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DOE OSTI2014

Coupled Mercury–Cell Sorption, Reduction, and Oxidation on Methylmercury Production by Geobacter sulfurreducens PCA

G. sulfurreducens PCA cells have been shown to reduce, sorb, and methylate Hg(II) species, but it is unclear whether this organism can oxidize and methylate dissolved elemental Hg(0) as shown for Desulfovibrio desulfuricans ND132. Using Hg(II) and Hg(0) separately as Hg sources in washed cell assays in phosphate buffered saline (pH 7.4), in this paper we report how cell-mediated Hg reduction and oxidation compete or synergize with sorption, thus affecting the production of toxic methylmercury by PCA cells. Methylation is found to be positively correlated to Hg sorption (r = 0.73) but negatively correlated to Hg reduction (r = -0.62). These reactions depend on the Hg and cell concentrations or the ratio of Hg to cellular thiols (-SH). Oxidation and methylation of Hg(0) are favored at relatively low Hg to cell–SH molar ratios (e.g., <1). Increasing Hg to cell ratios from 0.25 × 10 –19 to 25 × 10 –19 moles-Hg/cell (equivalent to Hg/cell–SH of 0.71 to 71) shifts the major reaction from oxidation to reduction. In the absence of five outer membrane c-type cytochromes, mutant ΔomcBESTZ also shows decreases in Hg reduction and increases in methylation. However, the presence of competing thiol-binding ions such as Zn 2+ leads to increased Hg reduction and decreased methylation. Finally, these results suggest that the coupled cell-Hg sorption and redox transformations are important in controlling the rates of Hg uptake and methylation by G. sulfurreducens PCA in anoxic environments.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2013

Synthesis and Application of an Environmentally Insensitive Cy3-Based Arsenical Fluorescent Probe to Identify Adaptive Microbial Responses Involving Proximal Dithiol Oxidation

Reversible disulfide oxidation between proximal cysteines in proteins represents a common regulatory control mechanism to modulate flux through metabolic pathways in response to changing environmental conditions. To enable in vivo measurements of cellular redox changes linked to disulfide bond formation, we have synthesized a cell-permeable monosubstituted cyanine dye derivatized with arsenic (i.e., TRAP_Cy3) to trap and visualize dithiols in cytosolic proteins. Alkylation of reactive thiols prior to displacement of the bound TRAP-Cy3 by ethanedithiol permits facile protein capture and mass spectrometric identification of proximal reduced dithiols to the exclusion of individual cysteines. Applying TRAP_Cy3 to evaluate cellular responses to increases in oxygen and light levels in the photosynthetic microbe Synechococcus sp. PCC 7002, we observe large decreases in the abundance of reduced dithiols in cellular proteins, which suggest redox-dependent mechanisms involving the oxidation of proximal disulfides. Under these same growth conditions that result in the oxidation of proximal thiols, there is a reduction in the abundance of post-translational oxidative modifications involving nitrotyrosine and methionine sulfoxide formation. These results suggest that the redox status of proximal cysteines respond to environmental conditions, acting to regulate metabolic flux and minimize the formation of reactive oxygen species to decrease oxidative protein damage.

Environmental Molecular Sciences Laboratory↗

DOE OSTI2019

Architecture-Controlled Ring-Opening Polymerization for Dynamic Covalent Poly(disulfide)s

A strategy is discussed for controlling the architecture of poly(disulfide)s by ring-opening polymerization. Aryl thiol initiators shift the ring-chain equilibrium to yield cyclic pol-ymers while alkyl thiols favor linear ones. Control over polymerization enables synthesis of high polymers (630 kDa) and catalytic depolymerization to recycle monomers. Our research offers a new avenue to create dynamic covalent poly-mers with controlled geometry and length, allowing better characterization of structure-property relationships to expand their materials potentials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

A substrate-driven allosteric switch that enhances PDI catalytic activity

Protein disulfide isomerase (PDI) is an oxidoreductase essential for folding proteins in the endoplasmic reticulum. The domain structure of PDI is a–b–b'–x–a', wherein the thioredoxin-like a and a' domains mediate disulfide bond shuffling and b and b' domains are substrate binding. The b' and a' domains are connected via the x-linker, a 19-amino-acid flexible peptide. Here we identify a class of compounds, termed bepristats, that target the substrate-binding pocket of b'. Bepristats reversibly block substrate binding and inhibit platelet aggregation and thrombus formation in vivo . Ligation of the substrate-binding pocket by bepristats paradoxically enhances catalytic activity of a and a' by displacing the x-linker, which acts as an allosteric switch to augment reductase activity in the catalytic domains. This substrate-driven allosteric switch is also activated by peptides and proteins and is present in other thiol isomerases. Our results demonstrate a mechanism whereby binding of a substrate to thiol isomerases enhances catalytic activity of remote domains.

Science & Technology - Other Topics↗

DOE OSTI2024

Ligand-modified nanoparticle surfaces influence CO electroreduction selectivity

Improving the kinetics and selectivity of CO 2 /CO electroreduction to valuable multi-carbon products is a challenge for science and is a requirement for practical relevance. Here we develop a thiol-modified surface ligand strategy that promotes electrochemical CO-to-acetate. We explore a picture wherein nucleophilic interaction between the lone pairs of sulfur and the empty orbitals of reaction intermediates contributes to making the acetate pathway more energetically accessible. Density functional theory calculations and Raman spectroscopy suggest a mechanism where the nucleophilic interaction increases the sp 2 hybridization of CO (ad) , facilitating the rate-determining step, CO* to (CHO)*. We find that the ligands stabilize the (HOOC–CH 2 )* intermediate, a key intermediate in the acetate pathway. In-situ Raman spectroscopy shows shifts in C–O, Cu–C, and C–S vibrational frequencies that agree with a picture of surface ligand-intermediate interactions. A Faradaic efficiency of 70% is obtained on optimized thiol-capped Cu catalysts, with onset potentials 100 mV lower than in the case of reference Cu catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Non-chemisorbed gold–sulfur binding prevails in self-assembled monolayers

Gold–thiol contacts are ubiquitous across the physical and biological sciences in connecting organic molecules to surfaces. When thiols bind to gold in self-assembled monolayers (SAMs) the fate of the hydrogen remains a subject of profound debate—with implications for our understanding of their physical properties, spectroscopic features and formation mechanism(s). Exploiting measurements of the transmission through a molecular junction, which is highly sensitive to the nature of the molecule–electrode contact, we demonstrate here that the nature of the gold–sulfur bond in SAMs can be probed via single-molecule conductance measurements. Critically, we find that SAM measurements of dithiol-terminated molecular junctions yield a significantly lower conductance than solution measurements of the same molecule. Through numerous control experiments, conductance noise analysis and transport calculations based on density functional theory, we show that the gold–sulfur bond in SAMs prepared from the solution deposition of dithiols does not have chemisorbed character, which strongly suggests that under these widely used preparation conditions the hydrogen is retained.

Inkpen, Michael S.↗

DOE OSTI2014

Organo-sulfur molecules enable iron-based battery electrodes to meet the challenges of large-scale electrical energy storage

Rechargeable iron-air and nickel-iron batteries are attractive as sustainable and inexpensive solutions for large-scale electrical energy storage because of the global abundance and eco-friendliness of iron, and the robustness of iron-based batteries to extended cycling. Despite these advantages, the commercial use of iron-based batteries has been limited by their low charging efficiency. This limitation arises from the iron electrodes evolving hydrogen extensively during charging. The total suppression of hydrogen evolution has been a significant challenge. We have found that organo-sulfur compounds with various structural motifs (linear and cyclic thiols, dithiols, thioethers and aromatic thiols) when added in milli-molar concentration to the aqueous alkaline electrolyte, reduce the hydrogen evolution rate by 90%. These organo-sulfur compounds form strongly adsorbed layers on the iron electrode and block the electrochemical process of hydrogen evolution. The charge-transfer resistance and double-layer capacitance of the iron/electrolyte interface confirm that the extent of suppression of hydrogen evolution depends on the degree of surface coverage and the molecular structure of the organo-sulfur compound. An unanticipated electrochemical effect of the adsorption of organo-sulfur molecules is "de-passivation" that allows the iron electrode to be discharged at high current values. The strongly adsorbed organo-sulfur compounds were also found to resist electro-oxidation even at the positive electrode potentials at which oxygen evolution can occur. Through testing on practical rechargeable battery electrodes we have verified the substantial improvements to the efficiency during charging and the increased capability to discharge at high rates. We expect these performance advances to enable the design of efficient, inexpensive and eco-friendly iron-based batteries for large-scale electrical energy storage.

Yang, B↗

DOE OSTI2019

Mechanistic studies of a “Declick” reaction

A kinetic analysis of a “declick” reaction is described. Compound 1, previously reported to couple an amine and a thiol (i.e. “click”) under mild aqueous conditions to create 2, undergoes release of the unaltered coupling partners upon triggering with dithiothreitol (DTT). In the study reported herein various aniline derivatives possessing para-electron donating and withdrawing groups were used as the amines. UV/vis spectroscopy of the declick reaction shows time-dependent spectra lacking isosbestic points, implying a multi-step mechanism. Global data fitting using numerical integration of rate equations and singular value decomposition afforded the spectra and time-dependence of each species, as well as rate constants for each step. The kinetic analysis reveals a multi-step process with an intermediate where both thiols of DTT have added prior to expulsion of the aniline leaving group, followed by rearrangement to the final product. Hammett plots show a negative rho value on two of the steps, indicating positive charge building (i.e. reduction of a negative charge) in the step leading to the intermediate and its ratedetermining breakdown. Overall, the kinetic study reported herein gives a complete mechanistic picture of the declick reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Distonic radical anion species in cysteine oxidation processes

Oxidation of cysteine residues constitutes an important regulatory mechanism in the function of biological systems. Much of this behavior is controlled by the specific chemical properties of the thiol side-chain group, where reac-tions with reactive oxygen species take place. Herein, we in-vestigated the entire cysteine oxidation cycle Cys-SH ? Cys-SOnH (n = 1, 2, 3) using cryogenic negative ion photoelectron spectroscopy and quantum-chemical calculations. The conven-tional view on the first reversible oxidation step (n = 1) is as-sociated with sulfinate species. Yet our results indicate that an alternative option exists in the form of novel distonic radical anion •OS-CH2CH(NH2)-COO- with an unpaired electron on the thiol group and excess negative charge on the carboxylate. Higher order oxidation states (n = 2, 3) are thought to be asso-ciated with irreversible oxidative damage, and our results show that excess negative charge in those cases migrates to -SOn- group. Furthermore, these species are stable towards 1e oxidation, as opposed to the n = 1 case that undergoes intra-molecular proton transfer. The molecular level insights report-ed in this work provide direct spectroscopic evidences towards unique chemical versatility of Cys-sulfenic acid (Cys-SOH) in post-translational modifications of protein systems. This research was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences (X.-B. W. and M. V.) and was performed at the EMSL, a national scientific user facility sponsored by DOE's Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory. Z. B. Q. acknowledges additional support from the National Natural Science Foundation of China (Grant No. 21873003 and 21503003) and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) (Grant No. U1501501) and Super Computation of Shenzhen.

Qin, Zhengbo↗

DOE OSTI2020

Peptoid-directed assembly of CdSe nanoparticles

The high information content of proteins drives their hierarchical assembly and complex function, including the organization of inorganic nanomaterials. Peptoids offer an organic scaffold very similar to proteins, but with a wider solubility range and easily tunable side chains and functional groups to create a variety of self-assembling architectures with atomic precision. If we could harness this paradigm and understand the factors that govern how they direct nucleation and assembly of inorganic materials to design order within such materials, new dimensions of function and fundamental science would emerge. In this work, peptoid tubes and sheets were explored as platforms to assemble colloidal quantum dots (QDs) and clusters. We have successfully synthesized CdSe QDs with difunctionalized capping ligands containing both carboxylic acid and thiol groups and mixed them with maleimide containing peptoids, to create an assembly of the QDs on the peptoid surface via a covalent linkage. This conjugation was seen to be successful with peptoid tubes, sheets and CdSe QDs and clusters. The particles were seen to have a high preference for the peptoid surface but non-specific interactions with carboxylic acid groups on the peptoids limited control over QD density via maleimide conjugation. Replacing the carboxylic acid groups with methoxy ethers, however, allowed for control over QD density as a function of maleimide concentration. 1 H NMR analysis demonstrated that binding of QDs to peptoids involved a subset of surface ligands bound through the carboxylate functional group, allowing the distal thiol to engage in a covalent linkage to the maleimide. Overall, we have shown the compatibility and control of CdSe-peptoid interactions via a covalent linkage with varying peptoid structures and CdSe particles to create complex hybrid structures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2020

Temperature-mediated molecular ladder self-assembly employing Diels–Alder cycloaddition

Dynamic covalent self-assembly processes often exhibit poor capacities for error-correction owing to the relatively low connectivity rearrangement rates of dynamic covalent interactions and the common use of reaction conditions where the equilibrium state remains fixed. Here, we report a dynamic covalent self-assembly technique employing temperature, a conventional, externally-applied stimulus, to mediate the hybridization of peptoid oligomers bearing maleimide- and furan-based pendant groups to afford molecular ladders incorporating Diels–Alder adduct-based rungs. By raising or lowing the reaction temperature, this system enables the equilibrium state to be readily varied without altering reagent concentrations. Both triethylamine and the Lewis acidic scandium triflate were examined as candidate reaction catalysts; however, only scandium triflate increased the rate of single strand conversion. Additionally, as the Diels–Alder cycloaddition reaction does not liberate a small molecule, a registry-dependent mass change was effected by employing a base-catalyzed thiol-Michael addition reaction between any un-reacted maleimide pendant groups and a low molecular weight thiol to enable the number of Diels–Alder adduct rungs to be readily determined by mass spectrometry. Finally, by employing a slow temperature ramp from high to low temperature, approximating the thermal cycle employed for nucleic acid hybridization, sequence-selective hybridization between model, tetra-functional precursor strands was demonstrated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Ion exchange behavior of astatine and bismuth

Astatine and bismuth sorption on several ion exchange resins from nitric acid media has been studied. This work covers commercially available resins Dowex 50 × 4, MP thiol, Dowex 1 × 8, and TEVA. One of the main advantages of using ion exchangers for astatine separation and purification is the absence of any organic media in the effluent in comparison with extraction chromatography resins. The behavior of the above-mentioned metals was investigated in up to 4 M HNO 3 solutions. The determined distribution coefficients are greater than 1 for all the resins studied, reaching 10 4 for MP thiol and TEVA resins. The interaction of nitric acid with an anion exchanger in Cl-form results in an exchange reaction, where AtO + chloro complexes can form. To understand the astatine behavior under these conditions, stability constants of AtOCl and AtOCl 2 – complexes have been reevaluated and discrepancy in the literature values has been eliminated. For each resin a thermodynamic model has been developed to suggest a possible mechanism of astatine sorption. Literature and new experimental data on bismuth sorption by studying resins in nitric acid media have been reviewed and a mathematical model to describe its behavior has been suggested. Furthermore, a ratio of corresponding fit functions of At and Bi assigned to the same resin and acidity has been used to estimate the separation factors of these elements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Neutral rhenium( i ) tricarbonyl complexes with sulfur-donor ligands: anti-proliferative activity and cellular localization

Rhenium(I) tricarbonyl complexes are widely studied for their cell imaging properties and anti-cancer and anti-microbial activities, but the complexes with S-donor ligands remain relatively unexplored. A series of six fac-[Re(NN)(CO) 3 (SR)] complexes, where (NN) is 2,2'-bipyridyl (bipy) or 1,10-phenanthroline (phen), and RSH is a series of thiocarboxylic acid methyl esters, have been synthesized and characterized. Cellular uptake and anti-proliferative activities of these complexes in human breast cancer cell lines (MDA-MB-231 and MCF-7) were generally lower than those of the previously described fac-[Re(NN)(CO) 3 (OH 2 )]+ complexes; however, one of the complexes, fac-[Re(CO) 3 (phen)(SC(Ph)CH 2 C(O)OMe)] (3b), was active (IC 50 ~ 10 μM at 72 h treatment) in thiol-depleted MDA-MB-231 cells. Moreover, unlike fac-[Re(CO) 3 (phen)(OH 2 )]+, this complex did not lose activity in the presence of extracellular glutathione. Taken together these properties show promise for further development of 3b and its analogues as potential anti-cancer drugs for co-treatment with thiol-depleting agents. Conversely, the stable and non-toxic complex, fac-[Re(bipy)(CO) 3 (SC(Me)C(O)OMe)] (1a), predominantly localized in the lysosomes of MDA-MB-231 cells, as shown by live cell confocal microscopy (λ ex = 405 nm, λ em = 470–570 nm). It is strongly localized in a subset of lysosomes (25 μM Re, 4 h treatment), as shown by co-localization with a Lysotracker dye. Longer treatment times with 1a (25 μM Re for 48 h) resulted in partial migration of the probe into the mitochondria, as shown by co-localization with a Mitotracker dye. These properties make complex 1a an attractive target for further development as an organelle probe for multimodal imaging, including phosphorescence, carbonyl tag for vibrational spectroscopy, and Re tag for X-ray fluorescence microscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

First principles investigations on the electronic structure of anchor groups on ZnO nanowires and surfaces

We report on density functional theory investigations of the electronic properties of monofunctional ligands adsorbed on ZnO-(1010) surfaces and ZnO nanowires using semi-local and hybrid exchange-correlation functionals. We consider three anchor groups, namely thiol, amino, and carboxyl groups. Our results indicate that neither the carboxyl nor the amino group modify the transport and conductivity properties of ZnO. In contrast, the modification of the ZnO surface and nanostructure with thiol leads to insertion of molecular states in the band gap, thus suggesting that functionalization with this moiety may customize the optical properties of ZnO nanomaterials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2015

Recapitulating the Structural Evolution of Redox Regulation in Adenosine 5′-Phosphosulfate Kinase from Cyanobacteria to Plants

In plants, adenosine 5'-phosphosulfate (APS) kinase (APSK) is required for reproductive viability and the production of 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as a sulfur donor in specialized metabolism. Previous studies of the APSK from Arabidopsis thaliana (AtAPSK) identified a regulatory disulfide bond formed between the N-terminal domain (NTD) and a cysteine on the core scaffold. This thiol switch is unique to mosses, gymnosperms, and angiosperms. To understand the structural evolution of redox control of APSK, we investigated the redox-insensitive APSK from the cyanobacterium Synechocystis sp. PCC 6803 (SynAPSK). Crystallographic analysis of SynAPSK in complex with either APS and a non-hydrolyzable ATP analog or APS and sulfate revealed the overall structure of the enzyme, which lacks the NTD found in homologs from mosses and plants. A series of engineered SynAPSK variants reconstructed the structural evolution of the plant APSK. Biochemical analyses of SynAPSK, SynAPSK H23C mutant, SynAPSK fused to the AtAPSK NTD, and the fusion protein with the H23C mutation showed that the addition of the NTD and cysteines recapitulated thiol-based regulation. Here, these results reveal the molecular basis for structural changes leading to the evolution of redox control of APSK in the green lineage from cyanobacteria to plants.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2017

The thioreduction component CcmG confers efficiency and the heme ligation component CcmH ensures stereo-specificity during cytochrome c maturation

In many Gram-negative bacteria, including Rhodobacter capsulatus, cytochrome c maturation (Ccm) is carried out by a membrane-integral machinery composed of nine proteins (CcmA to I). During this process, the periplasmic thiol-disulfide oxidoreductase DsbA is thought to catalyze the formation of a disulfide bond between the Cys residues at the apocytochrome c heme binding site (CXXCH). Subsequently, a Ccm-specific thioreductive pathway involving CcmG and CcmH reduces this disulfide bond to allow covalent heme ligation. Currently, the sequence of thioredox reactions occurring between these components and apocytochrome c, and the identity of their active Cys residues are unknown. In this work, we first investigated protein-protein interactions among the apocytochrome c, CcmG, and the heme ligation components CcmF, CcmH, and CcmI. We found that they all interact with each other, forming a CcmFGHI-apocytochrome c complex. Using purified wild-type CcmG, CcmH, and apocytochrome c, as well as their respective Cys mutant variants we determined the rates of thiol-disulfide exchange reactions between selected pairs of Cys residues from these proteins. We established that CcmG can efficiently reduce the disulfide bond of apocytochrome c and also resolve a mixed disulfide bond formed between apocytochrome c and CcmH. We further show that C45 of CcmH and C34 of apocytochrome c are most likely to form this mixed disulfide bond, which is consistent with the stereo-specificity of the heme-apocytochrome c ligation reaction. We conclude that CcmG confers efficiency, and CcmH ensures stereo-specificity during Ccm, and present a comprehensive model for thioreduction reactions that lead to heme-apocytochrome c ligation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

A XANES and Raman investigation of sulfur speciation and structural order in Murchison and Allende meteorites

Abstract Insoluble organic matter ( IOM ) and hydrothermally treated IOM extracted from two carbonaceous chondrites, Murchison and Allende, was studied using sulfur K‐edge XANES (X‐ray absorption near edge structure) and μ‐Raman spectroscopy, with the aim to understand their IOM 's sulfur speciation and structural order, and how aqueous alteration or thermal metamorphism may have transformed these materials. We found that the sulfur‐functional group chemistry of both the Murchison IOM and hydrothermally treated IOM samples have a large chemical variability ranging from oxidation states of S −2 to S +6 , and exhibit a transformation in their oxidation state after the hydrothermal treatment ( HT ) to produce thiophenes and thiol compounds. Sulfoxide and sulfite peaks are also present in Murchison. Sulfates considered intrinsic to Murchison are most likely preaccretionary in nature, and not a result of reactions with water at high temperatures on the asteroid parent body. We argue that the reduced sulfides may have formed in the CM parent body, while the thiophenes and thiol compounds are a result of the HT . Micro‐Raman spectra show the presence of aliphatic and aromatic moieties in Murchison's material as observed previously, which exhibits no change after HT . Because the Murchison IOM was modified, as seen by XANES analysis, absence of a change observed using micro‐Raman indicated that although the alkyl carbons of IOM were cleaved, the aromatic network was not largely modified after HT . By contrast, Allende IOM contains primarily disulfide and elemental sulfur, no organic sulfur, and shows no transformation after HT . This nontransformation of Allende IOM after HT would indicate that parent body alteration of sulfide to sulfate is not feasible up to temperatures of 300°C. The reduced sulfur products indicate extreme secondary chemical processing from the precursor compounds in its parent body at temperatures as high as 624°C, as estimated from μ‐Raman D band parameters. The Raman parameters in Allende IOM that was interpreted in terms of amorphous carbon with regions of large clusters of benzene rings, was transformed after the HT to those with fewer benzene rings.

Bose, M.↗

DOE OSTI2023

Redox regulation in chloroplast thylakoid lumen: The pmf changes everything, again

Abstract Photosynthesis is the foundation of life on Earth. However, if not well regulated, it can also generate excessive reactive oxygen species (ROS), which can cause photodamage. Regulation of photosynthesis is highly dynamic, responding to both environmental and metabolic cues, and occurs at many levels, from light capture to energy storage and metabolic processes. One general mechanism of regulation involves the reversible oxidation and reduction of protein thiol groups, which can affect the activity of enzymes and the stability of proteins. Such redox regulation has been well studied in stromal enzymes, but more recently, evidence has emerged of redox control of thylakoid lumenal enzymes. This review/hypothesis paper summarizes the latest research and discusses several open questions and challenges to achieving effective redox control in the lumen, focusing on the distinct environments and regulatory components of the thylakoid lumen, including the need to transport electrons across the thylakoid membrane, the effects of pH changes by the proton motive force (pmf) in the stromal and lumenal compartments, and the observed differences in redox states. These constraints suggest that activated oxygen species are likely to be major regulatory contributors to lumenal thiol redox regulation, with key components and processes yet to be discovered.

Plant Sciences↗