ResearchPad - very-important-paper https://www.researchpad.co Default RSS Feed en-us © 2020 Newgen KnowledgeWorks <![CDATA[(Electro‐)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex]]> https://www.researchpad.co/article/elastic_article_7144 N2 splitting into terminal nitrides by chemical and electrochemical reduction of [ReCl2{N(CHCHPtBu2)2}] is presented. Comparison of electrochemical data with that of the previously reported, related pincer complex [ReCl2{N(CH2CH2PtBu2)2}] allowed for identifying key parameters that control the efficiency of the reaction sequence, which defines reductive N2 splitting.John Wiley & Sons, Ltd.

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<![CDATA[Mercury‐Free Synthesis of Pincer [C^N^C]Au<sup>III</sup> Complexes by an Oxidative Addition/CH Activation Cascade]]> https://www.researchpad.co/article/elastic_article_7017 Non‐toxic route: A flexible photochemical approach with high functional‐group tolerance avoids the use of toxic mercury in the preparation of organogold(III) C^N^C pincer complexes. The precursors are readily available starting materials.

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<![CDATA[Improved Acid Resistance of a Metal–Organic Cage Enables Cargo Release and Exchange between Hosts]]> https://www.researchpad.co/article/elastic_article_6946 A strategy for improving the acid resistance of a tetrahedral cage has been developed by incorporating additional free pyridyl units on its vertices. The guest binding properties of the cage are also altered compared to the analogous tetrahedron without these peripheral groups, allowing the functions of complete cargo delivery and exchange between the two capsules by using acid and base as chemical stimuli.

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<![CDATA[Lamprey Parapinopsin (“UVLamP”): a Bistable UV‐Sensitive Optogenetic Switch for Ultrafast Control of GPCR Pathways]]> https://www.researchpad.co/article/Nbb1f6811-d486-46f3-8c29-27e2166277c0

Abstract

Optogenetics uses light‐sensitive proteins, so‐called optogenetic tools, for highly precise spatiotemporal control of cellular states and signals. The major limitations of such tools include the overlap of excitation spectra, phototoxicity, and lack of sensitivity. The protein characterized in this study, the Japanese lamprey parapinopsin, which we named UVLamP, is a promising optogenetic tool to overcome these limitations. Using a hybrid strategy combining molecular, cellular, electrophysiological, and computational methods we elucidated a structural model of the dark state and probed the optogenetic potential of UVLamP. Interestingly, it is the first described bistable vertebrate opsin that has a charged amino acid interacting with the Schiff base in the dark state, that has no relevance for its photoreaction. UVLamP is a bistable UV‐sensitive opsin that allows for precise and sustained optogenetic control of G protein‐coupled receptor (GPCR) pathways and can be switched on, but more importantly also off within milliseconds via lowintensity short light pulses. UVLamP exhibits an extremely narrow excitation spectrum in the UV range allowing for sustained activation of the Gi/o pathway with a millisecond UV light pulse. Its sustained pathway activation can be switched off, surprisingly also with a millisecond blue light pulse, minimizing phototoxicity. Thus, UVLamP serves as a minimally invasive, narrow‐bandwidth probe for controlling the Gi/o pathway, allowing for combinatorial use with multiple optogenetic tools or sensors. Because UVLamP activated Gi/o signals are generally inhibitory and decrease cellular activity, it has tremendous potential for health‐related applications such as relieving pain, blocking seizures, and delaying neurodegeneration.

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<![CDATA[One Guest or Two? A Crystallographic and Solution Study of Guest Binding in a Cubic Coordination Cage]]> https://www.researchpad.co/article/N19504d62-a41e-461d-90f9-c74ac0bb8e05

Abstract

A crystallographic investigation of a series of host–guest complexes in which small‐molecule organic guests occupy the central cavity of an approximately cubic M8L12 coordination cage has revealed some unexpected behaviour. Whilst some guests form 1:1 H⋅G complexes as we have seen before, an extensive family of bicyclic guests—including some substituted coumarins and various saturated analogues—form 1:2 H⋅G2 complexes in the solid state, despite the fact that solution titrations are consistent with 1:1 complex formation, and the combined volume of the pair of guests significantly exceeds the Rebek 55±9 % packing for optimal guest binding, with packing coefficients of up to 87 %. Re‐examination of solution titration data for guest binding in two cases showed that, although conventional fluorescence titrations are consistent with 1:1 binding model, alternative forms of analysis—Job plot and an NMR titration—at higher concentrations do provide evidence for 1:2 H⋅G2 complex formation. The observation of guests binding in pairs in some cases opens new possibilities for altered reactivity of bound guests, and also highlights the recently articulated difficulties associated with determining stoichiometry of supramolecular complexes in solution.

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<![CDATA[Monolayer Two‐dimensional Molecular Crystals for an Ultrasensitive OFET‐based Chemical Sensor]]> https://www.researchpad.co/article/Naee50b40-239d-4706-91b5-911c941436d6

Abstract

The sensitivity of conventional thin‐film OFET‐based sensors is limited by the diffusion of analytes through bulk films and remains the central challenge in sensing technology. Now, for the first time, an ultrasensitive (sub‐ppb level) sensor is reported that exploits n‐type monolayer molecular crystals (MMCs) with porous two‐dimensional structures. Thanks to monolayer crystal structure of NDI3HU‐DTYM2 (NDI) and controlled formation of porous structure, a world‐record detection limit of NH3 (0.1 ppb) was achieved. Moreover, the MMC‐OFETs also enabled direct detection of solid analytes of biological amine derivatives, such as dopamine at an extremely low concentration of 500 ppb. The remarkably improved sensing performances of MMC‐OFETs opens up the possibility of engineering OFETs for ultrasensitive (bio)chemical sensing.

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<![CDATA[Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO 2 (H 2 O) n − , n =1–10, in the C−O Stretch Region]]> https://www.researchpad.co/article/Nc926e130-88c2-4f8e-8aae-cb055fa680d9

Abstract

We investigate anionic [Co,CO2,nH2O] clusters as model systems for the electrochemical activation of CO2 by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250–2234 cm−1 using an FT‐ICR mass spectrometer. We show that both CO2 and H2O are activated in a significant fraction of the [Co,CO2,H2O] clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C−O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C−O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)2 . However, calculations find Co(HCOO)(OH) as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590–1730 cm−1 are most likely due to higher lying isomers of the composition Co(HOCO)(OH). Upon additional hydration, all species [Co,CO2,nH2O], n≥2, undergo IRMPD through loss of H2O molecules as a relatively weakly bound messenger. The main spectral features are the C−O stretching mode of the CO ligand around 1900 cm−1, the water bending mode mixed with the antisymmetric C−O stretching mode of the HCOO ligand around 1580–1730 cm−1, and the symmetric C−O stretching mode of the HCOO ligand around 1300 cm−1. A weak feature above 2000 cm−1 is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.

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<![CDATA[Periodic Nucleation of Calcium Phosphate in a Stirred Biocatalytic Reaction]]> https://www.researchpad.co/article/N9ef96d8e-57e9-45ec-9780-73be02f26255

Abstract

Highly ordered superstructures composed of inorganic nanoparticles appear in natural and synthetic systems, however the mechanisms of non‐equilibrium self‐organization that may be involved are still poorly understood. Herein, we performed a kinetic investigation of the precipitation of calcium phosphate using a process widely found in microorganisms: the hydrolysis of urea by enzyme urease. With high initial ratio of calcium ion to phosphate, periodic precipitation was obtained accompanied by pH oscillations in a well‐stirred, closed reactor. We propose that an internal pH‐regulated change in the concentration of phosphate ion is the driving force for periodicity. A simple model involving the biocatalytic reaction network coupled with burst nucleation of nanoparticles above a critical supersaturation reproduced key features of the experiments. These findings may provide insight to the self‐organization of nanoparticles in biomineralization and improve design strategies of biomaterials for medical applications.

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<![CDATA[Analysis of the Substrate Specificity of the SMYD2 Protein Lysine Methyltransferase and Discovery of Novel Non‐Histone Substrates]]> https://www.researchpad.co/article/N1b267aea-8619-4974-b175-1d0e67966c04

Abstract

The SMYD2 protein lysine methyltransferase methylates various histone and non‐histone proteins and is overexpressed in several cancers. Using peptide arrays, we investigated the substrate specificity of the enzyme, revealing a recognition of leucine (or weaker phenylalanine) at the −1 peptide site and disfavor of acidic residues at the +1 to +3 sites. Using this motif, novel SMYD2 peptide substrates were identified, leading to the discovery of 32 novel peptide substrates with a validated target site. Among them, 19 were previously reported to be methylated at the target lysine in human cells, strongly suggesting that SMYD2 is the protein lysine methyltransferase responsible for this activity. Methylation of some of the novel peptide substrates was tested at the protein level, leading to the identification of 14 novel protein substrates of SMYD2, six of which were more strongly methylated than p53, the best SMYD2 substrate described so far. The novel SMYD2 substrate proteins are involved in diverse biological processes such as chromatin regulation, transcription, and intracellular signaling. The results of our study provide a fundament for future investigations into the role of this important enzyme in normal development and cancer.

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<![CDATA[Modulation and Visualization of EF‐G Power Stroke During Ribosomal Translocation]]> https://www.researchpad.co/article/Nb6adb752-dc4f-443f-936a-a8b4fcc30d8d

Abstract

During ribosome translocation, the elongation factor EF‐G undergoes large conformational change while maintaining its contact with the moving tRNA. We previously measured a power stroke accompanying EF‐G catalysis, which was consistent with structural studies. However, the role of power stroke in translocation fidelity remains unclear. Here, we report quantitative measurements of the power strokes of structurally modified EF‐Gs by using two different techniques and reveal the correlation between power stroke and translocation efficiency and fidelity. We discovered that the reduced power stroke only lowered the percentage of translocation but did not introduce translocation error. The established force ‐structure–function correlation for EF‐G indicates that power stroke drives ribosomal translocation, but the mRNA reading frame is probably maintained by ribosome itself. Furthermore, the microscope detection method reported here can be simply implemented for other biochemical applications.

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<![CDATA[Luminescence and Light-Driven Energy and Electron Transfer from an Exceptionally Long-Lived Excited State of a Non-Innocent Chromium(III) Complex]]> https://www.researchpad.co/article/N07d691df-a63e-4a5c-bcb8-1cfe0589072e

Abstract

Photoactive metal complexes employing Earth‐abundant metal ions are a key to sustainable photophysical and photochemical applications. We exploit the effects of an inversion center and ligand non‐innocence to tune the luminescence and photochemistry of the excited state of the [CrN6] chromophore [Cr(tpe)2]3+ with close to octahedral symmetry (tpe=1,1,1‐tris(pyrid‐2‐yl)ethane). [Cr(tpe)2]3+ exhibits the longest luminescence lifetime (τ=4500 μs) reported up to date for a molecular polypyridyl chromium(III) complex together with a very high luminescence quantum yield of Φ=8.2 % at room temperature in fluid solution. Furthermore, the tpe ligands in [Cr(tpe)2]3+ are redox non‐innocent, leading to reversible reductive chemistry. The excited state redox potential and lifetime of [Cr(tpe)2]3+ surpass those of the classical photosensitizer [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine) enabling energy transfer (to oxygen) and photoredox processes (with azulene and tri(n‐butyl)amine).

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<![CDATA[Flavinium Catalysed Photooxidation: Detection and Characterization of Elusive Peroxyflavinium Intermediates]]> https://www.researchpad.co/article/N1f05c1ca-6a5d-4e88-83e1-398124eae6d0

Abstract

Flavin‐based catalysts are photoactive in the visible range which makes them useful in biology and chemistry. Herein, we present electrospray‐ionization mass‐spectrometry detection of short‐lived intermediates in photooxidation of toluene catalysed by flavinium ions (Fl+). Previous studies have shown that photoexcited flavins react with aromates by proton‐coupled electron transfer (PCET) on the microsecond time scale. For Fl+, PCET leads to FlH.+ with the H‐atom bound to the N5 position. We show that the reaction continues by coupling between FlH.+ and hydroperoxy or benzylperoxy radicals at the C4a position of FlH.+. These results demonstrate that the N5‐blocking effect reported for alkylated flavins is also active after PCET in these photocatalytic reactions. Structures of all intermediates were fully characterised by isotopic labelling and by photodissociation spectroscopy. These tools provide a new way to study reaction intermediates in the sub‐second time range.

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<![CDATA[Chemical Ligation and Isotope Labeling to Locate Dynamic Effects during Catalysis by Dihydrofolate Reductase† ]]> https://www.researchpad.co/article/5b01d7a1463d7e4e865d3bea

Abstract

Chemical ligation has been used to alter motions in specific regions of dihydrofolate reductase from E. coli and to investigate the effects of localized motional changes on enzyme catalysis. Two isotopic hybrids were prepared; one with the mobile N‐terminal segment containing heavy isotopes (2H, 13C, 15N) and the remainder of the protein with natural isotopic abundance, and the other one with only the C‐terminal segment isotopically labeled. Kinetic investigations indicated that isotopic substitution of the N‐terminal segment affected only a physical step of catalysis, whereas the enzyme chemistry was affected by protein motions from the C‐terminal segment. QM/MM studies support the idea that dynamic effects on catalysis mostly originate from the C‐terminal segment. The use of isotope hybrids provides insights into the microscopic mechanism of dynamic coupling, which is difficult to obtain with other studies, and helps define the dynamic networks of intramolecular interactions central to enzyme catalysis.

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<![CDATA[Quantitative Insights into the Fast Pyrolysis of Extracted Cellulose, Hemicelluloses, and Lignin]]> https://www.researchpad.co/article/5b434010463d7e25576032ad

Abstract

The transformation of lignocellulosic biomass into bio‐based commodity chemicals is technically possible. Among thermochemical processes, fast pyrolysis, a relatively mature technology that has now reached a commercial level, produces a high yield of an organic‐rich liquid stream. Despite recent efforts to elucidate the degradation paths of biomass during pyrolysis, the selectivity and recovery rates of bio‐compounds remain low. In an attempt to clarify the general degradation scheme of biomass fast pyrolysis and provide a quantitative insight, the use of fast pyrolysis microreactors is combined with spectroscopic techniques (i.e., mass spectrometry and NMR spectroscopy) and mixtures of unlabeled and 13C‐enriched materials. The first stage of the work aimed to select the type of reactor to use to ensure control of the pyrolysis regime. A comparison of the chemical fragmentation patterns of “primary” fast pyrolysis volatiles detected by using GC‐MS between two small‐scale microreactors showed the inevitable occurrence of secondary reactions. In the second stage, liquid fractions that are also made of primary fast pyrolysis condensates were analyzed by using quantitative liquid‐state 13C NMR spectroscopy to provide a quantitative distribution of functional groups. The compilation of these results into a map that displays the distribution of functional groups according to the individual and main constituents of biomass (i.e., hemicelluloses, cellulose and lignin) confirmed the origin of individual chemicals within the fast pyrolysis liquids.

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