and 11, with exceptional stability and resistance to other substances in water. For the three pollutants, bisphenol A, tetracycline and ciprofloxacin, the highest degradation efficiency can reach 99.3 %. Compared to traditional electro-Fenton systems, the novel process exhibited superior performances due to synergistic effects from feasible flow design, enhanced electrochemical reactivity and greater exposure of active sites. Through electron spin resonance and free-radical quenching experiments, it was determined that the primary species participating in the heterogeneous electro-Fenton process were the hydroxyl radical and superoxide anion. Specifically, the hydroxyl radical was identified as playing a pivotal role in the degradation of bisphenol A. After a continuous operation of 240 min, the electrode showed low metal ion leaching rate and good stability, with the concentrations of leached Fe and Co ions to be only 17 and 7 μg/L, respectively. Meanwhile, the removal efficiency was only decreased by 5.7 % during the process. Through density functional theory calculation and intermediate product analysis, three potential degradation pathways were proposed, with the cleavage of carbon bonds serving as the main degradation approach. The toxicity analysis demonstrated that the intermediates were generally less toxic than BPA during treatments. This research provides a fresh means for promoting a valence state cycling of ferric species in electro-Fenton system, which has potential applications in wastewater treatment containing emerging contaminants.

Iron mobility in subduction zone fluids at forearc depths and implications for mantle redox heterogeneity

The mobility of redox sensitive elements such as iron, carbon and sulfur in subduction zone fluids is important for redox transfer between the subducting slab and mantle wedge. However, the speciation of Fe during its mobility in subduction zone fluids at forearc depths is still unclear. Here, we studied the Fe isotope compositions of high-pressure (HP) fluid-metasomatized rocks (Mg-rich leucophyllite and gneiss, which is mineralogically transitional between leucophyllite and metagranite) from the Eastern Alps and found evidence for two distinct types of Fe species during Fe mobility in subduction zone fluids. The protolith of both types of rocks is metagranite, which shows δ56Fe values of 0.12–0.25‰. Compared to the metagranite, the transitional gneiss shows slightly higher Fe2O3t contents, lower Fe3+/∑Fe ratios and significantly lower δ56Fe values of −0.03–0.16‰, whereas the leucophyllite exhibits much lower Fe2O3t contents, comparable or slightly lower Fe3+/∑Fe ratios and generally higher δ56Fe values of up to 0.55‰. These observations indicate that the granitic protolith has experienced two types of fluid metasomatism. Combined with previous results, we infer the mobility of Fe in subduction zone fluids at forearc depths via two kinds of Fe species, Fe2+-(CO3), and Fe2+-(HS) or Fe2+-Cl. The Fe mobility was dominant in the form of the Fe2+-(CO3) complex in a carbonate-rich fluid during the formation of transitional gneiss, consistent with reported radiogenic Sr and isotopically light Mg isotope compositions, whereas Fe loss in the form of Fe2+-(HS) or Fe2+-Cl complexes via dissolution of biotite and phlogopite can account for the formation of leucophyllites. According to the tectonic evolution of the Eastern Alps, both types of fluids possibly originated from the dehydration of the sediment-serpentinite mélange produced by the subducting oceanic slab of the Alpine Penninic unit. The diversity of ferrous iron species in the subduction zone fluids at forearc depths probably reflects the variation in the redox properties of fluids derived from carbonate-rich sediment and serpentinite under such conditions. Our results provide an excellent example of tracing Fe mobility with distinct Fe species in subduction zone fluids.

Iron‐Catalyzed Asymmetric α‐Alkylation of 2‐Acylimidazoles via Dehydrogenative Radical Cross‐Coupling with Alkanes

AbstractThe direct α‐alkylation of acyclic carbonyls with nonactivated hydrocarbons through C(sp3)−H functionalization is both extremely promising and notably challenging, especially when attempting to achieve enantioselectivity using iron‐based catalysts. We have identified a robust chiral iron complex for the oxidative cross‐coupling of 2‐acylimidazoles with benzylic and allylic hydrocarbons, as well as nonactivated alkanes. The readily available and tunable N,N′‐dioxide catalysts of iron in connection with oxidants exhibit precise asymmetric induction (up to 99 % ee) with good compatibility in moderate to good yields (up to 88 % yield). This protocol provides an elegant and straightforward access to optically active acyclic carbonyl derivatives starting from simple alkanes without prefunctionalization. Density functional theory (DFT) calculations and control experiments were made to gain insight into the nature of C−C bond formation and the origin of enantioselectivity. We propose a radical‐radical cross‐coupling process enabled by the immediate interconversion between chiral ferric species and ferrous species.

Iron-Catalyzed Asymmetric α-Alkylation of 2-Acylimidazoles via Dehydrogenative Radical Cross-Coupling with Alkanes.

The direct α-alkylation of acyclic carbonyls with nonactivated hydrocarbons through C(sp3 )-H functionalization is both extremely promising and notably challenging, especially when attempting to achieve enantioselectivity using iron-based catalysts. We have identified a robust chiral iron complex for the oxidative cross-coupling of 2-acylimidazoles with benzylic and allylic hydrocarbons, as well as nonactivated alkanes. The readily available and tunable N,N'-dioxide catalysts of iron in connection with oxidants exhibit precise asymmetric induction (up to 99 % ee) with good compatibility in moderate to good yields (up to 88 % yield). This protocol provides an elegant and straightforward access to optically active acyclic carbonyl derivatives starting from simple alkanes without prefunctionalization. Density functional theory (DFT) calculations and control experiments were made to gain insight into the nature of C-C bond formation and the origin of enantioselectivity. We propose a radical-radical cross-coupling process enabled by the immediate interconversion between chiral ferric species and ferrous species.

Divergent Reactivity of Hemoglobin Isoforms α2Aβ2 and α2Dβ2 from Meleagris gallopavo in Native and Fatty Acid-Modified States.

Meleagris gallopavo (turkey) coexpresses distinct hemoglobin (Hb) isoforms, Hb α2Aβ2 (HbA) and α2Dβ2 (HbD), at a ratio of ∼3:1 (HbA:HbD). Herein, the reactivities of HbA and HbD were investigated in their native and free fatty acid (FFA)-modified states. Results indicated that HbD displays elevated autoxidation (kox) and an increased propensity to oxidize lipids in its reduced (oxy) and oxidized (met) forms. Interestingly, metHbD displayed less heme-globin cross-linking compared to HbA. Regarding FFA-modified Hb, we found that an FFA mixture and linoleic acid (LA) produced a bis-histidyl ferric (Bis-His) Hb species, decreasing the ability of Hb to oxidize lipids. Using molecular docking, we found LA to hydrogen bond with β Arg C6, found at the α1β2 interface, but the extent of Bis-His formation differs between HbA and HbD. Our findings suggest HbA displays elevated oxidative stability compared to HbD and that FFA may act as allosteric effectors of metHb.

Ultrasound-amplified polymetallic nanozyme-induced pyroptosis for lung cancer theranostics

For catalytic nanozyme and biomaterials in disease treatment, the iron-based catalysts suffer from low •OH generation performance due to the creation of a passive Fe (III) oxide layer and the prevention of the catalytic activation of H2O2 as a result of the oxidation of Fe (II) to Fe (III). Herein, polymetallic Cu0.5Mn0.5Fe2O4 (CMF) nanozyme has been rationally designed and engineered for massive generation of •OH through stabilizing the surface ferric species and providing enormous catalytically active sites. Furthermore, such a nanozyme with multienzyme-mimicking activities can amplify the reactive oxygen species production, elevate the oxidative stress level and induce cell pyroptosis. Importantly, exogenous ultrasound irradiation further achieves the goal of amplified and precise therapeutic effect again lung cancer. The in vitro and in vivo anti-tumor theranostic results confirm the accurate T1-T2 dual-mode magnetic resonance imaging and emphatic tumor inhibition effect of ultrasound-amplified CMF nanozymes. These results provide the paradigm of the utilization of polymetallic nanozymes as a therapeutic nanoplatform in treating apoptosis-resistant and pyroptosis sensitive tumors.

Efficient selective flotation separation of fluorite from calcite using ferrous and ferric species as combined depressant

It is known that the flotation separation of fluorite/calcite is extremely difficult because of their similar surface features. In this research, the combination of FeSO4·7H2O and Fe2 (SO4)3 (FF) as depressant for possible flotation separation of calcite/fluorite was systematically investigated, with sodium oleate (NaOL) as collector. Besides large gap between mono-mineral flotation results of fluorite and calcite, even for 1:1 mixed sample, CaF2 grade of 80.29 % and CaF2 recovery of 90.79 % of fluorite concentrate can be achieved with FeSO4·7H2O (300 mg/L) and Fe2 (SO4)3 (150 mg/L) used as combined depressant and NaOL (40 mg/L) as collector at pH = 6. In addition to the quantitative micro-flotation results on mono-mineral and artificially mixed sample, SEM-EDS, contact angle measurement, zeta potential measurement, FTIR and XPS analyses, as well as collector adsorption experiment, were carried out to clarify the selective inhibition mechanism. Characterizations confirmed that, under interaction with iron sulfates (FF), ferrous hydroxide, ferric hydroxide, hydroxyl iron oxide and green rust were in-suit generated on calcite surface due to the presence of the hydrolysis of OH− and CO32− on calcite surface, while such Fe-related components were barely discernible on fluorite surface. These different adsorption features for Fe (II) and Fe (III) species on mineral surfaces exactly hindered the complexation of NaOL on calcite surface, while maintained the adsorption of collector on fluorite surface, thereby to realize the selective separation of fluorite from calcite.

Iron hydrolysis and lithium uptake on mixed-bed ion exchange resin at alkaline pH

The use of ion exchange resins to remove ionic impurities from solution is prevalent in industrial process systems, including in the primary heat transport system (PHTS) purification circuit of nuclear power plants. Despite its extensive use in the nuclear industry, our general understanding of ion exchange cannot fully explain the complex chemistry in ion exchange beds, particularly when operated at or near their saturation limit. This work investigates the behaviour of mixed-bed ion exchange resin, saturated with species representative of corrosion products in a CANDU (Canadian Deuterium Uranium) reactor PHTS, particularly with respect to iron chemistry in the resin bed and the removal of lithium ions from solution. Experiments were performed under deaerated conditions, analogous to normal PHTS operation. The results show interesting iron chemistry, suggesting the hydrolysis of cation resin bound ferrous species and the subsequent formation of either a solid hydrolysis product or the soluble, anionic Fe(OH)3-.

Exploring the Antioxidant, Antidiabetic, and Antimicrobial Capacity of Phenolics from Blueberries and Sweet Cherries

(1) Background: Nowadays, special attention has been paid to red and purple fruits, including blueberries and sweet cherries, since they are highly attractive to consumers due to their organoleptic properties, standing out due to their vibrant red and purple colours and sweet flavour, and nutritional value. (2) Methods: The present study evaluated the phenolic profile of phenolic-enriched extracts from blueberries and sweet cherries and explored their antioxidant potential against DPPH, superoxide and nitric oxide radicals, and ferric species, and their potential to inhibit the α-glucosidase enzyme. Furthermore, their antimicrobial activity was also determined by microdilution method against four Gram-positive strains (Enterococcus faecalis ATCC 29212, Bacillus cereus ATCC 11778, Listeria monocytogenes LMG 16779, and Staphylococcus aureus ATCC 25923) and five Gram-negative strains (Salmonella enterica subsp. enterica ATCC 13311 serovar Typhimurium, Klebsiella pneumoniae ATCC 13883, Proteus mirabilis CECT 170, Serratia marcescens CECT 159, and Acinetobacter baumannii LMG 1025). (3) Results: By chromatographic techniques, eight anthocyanins were detected in blueberry coloured fraction and total extract, and five anthocyanins were detected in sweet cherry total extract and coloured fraction, while quercetin aglycone and chlorogenic acids were the dominant non-coloured compounds in blueberries and sweet cherries, respectively. All extracts demonstrated significant antioxidant properties, as well as the ability to inhibit the activity of α-glucosidase enzyme and the development of various microorganisms. (4) Conclusion: The obtained data evidence the promising biological potential of blueberries and sweet cherries, being highly correlated with the presence of phenolic compounds.

Highly applicable dual-cathode electro-Fenton system with self-adjusting pH and ferrous species

Due to the high dependence on the pH of influent water and the level of ferrous species, the applicability of the electro-Fenton (EF) system is poor. A highly applicable dual-cathode (DC) EF system with self-adjusting pH and ferrous species is proposed: gas diffusion electrode (GDE) for generation H2O2 and Fe/S doped multi-walled carbon nanotubes (Fe/S-MWCNT) modification active cathode (AC) for adjusting pH and iron species. The strong synergistic enhancement effect between two cathodes (synergy factor up to 90.3%) improves the catalytic activity of this composite system about 12.4 times higher than that of cathode alone. Impressively, AC has the ability of self-regulate to shift towards the optimal Fenton pH (around 3.0) without adding reagents. Even pH can be adjusted from 9.0 to 3.4 within 60 min. This characteristic gives the system a wide range of pH applications, while avoiding the disadvantage of the high cost of traditional EF in pre-acidification. Furthermore, DC has a high and stable ferrous species supply, and the iron leaching amount is about twice less than that of heterogeneous EF system. Long-term stability of the DC system and its easy activity regeneration exhibit the potential of environmental remediation in industrial application.

Kinetics of Hydroxyl Radical Production from Oxygenation of Reduced Iron Minerals and Their Reactivity with Trichloroethene: Effects of Iron Amounts, Iron Species, and Sulfate Reducing Bacteria.

Reactive oxygen species generated during the oxygenation of different ferrous species have been documented at groundwater field sites, but their effect on pollutant destruction remains an open question. To address this knowledge gap, a kinetic model was developed to probe mechanisms of •OH production and reactivity with trichloroethene (TCE) and competing species in the presence of reduced iron minerals (RIM) and oxygen in batch experiments. RIM slurries were formed by combining different amounts of Fe(II) and sulfide (with Fe(II):S ratios from 1:1 to 50:1) or Fe(II) and sulfate with sulfate reducing bacteria (SRB) added. Extents of TCE oxidation and •OH production were both greater with RIM prepared under more reducing conditions (more added Fe(II)) and then amended with O2. Kinetic rate constants from modeling indicate that •OH production from free Fe(II) dominates •OH production from solid Fe(II) and that TCE competes for •OH with Fe(II) and organic matter (OM). Competition with OM only occurs in experiments with SRB, which include cells and their exudates. Experimental results indicate that cells and/or exudates also provide electron equivalents to reform Fe(II) from oxidized RIM. Our work provides new insights into mechanisms and environmental significance of TCE oxidation by •OH produced from oxygenation of RIM. However, further work is necessary to confirm the relative importance of reaction pathways identified here and to probe potentially unaccounted for mechanisms that affect abiotic TCE oxidation in natural systems.

BAY58-2667 Activates Different Soluble Guanylyl Cyclase Species by Distinct Mechanisms that Indicate Its Principal Target in Cells is the Heme-Free Soluble Guanylyl Cyclase-Heat Shock Protein 90 Complex.

Nitric oxide (NO)-unresponsive forms of soluble guanylyl cyclase (sGC) exist naturally and in disease can disable NO-sGC-cGMP signaling. Agonists like BAY58-2667 (BAY58) target these sGC forms, but their mechanisms of action in living cells are unclear. We studied rat lung fibroblast-6 cells and human airway smooth muscle cells that naturally express sGC and HEK293 cells that we transfected to express sGC and variants. Cells were cultured to build up different forms of sGC, and we used fluorescence and FRET-based measures to monitor BAY58-driven cGMP production and any protein partner exchange or heme loss events that may occur for each sGC species. We found that: (i) BAY58 activated cGMP production by the apo-sGCβ-Hsp90 species after a 5-8 minute delay that was associated with apo-sGCβ exchanging its Hsp90 partner with an sGCα subunit. (ii) In cells containing an artificially constructed heme-free sGC heterodimer, BAY58 initiated an immediate and three times faster cGMP production. However, this behavior was not observed in cells expressing native sGC under any condition. (iii) BAY58 activated cGMP production by ferric heme sGC only after a 30-minute delay, coincident with it initiating a delayed, slow ferric heme loss from sGCβ We conclude that the kinetics favor BAY58 activation of the apo-sGCβ-Hsp90 species over the ferric heme sGC species in living cells. The protein partner exchange events driven by BAY58 account for the initial delay in cGMP production and also limit the speed of subsequent cGMP production in the cells. Our findings clarify how agonists like BAY58 may activate sGC in health and disease. SIGNIFICANCE STATEMENT: A class of agonists can activate cyclic guanosine monophosphate (cGMP) synthesis by forms of soluble guanylyl cyclase (sGC) that do not respond to NO and accumulate in disease, but the mechanisms of action are unclear. This study clarifies what forms of sGC exist in living cells, which of these can be activated by the agonists, and the mechanisms and kinetics by which each form is activated. This information may help to hasten deployment of these agonists for pharmaceutical intervention and clinical therapy.