Efficient preparation of unsymmetrical alkyl–aryl tellurides via a nickel-catalyzed reductive coupling strategy

A selective cross-coupling between unactivated alkyl bromides and diaryl tellurides through reductive nickel-catalysis is described, which efficiently led to unsymmetrical alkyl–aryl tellurides.

Reversibly responsive microemulsion triggered by redox reactions

HYPOTHESIS

Stimuli-responsive surfactants (also known as switchable surfactants) can undergo reversible conversions between active and inactive forms under particular stimuli, affecting surface and interfacial activity, aggregation structure, emulsification and solubilisation. Selenium-containing surfactants are of reversibly redox-responsive. Hence, microemulsions (MEs) stabilized by selenium-containing surfactants should reversibly respond to redox reactions.

EXPERIMENTS

The formation of MEs, consisting of sodium dodecylselanylpropyl sulfate (reduced form, SDSePS-Re) or its oxidized form (SDSePS-Ox), n-butanol, n-heptane, and water, was verified based on a pseudo-ternary phase diagram. Changes in molecular structure between SDSePS-Re and SDSePS-Ox were verified by nuclear magnetic resonance spectrometry and electrospray ionization mass spectrometry. The transition between SDSePS-Re- and SDSePS-Ox -based MEs was systematically characterized through electrical conductivity measurements, cryo-transmission electron microscopy and dynamic light-scattering.

FINDINGS

Both SDSePS-Re and SDSePS-Ox could stabilize the mixture of n-butanol-n-heptane-water to form MEs. A reversible transition between an SDSePS-Re-based ME and the corresponding SDSePS-Ox-based ME was achieved, which was realized by the oxidation of SDSePS-Re with H₂O₂ and then reduction with N₂H₄. Compared with SDSePS-Re, SDSePS-Ox has a lower surface activity, resulting in a difference in solubilization capacity of the oil between SDSePS-Re- and SDSePS-Ox -based MEs. After oxidation with H₂O₂, phase separation could be observed in some SDSePS-Re-based MEs; however, the SDSePS-Re-based MEs could be recovered after reduction of SDSePS-Ox-based MEs with N₂H₄.

Small Molecule Catalysts with Therapeutic Potential

Catalysts are employed in many areas of research and development where they combine high efficiency with often astonishing selectivity for their respective substrates. In biology, biocatalysts are omnipresent. Enzymes facilitate highly controlled, sophisticated cellular processes, such as metabolic conversions, sensing and signalling, and are prominent targets in drug development. In contrast, the therapeutic use of catalysts per se is still rather limited. Recent research has shown that small molecule catalytic agents able to modulate the redox state of the target cell bear considerable promise, particularly in the context of inflammatory and infectious diseases, stroke, ageing and even cancer. Rather than being "active" on their own in a more traditional sense, such agents develop their activity by initiating, promoting, enhancing or redirecting reactions between biomolecules already present in the cell, and their activity therefore depends critically on the predisposition of the target cell itself. Redox catalysts, for instance, preferably target cells with a distinct sensitivity towards changes in an already disturbed redox balance and/or increased levels of reactive oxygen species. Indeed, certain transition metal, chalcogen and quinone agents may activate an antioxidant response in normal cells whilst at the same time triggering apoptosis in cancer cells with a different pre-existing "biochemical redox signature" and closer to the internal redox threshold. In pharmacy, catalysts therefore stand out as promising lead structures, as sensor/effector agents which are highly effective, fairly selective, active in catalytic, i.e., often nanomolar concentrations and also very flexible in their structural design.

Selenazolinium Salts as "Small Molecule Catalysts" with High Potency against ESKAPE Bacterial Pathogens

In view of the pressing need to identify new antibacterial agents able to combat multidrug-resistant bacteria, we investigated a series of fused selenazolinium derivatives (1–8) regarding their in vitro antimicrobial activities against 25 ESKAPE-pathogen strains. Ebselen was used as reference compound. Most of the selenocompounds demonstrated an excellent in vitro activity against all S. aureus strains, with activities comparable to or even exceeding the one of ebselen. In contrast to ebselen, some selenazolinium derivatives (1, 3, and 7) even displayed significant actions against all Gram-negative pathogens tested. The 3-bromo-2-(1-hydroxy-1-methylethyl)[1,2]selenazolo[2,3-a]pyridinium chloride (1) was particularly active (minimum inhibitory concentrations, MICs: 0.31–1.24 µg/mL for MRSA, and 0.31–2.48 µg/mL for Gram-negative bacteria) and devoid of any significant mutagenicity in the Ames assay. Our preliminary mechanistic studies in cell culture indicated that their mode of action is likely to be associated with an alteration of intracellular levels of glutathione and cysteine thiols of different proteins in the bacterial cells, hence supporting the idea that such compounds interact with the intracellular thiolstat. This alteration of pivotal cysteine residues is most likely the result of a direct or catalytic oxidative modification of such residues by the highly reactive selenium species (RSeS) employed.

The Reactive Sulfur Species Concept: 15 Years On

Fifteen years ago, in 2001, the concept of “Reactive Sulfur Species” or RSS was advocated as a working hypothesis. Since then various organic as well as inorganic RSS have attracted considerable interest and stimulated many new and often unexpected avenues in research and product development. During this time, it has become apparent that molecules with sulfur-containing functional groups are not just the passive “victims” of oxidative stress or simple conveyors of signals in cells, but can also be stressors in their own right, with pivotal roles in cellular function and homeostasis. Many “exotic” sulfur-based compounds, often of natural origin, have entered the fray in the context of nutrition, ageing, chemoprevention and therapy. In parallel, the field of inorganic RSS has come to the forefront of research, with short-lived yet metabolically important intermediates, such as various sulfur-nitrogen species and polysulfides (S_x²⁻), playing important roles. Between 2003 and 2005 several breath-taking discoveries emerged characterising unusual sulfur redox states in biology, and since then the truly unique role of sulfur-dependent redox systems has become apparent. Following these discoveries, over the last decade a “hunt” and, more recently, mining for such modifications has begun—and still continues—often in conjunction with new, innovative and complex labelling and analytical methods to capture the (entire) sulfur “redoxome”. A key distinction for RSS is that, unlike oxygen or nitrogen, sulfur not only forms a plethora of specific reactive species, but sulfur also targets itself, as sulfur containing molecules, i.e., peptides, proteins and enzymes, preferentially react with RSS. Not surprisingly, today this sulfur-centred redox signalling and control inside the living cell is a burning issue, which has moved on from the predominantly thiol/disulfide biochemistry of the past to a complex labyrinth of interacting signalling and control pathways which involve various sulfur oxidation states, sulfur species and reactions. RSS are omnipresent and, in some instances, are even considered as the true bearers of redox control, perhaps being more important than the Reactive Oxygen Species (ROS) or Reactive Nitrogen Species (RNS) which for decades have dominated the redox field. In other(s) words, in 2017, sulfur redox is “on the rise”, and the idea of RSS resonates throughout the Life Sciences. Still, the RSS story isn’t over yet. Many RSS are at the heart of “mistaken identities” which urgently require clarification and may even provide the foundations for further scientific revolutions in the years to come. In light of these developments, it is therefore the perfect time to revisit the original hypotheses, to select highlights in the field and to question and eventually update our concept of “Reactive Sulfur Species”.

Organotellurium scaffolds for mass cytometry reagent development

Tellurophenes are promising biocompatible mass tags.

Special issue: redox active natural products and their interaction with cellular signalling pathways

During the last decade, research into natural products has experienced a certain renaissance. The urgent need for more and more effective antibiotics in medicine, the demand for ecologically friendly plant protectants in agriculture, “natural” cosmetics and the issue of a sustainable and healthy nutrition in an ageing society have fuelled research into Nature’s treasure chest of “green gold”. Here, redox active secondary metabolites from plants, fungi, bacteria and other (micro-)organisms often have been at the forefront of the most interesting developments. These agents provide powerful means to interfere with many, probably most cellular signaling pathways in humans, animals and lower organisms, and therefore can be used to protect, i.e., in form of antioxidants, and to frighten off or even kill, i.e., in form of repellants, antibiotics, fungicides and selective, often catalytic “sensor/effector” anticancer agents. Interestingly, whilst natural product research dates back many decades, in some cases even centuries, and compounds such as allicin and various flavonoids have been investigated thoroughly in the past, it has only recently become possible to investigate their precise interactions and mode(s) of action inside living cells. Here, fluorescent staining and labelling on the one side, and appropriate detection, either qualitatively under the microscope or quantitatively in flow cytometers and plate readers, on the other, enable researchers to obtain the various pieces of information necessary to construct a fairly complete puzzle of how such compounds act and interact in living cells. Complemented by the more traditional activity assays and Western Blots, and increasingly joined by techniques such as proteomics, chemogenetic screening and mRNA profiling, these cell based bioanalytical techniques form a powerful platform for “intracellular diagnostics”. In the case of redox active compounds, especially of Reactive Sulfur Species (RSS), such techniques have recently unraveled concepts such as the “cellular thiolstat”, yet considerably more research is required in order to gain a full understanding of why and how such compounds act—often selectively—in different organisms.

Selenium analogues of raloxifene as promising antiproliferative agents in treatment of breast cancer

Synthetic protocols for the preparation of selenium analogues of raloxifene were elaborated. General aim of the current research is to improve the positive impact of selenium atom introduction in drug design. Antiproliferative activity on CCL-8 (mouse sarcoma), MDA-MB-435s (human melanoma), MES-SA (human uterus sarcoma), MCF-7 (human breast adenocarcinoma), HT-1080 (human fibrosarcoma), MG-22A (mouse hepatoma) tumor cell lines, and normal cell line NIH 3T3 (mouse fibroblasts) was studied. Influence of aminoethoxy "tail" and benzoyl group position on SAR was discussed. Results of in vivo studies on BALB/c female mice with 4T1 cell induced breast cancer model showed that selenium analogue of raloxifene is able to suppress estrogen-depending tumor growth.

Binding of a cyclic organoselenium compound with gold nanoparticles (GNP) and its effect on electron transfer properties

Binding of a cyclic organoselenium compound, DL-trans-3,4-dihydroxy-1-selenolane (DHSred) with gold nanoparticles (GNP) of different sizes was studied by absorption spectroscopy, dynamic light scattering (DLS), transmission electron microscope (TEM), surface enhanced Raman spectroscopy (SERS) and zeta-potential (ζ) measurements. GNP of different size were synthesized by varying the reaction conditions and their size was determined by DLS and TEM techniques. The absorption spectral data showed red shift in the surface plasmon resonance (SPR) band indicating increase in the size of GNP on binding to DHSred. SERS studies confirmed that the binding of DHSred with GNP is through selenium center with planar orientation of DHSred on the GNP surface. The product of the number of binding sites (n) in GNP and the binding constant (K) was estimated for GNP of different particle size. The zeta potential (ζ) value of GNP decreased marginally in the presence of DHSred. Further, the binding of DHSred with GNP was found to enhance its reactivity with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) radicals (ABTS(·-)) and the reactivity increased with decrease in the GNP size. Such enhancement in the reducing ability may have a greater impact on the antioxidant activity of DHSred.

Intracellular diagnostics: hunting for the mode of action of redox-modulating selenium compounds in selected model systems

Redox-modulating compounds derived from natural sources, such as redox active secondary metabolites, are currently of considerable interest in the field of chemoprevention, drug and phytoprotectant development. Unfortunately, the exact and occasionally even selective activity of such products, and the underlying (bio-)chemical causes thereof, are often only poorly understood. A combination of the nematode- and yeast-based assays provides a powerful platform to investigate a possible biological activity of a new compound and also to explore the “redox link” which may exist between its activity on the one side and its chemistry on the other. Here, we will demonstrate the usefulness of this platform for screening several selenium and tellurium compounds for their activity and action. We will also show how the nematode-based assay can be used to obtain information on compound uptake and distribution inside a multicellular organism, whilst the yeast-based system can be employed to explore possible intracellular mechanisms via chemogenetic screening and intracellular diagnostics. Whilst none of these simple and easy-to-use assays can ultimately substitute for in-depth studies in human cells and animals, these methods nonetheless provide a first glimpse on the possible biological activities of new compounds and offer direction for more complicated future investigations. They may also uncover some rather unpleasant biochemical actions of certain compounds, such as the ability of the trace element supplement selenite to induce DNA strand breaks.

Synthesis of amphiphilic seleninic acid derivatives with considerable activity against cellular membranes and certain pathogenic microbes

Selenium compounds play a major role in Biology, where they are often associated with pronounced antioxidant activity or toxicity. Whilst most selenium compounds are not necessarily hazardous, their often selective cytotoxicity is interesting from a biochemical and pharmaceutical perspective. We have synthesized a series of amphiphilic molecules which combine a hydrophilic seleninic acid head group - which at the same time serves as thiol-specific warhead - with a hydrophobic tail. These molecules possess a surface activity similar to the one of SDS, yet their biological activity seems to exceed by far the one of a simple surfactant (e.g. SDS) or seleninic acid (e.g. phenyl seleninic acid). Such compounds effectively haemolyse Red Blood Cells and exhibit pronounced activity against Saccharomyces cerevisiae. From a chemical perspective, the seleninic warheads are likely to attack crucial cysteine proteins of the cellular thiolstat.

A new tellurium-containing amphiphilic molecule induces apoptosis in HCT116 colon cancer cells

BACKGROUND

Chalcogen-based redox modulators over the years have attracted considerable attention as anti-cancer agents. New selenium- and tellurium-containing compounds with a polar head group and aryl-groups of various lengths have recently been reported as biologically active in several organisms. In the present study, we used the most active of the tellurium compound DP41, and its selenium counterpart DP31 to investigate their effects on the human cancer cell line HCT116.

METHODS

Cells were treated with DP41 or DP31 and the formation of superoxide radicals was determined using dihydroethidium. Cell cycle analysis and apoptosis was determined by cytofluorimetry. Proteins involved in ER signaling and apoptosis were determined by Western blot analysis and fluorescence microscopy.

RESULTS

With 50μM of DP41, we observed an increase in O2(-) formation. There was, however, no such increase in O2(-) after treatment with the corresponding selenium compound under the same conditions. In the case of DP41, the production of O2(-) radicals was followed by an up-regulation of Nrf2, HO-1, phospho-eIF2α and ATF4. CHOP was also induced and cells entered apoptosis. Unlike the cancer cells, normal retinal epithelial ARPE-19 cells did not produce elevated levels of O2(-) radicals nor did they induce the ER signaling pathway or apoptosis.

CONCLUSIONS

The tellurium-containing compound DP41, in contrast to the corresponding selenium compound, induces O2(-) radical formation and oxidative and ER stress responses, including CHOP activation and finally apoptosis.

GENERAL SIGNIFICANCE

These results indicate that DP41 is a redox modulating agent with promising anti-cancer potentials.