The Reactive Sulfur Species Concept: 15 Years On

Reactivity of Small Oxoacids of Sulfur

Oxidation of sulfide to sulfate is known to consist of several steps. Key intermediates in this process are the so-called small oxoacids of sulfur (SOS)—sulfenic HSOH (hydrogen thioperoxide, oxadisulfane, or sulfur hydride hydroxide) and sulfoxylic S(OH)₂ acids. Sulfur monoxide can be considered as a dehydrated form of sulfoxylic acid. Although all of these species play an important role in atmospheric chemistry and in organic synthesis, and are also invoked in biochemical processes, they are quite unstable compounds so much so that their physical and chemical properties are still subject to intense studies. It is well-established that sulfoxylic acid has very strong reducing properties, while sulfenic acid is capable of both oxidizing and reducing various substrates. Here, in this review, the mechanisms of sulfide oxidation as well as data on the structure and reactivity of small sulfur-containing oxoacids, sulfur monoxide, and its precursors are discussed.

Synthesis and computer-aided SAR studies for derivatives of phenoxyalkyl-1,3,5-triazine as the new potent ligands for serotonin receptors 5-HT6

This research has provided the most active 5-HT₆R agents among 1,3,5-triazine derivatives investigated to date and has also identified the world's first selenium-containing 5-HT₆R ligands. The studies are focused on design, synthesis, biological evaluation and docking-supported SAR analysis for novel 5-HT₆R agents as derivatives of lead structure 4-(4-methylpiperazin-1-yl)-6-(phenoxymethyl)-1,3,5-triazin-2-amine (7). The lead modifications included an introduction of: (i) various small substituents at benzene ring, (ii) a branched ether linker or (iii) the ether oxygen replacement with other chalcogen (S, Se) or sulfonyl moiety. Hence, a series of new compounds (7-24) was synthesized and examined on their affinities for 5-HT₆R and selectivity, in respect to the 5-HT_1AR, 5-HT_2AR, 5-HT₇R and dopamine D₂ receptor, in the radioligand binding assays. For representative most active compounds functional bioassays and toxicity profile in vitro and antidepressant-like activity in vivo were examined. The 2-isopropyl-5-methylphenyl derivative (10) was found as the most active triazine 5-HT₆R antagonist (K_i = 11 nM). SAR analysis indicated, that an exchange of oxygen to selenium (7 vs. 22), and especially, to sulfur (7 vs. 19) was beneficial to increase both affinity and antagonistic action for 5-HT₆R. Surprisingly, an introduction of SO₂ caused a drastic decrease of the 5-HT₆R affinity, which was explained at a molecular level based on docking studies. All in vivo tested compounds (10, 18 and 21) did not show any risk of toxicity in the safety studies in vitro.

Oxidative stress and antioxidant treatment in patients with peripheral artery disease

Peripheral artery disease is an atherosclerotic disease of arterial vessels that mostly affects arteries of lower extremities. Effort induced cycles of ischemia and reperfusion lead to increased reactive oxygen species production by mitochondria. Therefore, the pathophysiology of peripheral artery disease is a consequence of metabolic myopathy, and oxidative stress is the putative major operating mechanism behind the structural and metabolic changes that occur in muscle. In this review, we discuss the evidence for oxidative damage in peripheral artery disease and discuss management strategies related to antioxidant supplementation. We also highlight the major pathways governing oxidative stress in the disease and discuss their implications in disease progression. Potential therapeutic targets and diagnostic methods related to these mechanisms are explored, with an emphasis on the Nrf2 pathway.

Redox Mechanisms in Neurodegeneration: From Disease Outcomes to Therapeutic Opportunities

SIGNIFICANCE

Once considered to be mere by-products of metabolism, reactive oxygen, nitrogen and sulfur species are now recognized to play important roles in diverse cellular processes such as response to pathogens and regulation of cellular differentiation. It is becoming increasingly evident that redox imbalance can impact several signaling pathways. For instance, disturbances of redox regulation in the brain mediate neurodegeneration and alter normal cytoprotective responses to stress. Very often small disturbances in redox signaling processes, which are reversible, precede damage in neurodegeneration. Recent Advances: The identification of redox-regulated processes, such as regulation of biochemical pathways involved in the maintenance of redox homeostasis in the brain has provided deeper insights into mechanisms of neuroprotection and neurodegeneration. Recent studies have also identified several post-translational modifications involving reactive cysteine residues, such as nitrosylation and sulfhydration, which fine-tune redox regulation. Thus, the study of mechanisms via which cell death occurs in several neurodegenerative disorders, reveal several similarities and dissimilarities. Here, we review redox regulated events that are disrupted in neurodegenerative disorders and whose modulation affords therapeutic opportunities.

CRITICAL ISSUES

Although accumulating evidence suggests that redox imbalance plays a significant role in progression of several neurodegenerative diseases, precise understanding of redox regulated events is lacking. Probes and methodologies that can precisely detect and quantify in vivo levels of reactive oxygen, nitrogen and sulfur species are not available.

FUTURE DIRECTIONS

Due to the importance of redox control in physiologic processes, organisms have evolved multiple pathways to counteract redox imbalance and maintain homeostasis. Cells and tissues address stress by harnessing an array of both endogenous and exogenous redox active substances. Targeting these pathways can help mitigate symptoms associated with neurodegeneration and may provide avenues for novel therapeutics. Antioxid. Redox Signal. 30, 1450-1499.

Pushing arterial-venous plasma biomarkers to new heights: A model for personalised redox metabolomics?

The chemical and functional interactions between Reactive Oxygen (ROS), Nitrogen (RNS) and Sulfur (RSS) species allow organisms to detect and respond to metabolic and environmental stressors, such as exercise and altitude exposure. Whether redox markers and constituents of this ‘Reactive Species Interactome’ (RSI) differ in concentration between arterial and venous blood is unknown. We hypothesised that such measurements may provide useful insight into metabolic/redox regulation at the whole-body level and would be consistent between individuals exposed to identical challenges. An exploratory study was performed during the Xtreme Alps expedition in 2010 in which four healthy individuals (2 male, 2 female) underwent paired arterial and central venous blood sampling before, during and after performance of a constant-work-rate cardiopulmonary exercise test, at sea level and again at 4559 m. Unexpectedly, plasma total free thiol and free cysteine concentrations remained substantially elevated at altitude throughout exercise with minimal arteriovenous gradients. Free sulfide concentrations changed only modestly upon combined altitude/exercise stress, whereas bound sulfide levels were lower at altitude than sea-level. No consistent signal indicative of the expected increased oxidative stress and nitrate→nitrite→NO reduction was observed with 4-hydroxynonenal, isoprostanes, nitrate, nitrite, nitroso species and cylic guanosine monophosphate. However, the observed arteriovenous concentration differences revealed a dynamic pattern of response that was unique to each participant. This novel redox metabolomic approach of obtaining quantifiable ‘metabolic signatures’ to a defined physiological challenge could potentially offer new avenues for personalised medicine.

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Exercise and high altitude (hypobaric hypoxia) significantly perturb redox balance.

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The role of thiols and reactive sulfur species in altitude acclimatization remains largely unknown.

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First measure of arteriovenous gradients of redox markers at altitude.

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Different individuals exposed to identical stresses display distinct redox response profiles.

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Redox metabolomics may offer new ways of personalizing medicine.

An Assessment of Computational Methods for Calculating Accurate Structures and Energies of Bio-Relevant Polysulfur/Selenium-Containing Compounds

The heavier chalcogens sulfur and selenium are important in organic and inorganic chemistry, and the role of such chalcogens in biological systems has recently gained more attention. Sulfur and, to a lesser extent selenium, are involved in diverse reactions from redox signaling to antioxidant activity and are considered essential nutrients. We investigated the ability of the DFT functionals (B3LYP, B3PW91, ωB97XD, M06-2X, and M08-HX) relative to electron correlation methods MP2 and QCISD to produce reliable and accurate structures as well as thermochemical data for sulfur/selenium-containing systems. Bond lengths, proton affinities (PA), gas phase basicities (GPB), chalcogen–chalcogen bond dissociation enthalpies (BDE), and the hydrogen affinities (HA) of thiyl/selenyl radicals were evaluated for a range of small polysulfur/selenium compounds and cysteine per/polysulfide. The S–S bond length was found to be the most sensitive to basis set choice, while the geometry of selenium-containing compounds was less sensitive to basis set. In mixed chalcogens species of sulfur and selenium, the location of the sulfur atom affects the S–Se bond length as it can hold more negative charge. PA, GPB, BDE, and HA of selenium systems were all lower, indicating more acidity and more stability of radicals. Extending the sulfur chain in cysteine results in a decrease of BDE and HA, but these plateau at a certain point (199 kJ mol⁻¹ and 295 kJ mol⁻¹), and PA and GPB are also decreased relative to the thiol, indicating that the polysulfur species exist as thiolates in a biological system. In general, it was found that ωB97XD/6-311G(2d,p) gave the most reasonable structures and thermochemistry relative to benchmark calculations. However, nuances in performance are observed and discussed.

Copper signalling: causes and consequences

Copper-containing enzymes perform fundamental functions by activating dioxygen (O2) and therefore allowing chemical energy-transfer for aerobic metabolism. The copper-dependence of O2 transport, metabolism and production of signalling molecules are supported by molecular systems that regulate and preserve tightly-bound static and weakly-bound dynamic cellular copper pools. Disruption of the reducing intracellular environment, characterized by glutathione shortage and ambient Cu(II) abundance drives oxidative stress and interferes with the bidirectional, copper-dependent communication between neurons and astrocytes, eventually leading to various brain disease forms. A deeper understanding of of the regulatory effects of copper on neuro-glia coupling via polyamine metabolism may reveal novel copper signalling functions and new directions for therapeutic intervention in brain disorders associated with aberrant copper metabolism.

Mitochondrial Specific H2S n Fluorogenic Probe for Live Cell Imaging by Rational Utilization of a Dual-Functional-Photocage Group

Reactive sulfur species play a very important role in modulating neural signal transmission. Hydrogen polysulfides (H₂S _n, n > 1) are recently suggested to be the actual signaling molecules. There are still few spatiotemporal controllable-based probes to detect H₂S _n. In this work, for the first time, we proposed the photocleavage product of the common photoremovable protecting group (2-nitrophenyl moiety) capable of trapping H₂S _n. Taking advantage of this, we constructed the probe H1 containing a photocontrollable group, a mitochondrial directing unit and a signal reporter fluorescein dye. H1 exhibited excellent fluorescence enhancement (50-fold) in response to H₂S _n under the aqueous buffer only after UV irradiation. H1 also showed high selectivity and sensitivity for H₂S _n over other reactive sulfur species, reactive oxygen species, and other analytes, especially biothoils including hydrogen sulfide, cysteine, homocysteine, and glutathione. We showed the utility of H1 to image H₂S _n in living cells with high spatiotemporal resolution.

Redox Signaling by Reactive Electrophiles and Oxidants

The concept of cell signaling in the context of nonenzyme-assisted protein modifications by reactive electrophilic and oxidative species, broadly known as redox signaling, is a uniquely complex topic that has been approached from numerous different and multidisciplinary angles. Our Review reflects on five aspects critical for understanding how nature harnesses these noncanonical post-translational modifications to coordinate distinct cellular activities: (1) specific players and their generation, (2) physicochemical properties, (3) mechanisms of action, (4) methods of interrogation, and (5) functional roles in health and disease. Emphasis is primarily placed on the latest progress in the field, but several aspects of classical work likely forgotten/lost are also recollected. For researchers with interests in getting into the field, our Review is anticipated to function as a primer. For the expert, we aim to stimulate thought and discussion about fundamentals of redox signaling mechanisms and nuances of specificity/selectivity and timing in this sophisticated yet fascinating arena at the crossroads of chemistry and biology.

Chemical fingerprints of cold physical plasmas - an experimental and computational study using cysteine as tracer compound

Reactive oxygen and nitrogen species released by cold physical plasma are being proposed as effectors in various clinical conditions connected to inflammatory processes. As these plasmas can be tailored in a wide range, models to compare and control their biochemical footprint are desired to infer on the molecular mechanisms underlying the observed effects and to enable the discrimination between different plasma sources. Here, an improved model to trace short-lived reactive species is presented. Using FTIR, high-resolution mass spectrometry, and molecular dynamics computational simulation, covalent modifications of cysteine treated with different plasmas were deciphered and the respective product pattern used to generate a fingerprint of each plasma source. Such, our experimental model allows a fast and reliable grading of the chemical potential of plasmas used for medical purposes. Major reaction products were identified to be cysteine sulfonic acid, cystine, and cysteine fragments. Less-abundant products, such as oxidized cystine derivatives or S-nitrosylated cysteines, were unique to different plasma sources or operating conditions. The data collected point at hydroxyl radicals, atomic O, and singlet oxygen as major contributing species that enable an impact on cellular thiol groups when applying cold plasma in vitro or in vivo.

Natural Nanoparticles: A Particular Matter Inspired by Nature

During the last couple of decades, the rapidly advancing field of nanotechnology has produced a wide palette of nanomaterials, most of which are considered as “synthetic” and, among the wider public, are often met with a certain suspicion. Despite the technological sophistication behind many of these materials, “nano” does not always equate with “artificial”. Indeed, nature itself is an excellent nanotechnologist. It provides us with a range of fine particles, from inorganic ash, soot, sulfur and mineral particles found in the air or in wells, to sulfur and selenium nanoparticles produced by many bacteria and yeasts. These nanomaterials are entirely natural, and, not surprisingly, there is a growing interest in the development of natural nanoproducts, for instance in the emerging fields of phyto- and phyco-nanotechnology. This review will highlight some of the most recent—and sometimes unexpected—advances in this exciting and diverse field of research and development. Naturally occurring nanomaterials, artificially produced nanomaterials of natural products as well as naturally occurring or produced nanomaterials of natural products all show their own, particular chemical and physical properties, biological activities and promise for applications, especially in the fields of medicine, nutrition, cosmetics and agriculture. In the future, such natural nanoparticles will not only stimulate research and add a greener outlook to a traditionally high-tech field, they will also provide solutions—pardon—suspensions for a range of problems. Here, we may anticipate specific biogenic factories, valuable new materials based on waste, the effective removal of contaminants as part of nano-bioremediation, and the conversion of poorly soluble substances and materials to biologically available forms for practical uses.