Structural Conversion Process Research Articles

Multi-stimuli-responsive luminescent switching properties of cycloplatinated(II) complexes bearing 2-phenylpyridine derivatives

Three new cycloplatinated(II) complexes, Pt(ppy)L (1), Pt[(CF3)ppy]L (2) and Pt[dF(CF3)ppy]L (3) based on 2-phenylpyridine (ppy), 2-phenyl-5-(trifluoromethyl)pyridine [(CF3)ppy], 2-(2,4-Difluorophenyl)-5-(trifluoromethyl)pyridine [dF(CF3)ppy] and 4-[2-(4-fluorophenyl)ethynyl]-2-pyridinecarboxylic acid (L) ligands were synthesized and characterized. Complex 1 has one metastable structure and exhibits reversible mechanoluminescent property. On the contrary, complexes 2 and 3 each have five types of metastable structures and exhibit multi-stimuli-responsive luminescent switching properties triggered by VOC vapors absorption, heating, and mechanical grinding. Moreover, complexes 2 and 3 display a series of untraditional luminescent switching behaviors such as two-step conversion processes, irreversible process and more than one thermally stable products. Systematic studies on crystal structure, luminescence spectra, PXRD and DFT theoretical calculation demonstrate that the luminescent switching properties of these complexes are attributed to the formation/elimination of intermolecular Pt-Pt interactions during structural conversion processes between metastable structures, which lead to the transformation of the lowest energy excited states between MLCT and MMLCT transitions. Furthermore, the unconventional luminescent switching behaviors are due to the high stability of the metastable structures. In addition, the studies also reveal that the stacking structure of complex can affect its luminescent switching behavior, while the good molecular planarity of the complex is not conducive to the luminescent switching properties.

Thermodynamics of amyloid formation

AbstractBackgroundIs amyloid formation an exothermic process with the potential to damage brain tissue by local hyperthermia? In our study we aim to find answers to the above question. Several proteins, i.e. lysozyme and amyloid‐β peptide, known to form amyloid structures are investigated primarily by differential scanning calorimetry (DSC) in order to determine their thermodynamic properties.MethodExperiments in DSC are complemented by a set of other biophysical methods in order to clarify molecular parameters for the evaluation of the heat capacity measurements.ResultFor this purpose a seeded amyloid formation assay has been designed. In this assay the structural conversion of a given amount of monomeric protein is accelerated and synchronized by the addition of preformed amyloid fibril fragments to result in reproducible heat bursts caused by amyloid formation.ConclusionFirst results are reported and the measured enthalpies are related to known enthalpies from protein structural conversion processes.

Millisecond Time-Resolved Solid-State NMR Initiated by Rapid Inverse Temperature Jumps

Elucidation of the detailed mechanisms by which biological macromolecules undergo major structural conversions, such as folding, complex formation, and self-assembly, is a central concern of biophysical chemistry that will benefit from new experimental methods. We describe a simple technique for initiating a structural conversion process by a rapid decrease in the temperature of a solution, i.e., a rapid inverse temperature jump. By pumping solutions through copper capillary tubes that are thermally anchored to heated and cooled blocks, solution temperatures can be switched from 95 to 30 °C (or lower) in about 0.8 ms. For time-resolved solid-state nuclear magnetic resonance (ssNMR), solutions can then be frozen rapidly by spraying into cold isopentane after a variable structural evolution time τe. As an initial demonstration, we use this "inverse T-jump" technique to characterize the kinetics and mechanism by which the 26-residue peptide melittin converts from its primarily disordered, monomeric state at 95 °C to its α-helical, tetrameric state at 30 °C. One- and two-dimensional ssNMR spectra of frozen solutions with various values of τe, recorded at 25 K with signal enhancements from dynamic nuclear polarization, show that both helical secondary structure and intermolecular contacts develop on the same time scale of about 6 ms. The dependences on τe of both intraresidue crosspeak patterns and inter-residue crosspeak volumes in two-dimensional spectra can be fit with a unidirectional dimerization model, consistent with dimerization being the rate-limiting step for melittin tetramer formation.

A Pacman-Like Titanium-Doped Cobalt Sulfide Hollow Superstructure for Electrocatalytic Oxygen Evolution.

Transition-metal sulfides (TMSs) are attractive oxygen evolution reaction (OER) electrocatalysts. Developing new strategies to improve their electrochemical performance of TMSs is of great significance. Herein, a unique pacman-like titanium-doped cobalt sulfide hollow superstructure (Ti-CoSx HSS) is fabricated as an OER electrocatalyst. Using a prearranged metal-organic framework (MOF)-on-MOF heterostructure as a precursor treated by one-pot sulfidation, a sequential structural conversion process leads to the formation of Ti-CoSx HSS, which is assembled by interconnected Ti-doped CoSx nanocages around a cake-like cavity. Benefiting from the architecture and compositional advantages, Ti-CoSx HSS exhibits excellent OER performance with an overpotential of 249mV at 10mA cm-2 and Tafel slope of 45.5mV dec-1 due to increased active site exposure, enhanced electron and mass transfer. This strategy enabled by MOF-on-MOF paves the way toward innovative MOF derivatives for various applications.

Unravelling the role of polyoxovanadates in electrocatalytic water oxidation reaction: Active species or precursors

Exploring the role of polyoxometalates (POMs) in electrocatalytic oxygen evolution reaction (OER) is greatly significant for developing efficient electrocatalyst towards the sustainable energy production. Herein, three polyoxovanadate (POV)-based inorganic–organic hybrid compounds with formula of [Ni2(BBTZ)(H2O)4]V4O12·2H2O(1), [Co2(BBTZ)(H2O)4]V4O12·2H2O(2), [Ni(BBTZ)2]V2O6·2H2O(3) (BBTZ = 1,4-bis-(1,2,4-triazol-1-ylmethyl)benzene) were synthesized as electrocatalysts to explore their roles for OER. Structural analysis showed that compounds 1–2 are isomorphic 2-D wave-like layer structures, while compound 3 features 3-D framework with a 5-fold interpenetrating metal–organic moiety. Experiments showed that the three compounds can keep their structural integrities after soaking in 1 M KOH electrolyte, and compound 1 exhibited the best catalytic activity towards OER with low overpotential of 353 mV at current density of 10 mA cm−2 and small Tafel slope of 77.8 mV dec−1. However, a series of technical characterizations clearly implied that there are structural conversion processes of compounds 1–3 on electrode surface accompanying the electrocatalytic reation. The organic ligands and most of polyoxovanadate species were electrochemically extracted under anodic potential, leaving the V-doping metal oxyhydroxides as OER-active species. This work confirmed that polyoxovanadates are not active sites but precursors for generating active species of OER, and suggested an available route for designing new nanosized vanadium-doped metal oxide electrocatalysts.

Topotactic conversion of Ge-rich IWW zeolite into IPC-18 under mild condition

Abstract A facile structural conversion process from Ge-rich IWW-type germanosilicate to its daughter structure IPC-18 zeolite has been developed under mild conditions of water medium and room temperature. The parent IWW zeolite with an extremely low Si/Ge ratio of 1.02, synthesized with 1,4-bis-(dimethyl-1-adamantylammonium)-butane as a novel structural directing agent, was critical for the success of this specific structural transformation. The high Ge content favored the breaking of double-four-ring (d4r) subunits under mild conditions and simultaneously the mild H2O medium suppressed the Si-assisted structural repairing. The HRTEM images collected along the (001) and (010) orientations of IWW and IPC-18 zeolite revealed the occurrence of structural transformation. The formation of IPC-18 zeolite structure was confirmed by the PXRD patterns and selected area electron diffraction patterns. The obtained IPC-18 zeolite demonstrated promising catalytic performance in the hydration of ethylene oxide to ethylene glycol with a high selective of 96%.

Application of millisecond time-resolved solid state NMR to the kinetics and mechanism of melittin self-assembly

Common experimental approaches for characterizing structural conversion processes such as protein folding and self-assembly do not report on all aspects of the evolution from an initial state to the final state. Here, we demonstrate an approach that is based on rapid mixing, freeze-trapping, and low-temperature solid-state NMR (ssNMR) with signal enhancements from dynamic nuclear polarization (DNP). Experiments on the folding and tetramerization of the 26-residue peptide melittin following a rapid pH jump show that multiple aspects of molecular structure can be followed with millisecond time resolution, including secondary structure at specific isotopically labeled sites, intramolecular and intermolecular contacts between specific pairs of labeled residues, and overall structural order. DNP-enhanced ssNMR data reveal that conversion of conformationally disordered melittin monomers at low pH to α-helical conformations at neutral pH occurs on nearly the same timescale as formation of antiparallel melittin dimers, about 6 to 9 ms for 0.3 mM melittin at 24 °C in aqueous solution containing 20% (vol/vol) glycerol and 75 mM sodium phosphate. Although stopped-flow fluorescence data suggest that melittin tetramers form quickly after dimerization, ssNMR spectra show that full structural order within melittin tetramers develops more slowly, in ∼60 ms. Time-resolved ssNMR is likely to find many applications to biomolecular structural conversion processes, including early stages of amyloid formation, viral capsid formation, and protein-protein recognition.

A perspective of chalcogenide semiconductor-noble metal nanocomposites through structural transformations

Abstract Intense efforts have been devoted to the synthesis of heterogeneous nanocomposites consisting of chalcogenide semiconductors and noble metals, which usually exhibit enhanced properties owing to the synergistic effect between their different material domains. Tailoring the structure of the metal domains in the nanocomposites may lead to further improvements of its performance for a given application. This review therefore highlights the strategies based on a structural conversion process for the fabrication of nanocomposites consisting of chalcogenide semiconductors and noble metals with various internal structures, e.g., hollow or cage-bell. This strategy relies on a unique inside-out diffusion phenomenon of Ag in core-shell nanoparticles with Ag residing at core or inner shell region. In the presence of sulfur or selenium precursors, the diffused Ag are converted into Ag2S or Ag2Se, which is connected with the remaining noble metal parts, forming nanocomposites consisting of silver chalcogenide and noble metal nanoparticles with hollow or cage-bell structures. We would focus on the introduction of the fundamentals, principles, electrocatalytic applications as well as perspectives of the chalcogenide semiconductor-noble metal nanocomposites derived from their core-shell precursors so as to provide the readers insights in designing efficient nanocomposites for electrocatalysis.

Study on structural conversion of dihydrochelerythrine in different solvents

Dihydrochelerythrine was isolated from the ethanol extract of Corydalis yanhusuo by chromatographic and recrystallization techniques. To our knowledge, this is the first report that dihydrochelerythrine was found to be unstable. The NMR, HPLC, and LC-MS were applied to monitor the structural conversion process of dihydrochelerythrine. The results showed that when dissolved in polar deuteration solvent (e.g., DMSO-d₆ & MeOD), dihydrochelerythrine is directly converted to chelerythrine gradually. However, if used non-polar reagent (e.g.,CD₂Cl₂), the sample of dihydrochelerythrine undergoes the formation of pseudobase, chelerythrine, and bimolecular ether then followed by oxidation to oxychelerythrine as the major final product. Which leads to this phenomenon maybe is that the C-6 in dihydrochelerythrine is highly reactive to nucleophiles, and is easily converted to different derivatives in different solvents attributed to the solvent effect. This finding will contribute to the extraction and isolation, bioactivity screening, and quality evaluation of medicinal materials containing dihydrochelerythrine and other similar derivatives.

Understanding the formation of nanocomposites consisting of silver sulfide and platinum hollow nanostructures

Structural conversion using core-shell silver (Ag)-platinum (Pt) nanoparticles as starting materials is a promising way to produce nanocomposites consisting of silver sulfide (Ag2S) and hollow Pt nanostructures (Ag2S-hPtNCs). However, the mechanism behind the structural conversion process is yet to be understood. In this study, we use transmission electron microscope (TEM) as a typical tool to characterize the products formed in different structural conversion processes, and interpret the experimental observations by proposing mechanisms behind the reactions between core-shell Ag-Pt nanoparticles and sulfur precursors from different sources. We then design additional experiments to rationalize the formation mechanism we proposed. The mechanistic understanding of the structural conversion from bimetallic core-shell nanostructures to heterogeneous nanocomposites may extend the fabrication of heterogeneous nanocomposites for a given technological application.

Structural and morphological evolution of an octahedral KNbO3 mesocrystal via self-assembly-topotactic conversion process

A meaningful KNbO3 (KN) mesocrystal with octahedral morphology was formed using classical crystallizations including stacking and topotactic structural conversion processes.

Tuning of Exchange Coupling and Switchable Magnetization Dynamics by Displacing the Bridging Ligands Observed in Two Dimeric Manganese(III) Compounds

Two Mn(III)-based dimers, [Mn2(bpad)2(CH3O)4]n (1) and [Mn2(bpad)2(pa)2]n·2H2O (2) (Hbpad = N3-benzoylpyridine-2-carboxamidrazone, H2pa = phthalic acid), have been assembled from a tridentate Schiff-base chelator and various anionic coligands. Noteworthily, compound 1 could be identified as a reaction precursor to transform to 2 in the presence of phthalic acid, resulting in a rarely structural conversion process in which the bridges between intradimer Mn(III) ions alter from methanol oxygen atom with μ2-O mode in 1 (Mn Mn distance of 3.046 Å) to syn-anti carboxylate in 2 (Mn Mn distance of 4.043 Å), while the Mn(III) centers retain hexa-coordinated geometries with independently distorted octahedrons in two compounds. The dc magnetic determinations reveal that ferromagnetic coupling between two metal centers with J = 1.31 cm−1 exists in 1, whereas 2 displays weak antiferromagnetic interactions with the coupling constant J of −0.56 cm−1. Frequency-dependent ac susceptibilities in the absence of dc field for 1 suggest slow relaxation of the magnetization with an energy barrier of 13.9 K, signifying that 1 features single-molecule magnet (SMM) behavior. This work presents a rational strategy to fine-tune the magnetic interactions and further magnetization dynamics of the Mn(III)-containing dinuclear units through small structural variations driven by the ingenious chemistry.

Good and bad sides of TGFβ-signaling in myocardial infarction.

Myocardial infarction is a prevailing cause of death in industrial countries. In spite of the good opportunities we have nowadays in interventional cardiology to reopen the clotted coronary arteries for reperfusion of ischemic areas, post-infarct remodeling emerges and contributes to unfavorable structural conversion processes in the myocardium, finally resulting in heart failure. The growth factor TGFβ is upregulated during these processes. In this review, an overview on the functional role of TGFβ signaling in the process of cardiac remodeling is given, as it can influence apoptosis, fibrosis and hypertrophy thereby predominantly aggravating ischemia/reperfusion injury.

The structural conversion of multinuclear titanium(IV) μ-oxo-complexes.

For the first time we report the structural conversion processes of hexanuclear μ-oxo-Ti(IV) complexes into tetranuclear ones. Single-crystal X-ray diffraction studies reveal that metastable hexanuclear μ-oxo complexes ([Ti6O6(O(t)Bu)(O2CR')6]) are formed in the first stage of reactions between [Ti(O(t)Bu)4] and branched carboxylic acids R'COOH (R' = C(Me)2Et, CH2(t)Bu, (t)Bu). In the next stage they convert into tetranuclear μ-oxo-Ti(IV) complexes of the formula [Ti4O4(O(t)Bu)4(O2CR')4]. Spectroscopic investigations ((13)C NMR, IR, and MS) proved that the conversion of hexanuclear clusters relies on the attachment of smaller units (e.g., [Ti3O(O(t)Bu)8(O2CR')2] or [Ti4O2(O(t)Bu)6(O2CR')6]) and intermediate species formation (e.g., [Ti9O8(O(t)Bu)14(O2CR')6]). The decomposition of intermediate systems in the next reaction stage leads to the formation of tetranuclear clusters. The type of solvent used in the synthesis of multinuclear oxo-Ti(IV) complexes is an important factor, which influences the kind of clusters formed.

The Stability Study of Pentacyano(4-mercaptopyridine)ruthenate(II) Self-Assembled Monolayer onto Gold Surface

The stability of the SAM formed by 4-mercaptopyridine (pyS) coordinated to the [Ru(CN)5]3− moiety (RupyS) has been evaluated by SERS and LSV as function of the electrode immersion time in the modifier solution. The cathodic shift observed for the potential assigned to the RS–Au reductive electrode reaction of the RupySAu, comparatively to the pySAu, hints that the π back-bonding effect enhances the chemisorption process. Contrary to that observed for the SAM of pyS on gold, the SERS spectra profiles acquired for the RupyS monolayer formed onto gold after longer immersion times indicated the absence of the structural conversion process.