Red-shifted activity-based sensors for ethylene via direct conjugation of fluorophore to metal-carbene.

A number of Activity-Based Sensors (ABS) for relatively unreactive small molecules, such as ethylene, necessitates a transition metal for reaction under ambient conditions. Olefin metathesis has emerged as one of the primary strategies to achieve ethylene detection, and other transition metals are used for similarly challenging-to-detect analytes. However, limited studies exist investigating how fluorophore-metal attachment impacts photophysical properties of such ABS. Two new probes were prepared with the chelating benzlidene Ru-ligand directly conjugated to a BODIPY fluorophore and the photophysical properties of the new conjugated ABS were evaluated.

Detection of Ethylene with Defined Metal Complexes: Strategies and Recent Advances.

Despite its relative simplicity, ethylene is an interesting molecule with wide-ranging impact in modern chemistry and biology. Stemming from ethylene's role as a critical plant hormone, there has been significant effort to develop selective and sensitive molecular sensors for ethylene. Late transition metal complexes have played an important role in detection strategies due to ethylene's lack of structural complexity and limited reactivity. Two main approaches to ethylene detection are identified: (1) coordination-based sensors, wherein ethylene binds reversibly to a metal center, and (2) activity-based sensors, wherein ethylene undergoes a reaction at a metal center, resulting in the formation and destruction of covalent bonds. Herein, we describe the advantages and disadvantages of various approaches, and the challenges remaining for sensor development.

Steric Modulation of CAACs Controls Orientation and Ethenolysis Performance.

In this issue of Chem Catalysis, Sytniczuk, Kajetanowicz, and Grela report sterically tuned Cyclic(Alkyl)(Amino)Carbene (CAAC) ligands to protect the requisite Ru-methylidene ([Ru]=CH2) intermediate present during ethenolysis of renewable fatty acid methyl esters (FAME). Surprising structural characteristics of the Ru-CAAC complexes resulted in TON up to 740,000 and sub-ppm catalyst loadings.

Access to Substituted Indenol Ethers via a Regioselective Intermolecular Carbopalladation Cascade.

Alkyne carbopalladation reactions can rapidly generate multiple new C-C bonds; however, regioselectivity is challenging for intermolecular variants. Using ynol ethers, we observe complete regiocontrol of migratory insertion followed by a second migratory insertion with a pendant alkene to turn-over the catalytic cycle. The resulting products are oligosubstituted 1-indenol ethers with defined stereochemistry based on the initial alkene geometry. Blocking ß-hydride elimination allowed for C-H and C-C reductive elimination steps for catalyst turnover.

Ligand-Directed Approach to Activity-Based Sensing: Developing Palladacycle Fluorescent Probes That Enable Endogenous Carbon Monoxide Detection.

Carbon monoxide (CO) is an emerging gasotransmitter and reactive carbon species with broad anti-inflammatory, cytoprotective, and neurotransmitter functions along with therapeutic potential for the treatment of cardiovascular diseases. The study of CO chemistry in biology and medicine relative to other prominent gasotransmitters such as NO and H2S remains challenging, in large part due to limitations in available tools for the direct visualization of this transient and freely diffusing small molecule in complex living systems. Here we report a ligand-directed activity-based sensing (ABS) approach to CO detection through palladium-mediated carbonylation chemistry. Specifically, the design and synthesis of a series of ABS probes with systematic alterations in the palladium-ligand environment (e.g., sp3-S, sp3-N, sp2-N) establish structure-activity relationships for palladacycles to confer selective reactivity with CO under physiological conditions. These fundamental studies led to the development of an optimized probe, termed Carbon Monoxide Probe-3 Ester Pyridine (COP-3E-Py), which enables imaging of CO release in live cell and brain settings, including monitoring of endogenous CO production that triggers presynaptic dopamine release in fly brains. This work provides a unique tool for studying CO in living systems and establishes the utility of a synthetic methods approach to activity-based sensing using principles of organometallic chemistry.

Carbon Monoxide, a Retrograde Messenger Generated in Postsynaptic Mushroom Body Neurons, Evokes Noncanonical Dopamine Release.

Dopaminergic neurons innervate extensive areas of the brain and release dopamine (DA) onto a wide range of target neurons. However, DA release is also precisely regulated. In Drosophila melanogaster brain explant preparations, DA is released specifically onto α3/α'3 compartments of mushroom body (MB) neurons that have been coincidentally activated by cholinergic and glutamatergic inputs. The mechanism for this precise release has been unclear. Here we found that coincidentally activated MB neurons generate carbon monoxide (CO), which functions as a retrograde signal evoking local DA release from presynaptic terminals. CO production depends on activity of heme oxygenase in postsynaptic MB neurons, and CO-evoked DA release requires Ca2+ efflux through ryanodine receptors in DA terminals. CO is only produced in MB areas receiving coincident activation, and removal of CO using scavengers blocks DA release. We propose that DA neurons use two distinct modes of transmission to produce global and local DA signaling.SIGNIFICANCE STATEMENT Dopamine (DA) is needed for various higher brain functions, including memory formation. However, DA neurons form extensive synaptic connections, while memory formation requires highly specific and localized DA release. Here we identify a mechanism through which DA release from presynaptic terminals is controlled by postsynaptic activity. Postsynaptic neurons activated by cholinergic and glutamatergic inputs generate carbon monoxide, which acts as a retrograde messenger inducing presynaptic DA release. Released DA is required for memory-associated plasticity. Our work identifies a novel mechanism that restricts DA release to the specific postsynaptic sites that require DA during memory formation.

Accessing 4-oxy-substituted isoquinolinones via C-H activation and regioselective migratory insertion with electronically biased ynol ethers.

The rhodium-catalyzed C-H activation and annulation with ynol ethers to directly provide 4-oxy substituted isoquinolinones is reported. The polarized nature of ynol ethers provides an electronic bias for controlling the regioselectivity of the migratory insertion process. While the highly reactive nature of ynol ethers presents a challenge, mild conditions were found to provide product in moderate to good yield. Utility was demonstrated by application in the synthesis of a prolyl-4-hydroxylase inhibitor framework.

Olefin Metathesis-Based Fluorescent Probes for the Selective Detection of Ethylene in Live Cells.

Ethylene is an important plant hormone that is involved in a variety of developmental processes including agriculturally important ripening of certain fruits. Owing to its significant roles, a number of approaches have previously been developed to detect ethylene via molecular interactions. However, there are no current approaches for detection that are selective via a discrete homogeneous molecular interaction. Here we report two profluorescent chemodosimeters for the selective detection of the plant hormone ethylene. The approach consists of a BODIPY fluorophore with a pendant ruthenium recognition element based on a Hoveyda-Grubbs second generation catalysts. A marked increase in fluorescence is observed upon exposure to ethylene and selectivity is observed for ethylene over other alkenes, providing a unique approach toward ethylene detection. Imaging in live cells demonstrated that ethylene could be detected from multiple relevant sources.

Chlamydomonas reinhardtii LFO1 Is an IsdG Family Heme Oxygenase.

Heme is essential for respiration across all domains of life. However, heme accumulation can lead to toxicity if cells are unable to either degrade or export heme or its toxic by-products. Under aerobic conditions, heme degradation is performed by heme oxygenases, enzymes which utilize oxygen to cleave the tetrapyrrole ring of heme. The HO-1 family of heme oxygenases has been identified in both bacterial and eukaryotic cells, whereas the IsdG family has thus far been described only in bacteria. We identified a hypothetical protein in the eukaryotic green alga Chlamydomonas reinhardtii, which encodes a protein containing an antibiotic biosynthesis monooxygenase (ABM) domain consistent with those associated with IsdG family members. This protein, which we have named LFO1, degrades heme, contains similarities in predicted secondary structures to IsdG family members, and retains the functionally conserved catalytic residues found in all IsdG family heme oxygenases. These data establish LFO1 as an IsdG family member and extend our knowledge of the distribution of IsdG family members beyond bacteria. To gain further insight into the distribution of the IsdG family, we used the LFO1 sequence to identify 866 IsdG family members, including representatives from all domains of life. These results indicate that the distribution of IsdG family heme oxygenases is more expansive than previously appreciated, underscoring the broad relevance of this enzyme family. IMPORTANCE This work establishes a protein in the freshwater alga Chlamydomonas reinhardtii as an IsdG family heme oxygenase. This protein, LFO1, exhibits predicted secondary structure and catalytic residues conserved in IsdG family members, in addition to a chloroplast localization sequence. Additionally, the catabolite that results from the degradation of heme by LFO1 is distinct from that of other heme degradation products. Using LFO1 as a seed, we performed phylogenetic analysis, revealing that the IsdG family is conserved in all domains of life. Additionally, C. reinhardtii contains two previously identified HO-1 family heme oxygenases, making C. reinhardtii the first organism shown to contain two families of heme oxygenases. These data indicate that C. reinhardtii may have unique mechanisms for regulating iron homeostasis within the chloroplast.

A boronate-caged [¹8F]FLT probe for hydrogen peroxide detection using positron emission tomography.

Reactive oxygen species (ROS) play important roles in the development and progression of cancer and other diseases, motivating the development of translatable technologies for biological ROS imaging. Here we report Peroxy-Caged-[(18)F]Fluorodeoxy thymidine-1 (PC-FLT-1), an oxidatively immolative positron emission tomography (PET) probe for H2O2 detection. PC-FLT-1 reacts with H2O2 to generate [(18)F]FLT, allowing its peroxide-dependent uptake and retention in proliferating cells. The relative uptake of PC-FLT-1 was evaluated using H2O2-treated UOK262 renal carcinoma cells and a paraquat-induced oxidative stress cell model, demonstrating ROS-dependent tracer accumulation. The data suggest that PC-FLT-1 possesses promising characteristics for translatable ROS detection and provide a general approach to PET imaging that can be expanded to the in vivo study of other biologically relevant analytes.

Design and synthesis of new hybrid molecules that activate the transcription factor Nrf2 and simultaneously release carbon monoxide.

The transcription factor Nrf2 and its downstream target heme oxygenase-1 (HO-1) are essential protective systems against oxidative stress and inflammation. The products of HO-1 enzymatic activity, biliverdin and carbon monoxide (CO), actively contribute to this protection, suggesting that exploitation of these cellular systems may offer new therapeutic avenues in a variety of diseases. Starting from a CO-releasing compound and a chemical scaffold exhibiting electrophilic characteristics (esters of fumaric acid), we report the synthesis of hybrid molecules that simultaneously activate Nrf2 and liberate CO. These hybrid compounds, which we termed "HYCOs", release CO to myoglobin and activate the CO-sensitive fluorescent probe COP-1, while also potently inducing nuclear accumulation of Nrf2 and HO-1 expression and activity in different cell types. Thus, we provide here the first example of a new class of pharmacologically active molecules that target the HO-1 pathway by combining an Nrf2 activator coordinated to a CO-releasing group.

Wacker-type oxidation of internal alkenes using Pd(Quinox) and TBHP.

The Pd-catalyzed TBHP-mediated Wacker-type oxidation of internal alkenes is reported. The reaction uses 2-(4,5-dihydro-2-oxazolyl)quinoline (Quinox) as ligand and TBHP(aq) as oxidant to deliver single ketone constitutional isomer products in a predictable fashion from electronically biased olefins. This methodology is showcased through its application on an advanced intermediate in the total synthesis of the antimalarial drug artemisinin.

A reaction-based fluorescent probe for selective imaging of carbon monoxide in living cells using a palladium-mediated carbonylation.

Carbon monoxide is a member of the gasotransmitter family, which also includes NO and H(2)S, and has been implicated in a variety of pathological and physiological conditions. Whereas exogenous therapeutic additions of CO to tissues and whole animals have been well-studied, the real-time spatial and temporal tracking of CO at the cellular level remains an open challenge. Here we report a new type of turn-on fluorescent probe for selective CO detection based on palladium-mediated carbonylation reactivity. CO Probe 1 (COP-1) is capable of detecting CO both in aqueous buffer and in live cells with high selectivity over a range of biologically relevant reactive small molecules, providing a potentially powerful approach for interrogating its chemistry in biological systems.

On the mechanism of the palladium-catalyzed tert-butylhydroperoxide-mediated Wacker-type oxidation of alkenes using quinoline-2-oxazoline ligands.

The mechanism of the tert-butylhydroperoxide-mediated, Pd(Quinox)-catalyzed Wacker-type oxidation was investigated to evaluate the hypothesis that a selective catalyst-controlled oxidation could be achieved by rendering the palladium coordinatively saturated using a bidentate amine ligand. The unique role of the Quinox ligand framework was probed via systematic ligand modifications. The modified ligands were evaluated through quantitative Hammett analysis, which supports a "push-pull" relationship between the electronically asymmetric quinoline and oxazoline ligand modules.

Catalyst-controlled Wacker-type oxidation of homoallylic alcohols in the absence of protecting groups.

Homoallylic alcohols are oxidized to ß-hydroxy ketones using a TBHP-mediated Pd-catalyzed Wacker-type oxidation. The use of a bidentate ligand, quinoline-2-oxazoline (Quinox), and TBHP((aq)) as the terminal oxidant provides good yields of the desired products with reaction times significantly reduced as compared to the Tsuji-Wacker oxidation. Additionally, bis- and tris-homoallylic alcohols are oxidized to provide cyclic peroxyketals, presumably via nucleophilic attack of the methyl ketone product.

A general and efficient catalyst system for a Wacker-type oxidation using TBHP as the terminal oxidant: application to classically challenging substrates.

Utilizing the rapidly synthesized Quinox ligand and commercially available aqueous TBHP, a Wacker-type oxidation has been developed, which efficiently converts the traditionally challenging substrate class of protected allylic alcohols to the corresponding acyloin products. Additionally, the catalytic system is general for several other substrate classes, converting terminal olefins to methyl ketones, with short reaction times. The system is scalable (20 mmol) and can be performed with a reduced catalyst loading of 1 mol%. Enantioenriched substrates undergo oxidation with complete retention of enantiomeric excess.