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The gas-phase fragmentation behavior of self-assembled metallo-supramolecular rhombs based on an unusual chiral [2.2]paracyclophane bis(pyridine) ligand is studied by collision-induced dissociation mass spectrometry. The fragmentation patterns strongly depend on the charge state of the respective mass-selected aggregate. For the doubly charged ions, simple symmetric fragmentation is observed in full accordance with previous results reported for related metallo-supramolecular species. The triply charged species cleaves unsymmetrically which can be rationalized by a preferred formation of ions with low charge density. CID of the quadruply charged rhomb reveals a complex fragmentation. Besides ligand oxidation to the radical cation, facile cleavage of the central covalently bound part of the [2.2]paracyclophane ligand takes place which is even preferred over rupture of the weak dative pyridine-Pd bond. [ABSTRACT FROM AUTHOR]

We implement sensitized alkoxyamines as "photodynamic covalent bonds"--bonds that, while being stable in the dark at ambient temperatures, upon photoexcitation efficiently dissociate and recombine to the bound state in a fast thermal reaction. This type of bond allows for the photochemically induced exchange of molecular building blocks and resulting constitutional variation within dynamic reaction networks. To this end, alkoxyamines are coupled to a xanthone unit as triplet sensitizer enabling their reversible photodissociation into two radical species. By studying the photochemical properties of three generations of sensitized alkoxyamines it became clear that the nature and efficiency of triplet energy transfer from the sensitizer to the alkoxyamine bond as well as the reversibility of photodissociation crucially depends on the structure of the nitroxide terminus. By employing the thus designed photodynamic covalent bonding motif, we demonstrate how to use light energy to shift a dynamic covalent reaction network away from its thermodynamic minimum into a photostationary state. The network could be repeatedly switched between its minimum and kinetically trapped out-of-equilibrium state by thermal scrambling and selective photoactivation of sensitized alkoxyamines, respectively. [ABSTRACT FROM AUTHOR]

A small library of diarylsydnones (DASyds) was constructed based on aryl-pairing combinations and subjected to click reaction toward alkenes under photoirradiation with high efficiency. We were able to demonstrate the utility of DASyds for highly fluorescent turn-on ligation targeting the trans-cyclooct-4-en-1-ol moieties on protein. [ABSTRACT FROM AUTHOR]

Rapid progress has been witnessed in the past decade in the fields of covalent organic frameworks (COFs) and dynamic nuclear polarization (DNP). In this contribution, we bridge these two fields by constructing radical-embedded COFs as promising DNP agents. Via polarization transfer from unpaired electrons to nuclei, DNP realizes significant enhancement of NMR signal intensities. One of the crucial issues in DNP is to screen for suitable radicals to act as efficient polarizing agents, the basic criteria for which are homogeneous distribution and fixed orientation of unpaired electrons. We therefore envisioned that the crystalline and porous structures of COFs, if evenly embedded with radicals, may work as a new "crystalline sponge" for DNP experiments. As a proof of concept, we constructed a series of proxyl-radical-embedded COFs (denoted as PR(x)-COFs) and successfully applied them to achieve substantial DNP enhancement. Benefiting from the bottom-up and multivariate synthetic strategies, proxyl radicals have been covalently reticulated, homogeneously distributed, and rigidly embedded into the crystalline and mesoporous frameworks with adjustable concentration (x%). Excellent performance of PR(x)-COFs has been observed for DNP 1H, 13C, and 15N solid-state NMR enhancements. This contribution not only realizes the direct construction of radical COFs from radical monomers, but also explores the new application of COFs as DNP polarizing agents. Given that many radical COFs can therefore be rationally designed and facilely constructed with well-defined composition, distribution, and pore size, we expect that our effort will pave the way for utilizing radical COFs as standard polarizing agents in DNP NMR experiments. [ABSTRACT FROM AUTHOR]

Recently, porous framework materials with various network-type structures have been constructed via several different approaches, such as coordination interactions, reversible covalent bonds, and non-covalent interactions. Here, we have combined the concepts of supramolecular coordination complex (SCC) and metal--organic framework to offer a new strategy to construct a diamondoid supramolecular coordination framework (SCF) from an adamantanoid supramolecular coordination cage as the tetrahedral node and a difunctional Pt(II) ligand as the linear linker via stepwise orientation-induced supramolecular coordination. The adamantanoid supra-molecular coordination cage has four uncoordinated pyridyl groups, which serve as the four vertexes of the tetrahedral geometry in the diamondoid framework. As a result, this diamondoid SCF exhibits an adamantanoid-to-adamantanoid substructure with two sets of pores, including the interior cavity of the adamantanoid cage and the extended adamantanoid space between the individual cages in the framework. In addition, the shape-controllable and highly ordered self-assembly of nanometer-sized diamondoid SCF is observed as micrometer-sized regular octahedrons by evaporation under heating in DMSO. This study demonstrates the potential application of supramolecular coordination complexes in the precise construction of highly regulated porous framework materials. [ABSTRACT FROM AUTHOR]

Antibiotic resistance of bacterial pathogens poses an increasing threat to the wellbeing of our society and urgently calls for new strategies for infection diagnosis and antibiotic discovery. The antibiotic resistance problem at least partially arises from extensive use of broad-spectrum antibiotics. Ideally, for the treatment of infection, one would like to use a narrow-spectrum antibiotic that specifically targets and kills the disease-causing strain. This is particularly important considering the commensal bacterial species that are beneficial and sometimes even critical to the health of a human being. In this contribution, we describe a phage display platform that enables rapid identification of peptide probes for specific bacterial strains. The phage library described herein incorporates 2-acetylphenyl-boronic acid moieties to elicit dynamic covalent binding to the bacterial cell surface. Screening of the library against live bacterial cells yields submicromolar and highly specific binders for clinical strains of Staphylococcus aureus and Acinetobacter baumannii that display antibiotic resistance. We further show that the identified peptide probes can be readily converted to bactericidal agents that deliver generic toxins to kill the targeted bacterial strain with high specificity. The phage display platform described here is applicable to a wide array of bacterial strains, paving the way to facile diagnosis and development of strain-specific antibiotics. [ABSTRACT FROM AUTHOR]

In the dynamic synthesis of covalent organic frameworks and molecular cages, the typical synthetic approach involves heuristic methods of discovery. While this approach has yielded many remarkable products, the ability to predict the structural outcome of subjecting a multitopic precursor to dynamic covalent chemistry (DCC) remains a challenge in the field. The synthesis of covalent organic cages is a prime example of this phenomenon, where precursors designed with the intention of affording a specific product may deviate dramatically when the DCC synthesis is attempted. As such, rational design principles are needed to accelerate discovery in cage synthesis using DCC. Herein, we test the hypothesis that precursor bite angle contributes significantly to the energy landscape and product distribution in multitopic alkyne metathesis (AM). By subjecting a series of precursors with varying bite angles to AM, we experimentally demonstrate that the product distribution, and convergence toward product formation, is strongly dependent on this geometric attribute. Surprisingly, we discovered that precursors with the ideal bite angle (60°) do not afford the most efficient pathway to the product. The systematic study reported here illustrates how seemingly minor adjustments in precursor geometry greatly affect the outcome of DCC systems. This research illustrates the importance of fine-tuning precursor geometric parameters in order to successfully realize desirable targets. [ABSTRACT FROM AUTHOR]

Within the current study, a novel approach for the detailed determination of the scratch healing efficiency in mussel-inspired polymer films is presented. For this purpose, a sensor molecule was incorporated into a self-healing polymer network based on reversible zinc-histidine interactions. The fluorescence of the sensor molecule was monitored enabling a detailed depth- and time-resolved determination of the healing efficiency by means of confocal laser scanning microscopy (CLSM). Finally, this concept represents an efficient and detailed approach for the determination of the scratch self-healing efficiency in polymer films and can also be applied for other scratch self-healing systems, which are based on reversible dynamic bonds. [ABSTRACT FROM AUTHOR]

A macrocycle through a dynamic covalent approach relying on reversible hydrazone formation in acidic aqueous solutions at elevated temperatures is constructed. By decreasing the acidity of the solution and lowering the temperature, the structure becomes kinetically inert. The macrocycle is capable of hosting hydrophobic aromatic guest molecules in water. [ABSTRACT FROM AUTHOR]

A "Janus" type of hexa-cata-hexabenzocoronene with three triptyceno subunits fused symmetrically on the periphery of coronene has been synthesized using a covalent self-assembly strategy. The triptyceno subunits form a nanosized nest on one side of the aromatic plane with space-matching fullerenes such as C60 and C70 to afford shape-complementary supramolecular complexes. The formation of the complexes in solution was confirmed by 1H NMR and fluorescence titration. Four complexes with C60 or C70 were obtained and studied by single-crystal X-ray diffraction analysis. In the crystal structure, the host shows a proper tunability to adjust its conformation in accordance with the shape of the guest. The different stoichiometric ratios and various stacking patterns of the complexes suggest the diversity of this nonplanar polyaromatic host in complexation with fullerenes. [ABSTRACT FROM AUTHOR]

Protein function critically depends on structure. However, current analytical tools to monitor consistent higher-order structure with high sensitivity, as for instance required in the development of biopharmaceuticals, are limited. To complement existing assays, we present the analytical cascade of enzymes (ACE), a method based on enzymatic modifications of target proteins, which serve to exponentially amplify structural differences between them. The method enables conformational and chemical fingerprinting of closely related proteins, allowing for the sensitive detection of heterogeneities in protein preparations with high precision. Using this method, we detect protein variants differing in conformation only, as well as structural changes induced by diverse covalent modifications. Additionally, we employ this method to identify the nature of structural variants. Moreover, the ACE method should help to address the limited reproducibility in fundamental research, which partly relates to sample heterogeneities. [ABSTRACT FROM AUTHOR]

Continuous fiber reinforced thermoset composites are highly important in a variety of industrial applications because of their ultrahigh specific strength. However, conventional shaping techniques for such materials can hardly achieve very sophisticated shapes. In this paper, we present an unconventional method to enable complex shapes for continuous fiber reinforced thermoset epoxy composites that provided a capability of topology change due to the dynamic ester bonds. Thus, the corresponding composites providing largely enhanced mechanical strength can be endowed with complex permanent shapes via folding. At lower temperatures, the exchange reaction was frozen and the epoxy networks provided ideal shape memory behavior. Accordingly, the composites with complex permanent shapes enabled a self-folding shape recovery process. In addition, a laser half-cutting technique was applied to build controllable defects within the composites. As a result, more complex folding geometries which were extremely challenging for handwork can be easily fabricated. [ABSTRACT FROM AUTHOR]

Covalent drug discovery has undergone a resurgence in recent years due to comprehensive optimization of the structure-activity relationship (SAR) and the structure-reactivity relationship (SRR) for covalent drug candidates. The natural product oridonin maintains an impressive pharmacological profile through its covalent enone warhead on the D-ring and has attracted substantial SAR studies to characterize its potential in the development of new molecular entities for the treatment of various human cancers and inflammation. Herein, for the first time, we report the excessive reactivity of this covalent warhead and mediation of the covalent binding capability through a Rh2(esp)2-catalyzed mild and concise regio- and stereospecific aziridination approach. Importantly, aziridonin 44 (YD0514), with a more-druglike irreversible covalent warhead, has been identified to significantly induce apoptosis and inhibit colony formation against triple-negative breast cancer with enhanced antitumor effects in vitro and in vivo while displaying lower toxicity to normal human mammary epithelial cells in comparison to oridonin. [ABSTRACT FROM AUTHOR]

The effective control of crystallinity of covalent organic frameworks (COFs) and the optimization of their performances related to the crystallinity have been considered as big challenges. COFs bearing flexible building blocks (FBBs) generally own larger lattice sizes and broader monomer sources, which may endow them with unprecedented application values. Herein, we report the oriented synthesis of a series of two-dimensional (2D) COFs from FBBs with different content of intralayer hydrogen bonds. Studies of H-bonding effects on the crystallinity and adsorption properties indicate that partial structure of the COFs is "locked" by the H-bonding interaction, which consequently improves their microscopic order degree and crystallinity. Thus, the regulation of crystallinity can be effectively realized by controlling the content of hydrogen bonds in COFs. Impressively, the as-prepared COFs show excellent and reversible adsorption performance for volatile iodine with capacities up to 543 wt %, much higher than all previously reported adsorbents, although the variation tendency of adsorption capacities is opposite to their crystallinity. This study provides a general guidance for the design and construction of highly/appropriately crystalline COFs and ultrahigh-capacity iodine adsorbents. [ABSTRACT FROM AUTHOR]

Understanding atomic-level mechanisms of methane adsorption in nanoporous materials is of great importance to increase their methane storage capacity targeting energy sources with low carbon emission. In this work, we considered layered covalent organic frameworks (COFs) with low density and revealed the effect of slipping and chemical functionalization on their methane adsorption and separation properties. We performed grand canonical Monte Carlo simulations studies of methane (CH4) adsorption and carbon-dioxide:methane (CO2:CH4) separation in various slipped structures of TpPa1, TpBD, PI-COFs, and functionalized TpPa1 and TpBD COFs as well. We observed that the slipping improves the total CH4 uptake by 1.1-1.5 times, while functionalization does not have a significant effect on CH4 uptake. We also observed improvement in CO2:CH4 selectivity due to slipping, whereas functionalization results in decrease in the selectivity. In all considered COFs, we found the highest CH4 delivery capacity of 141 cm³ (STP) cm-3 at 65 bar and selectivity of ~25 at 1 bar in 60-AB slipped structure of TpBD COF. We analyzed the molecular details of CH4 adsorption using binding energy, heat of adsorption, pore characteristics, and expectation energy landscape. Our results show that COFs with increasing profile of heat of adsorption with pressure have the higher CH4 delivery capacity. In these COFs, we found proximity (~4-6 Å) of CH4 binding sites, resulting in higher CH4-CH4 interactions and hence the increasing profile of CH4 heat of adsorption. [ABSTRACT FROM AUTHOR]

Covalent organic frameworks (COFs) are an emerging class of porous crystalline polymers with wide range of potential applications. However, the availability of three-dimensional (3D) COFs is still limited, and their synthesis is confined to the high-temperature solvothermal method. Here, we report for the first time a general and simple strategy to produce a series of 3D ionic liquid (IL)-containing COFs (3D-IL-COFs) by using IL as a green solvent. The syntheses are carried out at ambient temperature and pressure accompanied by a high reaction speed (e.g., only three mins for 3D-IL-COF-1), and the IL can be reused without activity loss. Furthermore, the 3-DILCOFs show impressive performance in the separation of CO2/N2 and CO2/CH4. This research thus presents a potential pathway to green large-scale industrial production of COFs. [ABSTRACT FROM AUTHOR]

The structural uniqueness of covalent organic frameworks (COFs) has brought these new materials great potential for advanced applications. One of the key aspects yet to be developed is how to improve the robustness of covalently linked reticular frameworks. In order to make the best use of π-conjugated structures, we develop herein a "killing two birds with one stone" strategy and construct a series of ultrastable benzoxazole-based COFs (denoted as LZU-190, LZU-191, and LZU-192) as metal-free photocatalysts. Benefiting from the formation of benzoxazole rings through reversible/irreversible cascade reactions, the synthesized COFs exhibit permanent stability in the presence of strong acid (9 M HCl), strong base (9 M NaOH), and sunlight. Meanwhile, reticulation of the benzoxazole moiety into the π-conjugated COF frameworks decreases the optical band gap and therefore increases the capability for visible-light absorption. As a result, the excellent photoactivity and unprecedented recyclability of LZU-190 (for at least 20 catalytic runs, each with a product yield of 99%) have been illustrated in the visible-light-driven oxidative hydroxylation of arylboronic acids to phenols. This contribution represents the first report on the photocatalytic application of benzoxazole-based structures, which not only sheds new light on the exploration of robust organophotocatalysts from small molecules to extended frameworks but also offers in-depth understanding of the structure-activity relationship toward practical applications of COF materials. [ABSTRACT FROM AUTHOR]

We describe an approach to regulate the cellular uptake of small gold nanoparticles using supramolecular chemistry. The strategy relies on the functionalization of AuNPs with negatively charged pyranines, which largely hamper their penetration in cells. Cellular uptake can be activated in situ through the addition of cationic covalent cages that specifically recognize the fluorescent pyranine dyes and counterbalance the negative charges. The high selectivity and reversibility of the host-guest recognition activates cellular uptake, even in proteinrich biological media, as well as its regulation by rational addition of either cage or pyranine. [ABSTRACT FROM AUTHOR]

As one of the most widely used chemical concepts whose origin can be traced back to Lewis theory of bonding a century ago, a covalent bond involves sharing one or more pairs of electrons between atoms. A strong covalent interaction (SCI) is such a covalent bond that two or more electron pairs are shared, yielding a double, triple, quadruple, or even higher bond order. Despite its ubiquity and usefulness, a robust and generally applicable approach to accurately identify strong covalent interactions and determine their bond orders is still lacking. In this work, an SCI index is proposed from density functional theory using the Pauli energy, which is the contribution of the Pauli exclusion principle to the kinetic energy. Illustrative examples from organic, inorganic, and organometallic systems were provided. Its close relationship with the electron localization function (ELF) was elucidated. Both ELF and SCI generate similar results. Two complexes with a quintuple metal-metal bond have been confirmed. A stronger than quintuple bond has been showcased. This work should provide a robust approach to determine bond orders for strong covalent interactions in complex systems, pinpoint the physiochemical origin of strong covalent interactions, and rationalize the usefulness of both SCI and ELF. These tools should be able to be applied to other systems in different fields to effectively appreciate strong covalent interactions. [ABSTRACT FROM AUTHOR]

Post-translational modifications by the covalent attachment of Rub1 (NEDD8), ubiquitin, SUMO, and other small signaling proteins have profound impacts on the functions and fates of cellular proteins. Investigations of the relationship of these bioactive structures and their functions are limited by analytical methods that are scarce and tedious. A novel strategy is reported here for the analysis of branched proteins by top-down mass spectrometry and illustrated by application to four recombinant proteins and one synthetic peptide modified by covalent bonds with ubiquitin or Rub1. The approach allows an analyte to be recognized as a branched protein; the participating proteins to be identified; the site of conjugation to be defined; and other chemical, native, and recombinant modifications to be characterized. In addition to the high resolution and high accuracy provided by the mass spectrometer, success is based on sample fragmentation by electron-transfer dissociation assisted by collisional activation and on software designed for graphic interpretation and adapted for branched proteins. The strategy allows for structures of unknown, two-component branched proteins to be elucidated directly the first time and can potentially be extended to more complex systems. [ABSTRACT FROM AUTHOR]

Two-dimensional covalent organic framework (COF) materials can serve as excellent candidates for gas storage due to their high density of periodically arranged pores and channels, which can be tethered with functional groups. However, post-functionalization tends to disturb the structure of the COF; thus, it is attractive to develop synthetic approaches that generate built-in functionalities. Herein, we develop a new strategy for the construction of 2D-COFs with built-in, unreacted periodic bonding networks by solvent-directed divergent synthesis. Tetraphenylethane (TPE), which combines both π-rigidity for stacking and rotational flexibility, is selected as the central core for COF construction. By solvent control, two distinct COF structures could be constructed, arising from a [4 + 4] condensation pathway (TPE-COF-I) or an unusual [2 + 4] pathway (TPE-COF-II). TPE-COF-II contains unreacted linker units arranged around its pores and shows greatly enhanced carbon dioxide adsorption performance (23.2 wt %, 118.8 cm³ g-1 at 1 atm, 273 K), which is among the best COF materials for CO2 adsorption reported to date. [ABSTRACT FROM AUTHOR]

Single-walled carbon nanotubes (SWCNT) have been covalently cross-linked via a reductive functionalization pathway, utilizing negatively charged carbon nanotubides (KC4). We have compared the use of difunctional linkers acting as molecular pillars between the nanotubes, namely, p-diiodobenzene, pdiiodobiphenyl, benzene-4,4'-bis(diazonium), and 1,1'-biphenyl-4,4'-bis- (diazonium) salts as electrophiles. We have employed statistical Raman spectroscopy (SRS), a forefront characterization tool consisting of thermogravimetric analysis coupled with gas chromatography and mass spectrometry (TGGC- MS) and aberration-corrected high-resolution transmission electron microscopy imaging series at 80 kV to unambiguously demonstrate the covalent binding of the molecular linkers. The present study shows that the SWCNT functionalization using iodide derivatives leads to the best results in terms of bulk functionalization homogeneity (Hbulk) and degree of addition. Phenylene linkers yield the highest degree of functionalization, whereas biphenylene units induce a higher surface area with an increase in the thermal stability and an improved electrochemical performance in the oxygen reduction reaction (ORR). This work illustrates the importance of molecular engineering in the design of novel functional materials and provides important insights into the understanding of basic principles of reductive cross-linking of carbon nanotubes. [ABSTRACT FROM AUTHOR]

Covalent organic frameworks (COFs) are porous crystalline materials that are entirely composed of organic building blocks and can be assembled straightforwardly from solution. The main synthetic challenge associated with COFs, compared to other porous materials such as zeolites or metal-organic frameworks, is their poor long-range order; typical sizes of crystal domains do not exceed a few tens of nanometers. Here, we develop a model of the molecular constituents of COF-5 and follow the early stages of its assembly dynamics from dilute solution. Our simulations indicate that under typical experimental conditions COF-5 formation happens not through nucleation but far from equilibrium through spinodal decomposition. This rapid assembly mode leads to a plethora of defects that are difficult to anneal and that are likely responsible for the limited crystallinity observed in the synthesis of many COFs. We analyze the driving forces for COF-5 formation and find that stacking interactions between aromatic molecular constituents are too strong. When these interactions are weakened, assembly proceeds through single nucleation events followed by slow growth. The COF-5 crystallites obtained in this way are essentially defect-free. These results suggest experimental strategies for growing COFs with enhanced crystalline quality. [ABSTRACT FROM AUTHOR]

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the six unique cyclic quaternary complexes FH:FH:FH:FH, FH:FH:FH:FCl, FH:FH:FCl:FCl, FH:FCl:FH:FCl, FH:FCl:FCl:FCl, and FCl:FCl:FCl:FCl stabilized by F-H...F hydrogen bonds and F-Cl...F halogen bonds. The binding energies of these complexes decrease as the number of FH molecules decreases, and therefore as the number of hydrogen bonds decreases, indicating that hydrogen bonds are primarily responsible for stabilities. Nonadditivities of binding energies are synergistic for complexes with 4, 3, and 2 FH molecules, but antagonistic for those with 1 and 0 FH molecules. In addition to depending on changes in F-F, F-H, and F-Cl distances, complex binding energies are also influenced by two sets of angular parameters. These include the external F-F-F angles which must sum to 360° in these cyclic structures, and the internal H-F-F angles for hydrogen bonds and F-Cl-F angles for halogen bonds, which measure the deviation from linearity of these bonds. Transition structures present the barriers to converting an equilibrium structure to an equivalent equilibrium structure on the potential surfaces. These barriers increase as the number of FH molecules decreases. EOM-CCSD spin-spin coupling constants 2hJ(F-F) across hydrogen bonds in complexes tend to increase with decreasing F-F distance. They increase dramatically in transition structures, but show no dependence on the F-F distance. The one-bond coupling constants 1hJ(F-H) are relatively small and negative in complexes, increase dramatically, and are positive in transition structures. 1J(F-H) values are greatest for the covalent F-H bond. Coupling constants 1xJ(F-Cl) across halogen bonds are relatively small and positive in complexes, and increase dramatically in transition structures. The largest values of 1J(F-Cl) are found for covalent bonds. [ABSTRACT FROM AUTHOR]

The chemical formation process has been studied from relaxation holes, Δh(u), resulting from the difference between the radial intracule density and the nonrelaxed counterpart, which is obtained from atomic radial intracule densities and the pair density constructed from the overlap of the atomic densities. Δh(u) plots show that the internal reorganization of electron pairs prior to bond formation and the covalent bond formation from electrons in separate atoms are completely recognizable processes from the shape of the relaxation hole, Δh(u). The magnitude of Δh(u), the shape of Δh(u) ∀ u < Req, and the distance between the minimum and the maximum in Δh(u) provide further information about the nature of the chemical bond formed. A computational affordable approach to calculate the radial intracule density from approximate pair densities has been also suggested, paving the way to study electron-pair distributions in larger systems. [ABSTRACT FROM AUTHOR]

Covalent organic frameworks (COFs) represent an emerging class of crystalline porous materials that are constructed by the assembly of organic building blocks linked via covalent bonds. Several strategies have been developed for the construction of new COF structures; however, a facile approach to fabricate hierarchical COF architectures with controlled domain structures remains a significant challenge, and has not yet been achieved. In this study, a dynamic covalent chemistry (DCC)-based postsynthetic approach was employed at the solid-liquid interface to construct such structures. Two-dimensional imine-bonded COFs having different aromatic groups were prepared, and a homogeneously mixed-linker structure and a heterogeneously core-shell hollow structure were fabricated by controlling the reactivity of the postsynthetic reactions. Solid-state nuclear magnetic resonance (NMR) spectroscopy and transmission electron microscopy (TEM) confirmed the structures. COFs prepared by a postsynthetic approach exhibit several functional advantages compared with their parent phases. Their Brunauer-Emmett-Teller (BET) surface areas are 2-fold greater than those of their parent phases because of the higher crystallinity. In addition, the hydrophilicity of the material and the stepwise adsorption isotherms of H2O vapor in the hierarchical frameworks were precisely controlled, which was feasible because of the distribution of various domains of the two COFs by controlling the postsynthetic reaction. The approach opens new routes for constructing COF architectures with functionalities that are not possible in a single phase. [ABSTRACT FROM AUTHOR]

We numerically investigate the self-dynamics and collective dynamics of a simple model for vitrimers-polymeric covalent networks that have the ability to dynamically rearrange the bond structure via exchange reactions, preserving the total connectivity. Specifically, we study a binary mixture of tetrafunctional and bifunctional particles by means of molecular dynamics simulations that naturally incorporate the bond-swapping mechanism. We specifically focus on the dynamics at small wavevector q by simulating 800 000 particles. We observe two distinct collective relaxation processes: (i) a fast vibrational damped mode and (ii) a slow network restructuring dynamics. Unexpectedly, the slow process is characterized by a wavevector-independent (q0) mode originating from the swap motion of the bonds. [ABSTRACT FROM AUTHOR]

Organic paramagnetic and electroactive molecules are attracting interest as core components of molecular electronic and spintronic devices. Currently, further progress is hindered by the modest stability and reproducibility of the molecule/electrode contact. We report the synthesis of a persistent organic radical bearing one and two terminal alkyne groups to form Au-C s bonds. The formation and stability of self-assembled monolayers and the electron transport through single-molecule junctions at room temperature have been studied. The combined analysis of both systems demonstrates that this linker forms a robust covalent bond with gold and a better-defined contact when compared to traditional sulfur-based linkers. Density functional theory and quantum transport calculations support the experimental observation highlighting a reduced variability of conductance values for the C-Au based junction. Our findings advance the quest for robustness and reproducibility of devices based on electroactive molecules. [ABSTRACT FROM AUTHOR]

The integration of mechanophores, motifs that transduce mechanical forces into chemical reactions, allows creating materials with stress-dependent properties. Typical mechanophores are activated by cleaving weak covalent bonds, but these reactions can also be triggered by other stimuli, and this renders the behavior unspecific. Here we show that this problem can be overcome by extending the molecular-shuttle function of rotaxanes to mechanical activation. A mechanically interlocked mechanophore composed of a fluorophore-carrying macrocycle and a dumbbell-shaped molecule containing a matching quencher was integrated into a polyurethane elastomer. Deformation of this polymer causes a fluorescence turn-on, due to the spatial separation of fluorophore and quencher. This process is specific, efficient, instantly reversible, and elicits an easily detectable optical signal that correlates with the applied force. [ABSTRACT FROM AUTHOR]

Covalent organic frameworks (COFs) are crystalline, highly porous, two- or three-dimensional polymers with tunable topology and functionalities. Because of their higher chemical stabilities in comparison to their boron-linked counterparts, imine or β-ketoenamine linked COFs have been utilized for a broad range of applications, including gas storage, heterogeneous catalysis, energy storage devices, or proton-conductive membranes. Herein, we report the synthesis of highly porous and chemically stable acetylene (-C≡C-) and diacetylene (-C≡C-C≡C-) functionalized β-ketoenamine COFs, which have been applied as photocatalyst for hydrogen generation from water. It is shown that the diacetylene moieties have a profound effect as the diacetylene-based COF largely outperforms the acetylene-based COF in terms of photocatalytic activity. As a combined effect of high porosity, easily accessible diacetylene (-C≡C-C≡C-) functionalities and considerable chemical stability, an efficient and recyclable heterogeneous photocatalytic hydrogen generation is achieved. [ABSTRACT FROM AUTHOR]

In spite of their great promise for enantioselective processes due to the rich host-guest chemistry, it remains a challenge to construct covalent organic frameworks (COFs) with chiral three-dimensional (3D) structures. Here we report bottom-up synthesis of the first example of 3D chiral COFs by imine condensation of an enantiopure 2-fold symmetric TADDOL-derived tetraaldehyde with a tetrahedral tetra(4-anilyl)methane. After postsynthetic oxidation of imine linkages, the framework is transformed into an amide-linked COF with retention of crystallinity and permanent porosity as well as enhanced chemical stability. The resultant isostructural COFs feature a 4-fold interpenetrated diamondoid open framework with tubular channels decorated with chiral dihydroxy auxiliaries. Both COFs can be used as chiral stationary phases for high performance liquid chromatography to enantioseparate racemic alcohols, and the oxidized COF shows superior separation performance compared to the pristine framework. [ABSTRACT FROM AUTHOR]

In the drive toward green and sustainable methodologies for chemicals manufacturing, biocatalysts are predicted to have much to offer in the years to come. That being said, their practical applications are often hampered by a lack of long-term operational stability, limited operating range, and a low recyclability for the enzymes utilized. Herein, we show how covalent organic frameworks (COFs) possess all the necessary requirements needed to serve as ideal host materials for enzymes. The resultant biocomposites of this study have shown the ability boost the stability and robustness of the enzyme in question, namely lipase PS, while also displaying activities far outperforming the free enzyme and biocomposites made from other types of porous materials, such as mesoporous silica and metal-organic frameworks, exemplified in the kinetic resolution of the alcohol assays performed. The ability to easily tune the pore environment of a COF using monomers bearing specific functional groups can improve its compatibility with a given enzyme. As a result, the orientation of the enzyme active site can be modulated through designed interactions between both components, thus improving the enzymatic activity of the biocomposites. Moreover, in comparison with their amorphous analogues, the well-defined COF pore channels not only make the accommodated enzymes more accessible to the reagents but also serve as stronger shields to safeguard the enzymes from deactivation, as evidenced by superior activities and tolerance to harsh environments. The amenability of COFs, along with our increasing understanding of the design rules for stabilizing enzymes in an accessible fashion, gives great promise for providing "off the shelf" biocatalysts for synthetic transformations. [ABSTRACT FROM AUTHOR]

Syntheses of porous organic cages (POCs) represent an important synthetic puzzle in dynamic covalent chemistry-based self-sorting. Improved understanding of the formation mechanisms of POCs can lead to control and rational design of cages with desired functionality. Herein, we explore the formation mechanisms of imine-based POCs using time-resolved electrospray mass spectrometry and electronic structure calculations at the density functional theory and correlated molecular orbital theory levels. We found that the synthesis of the [4 + 6] cycloimine cage CC3-R and the [2 + 3] cycloimine cage CC-pentane both proceed through similar intermediates via a series of consecutive reactions. The proposed reaction mechanisms are supported by electronic structure calculations. On the basis of our observations from both experiments and calculations, we propose a comprehensive method for designing and predicting new POC species. In addition, the observation of stable incomplete cages during CC3-R synthesis inspired us to design intentionally defective cages. These missing-linker-type molecular defects were installed into CC3-R via nonsolvent induced crystallization. The defective CC3-R materials were found to have enhanced CO2 interaction and improved CO2 uptake capacity due to the additional functional groups present within the CC3 crystals. [ABSTRACT FROM AUTHOR]

The collagen structure in skins is significantly influenced by the cross-linking chemistry adopted during leather processing. We have developed an in situ technique to measure real-time collagen structure changes using synchrotron-based small-angle X-ray scattering (SAXS). Three common mineral tanning systems, basic chromium sulfate (BCS), zirconium sulfate (ZIR) and an aluminosilicate-based reagent (ALS) were used to stabilize collagen in ovine skin. Studying the molecular changes by in situ SAXS revealed a range of tanning mechanisms: a complex combination of covalent cross-linking, electrostatic interactions and hydrogen bonding by BCS, hydrogen bonding interactions by ZIR, and the formation of colloidal aggregates by ALS. These results unravel the mechanisms of producing leathers with different properties, explaining why ZIR produces denser leathers while ALS produces softer leathers compared to conventional BCS leathers. ZIR and ALS are environment-friendly alternatives to BCS, and understanding their mechanisms is important for a more sustainable future for the leather industry. [ABSTRACT FROM AUTHOR]

ABSTRACT The field of protein conjugation most commonly refers to the chemical, enzymatic, or chemoenzymatic formation of new covalent bonds between two polypeptides, or between a single polypeptide and a new molecule (polymer, small molecule, nucleic acid, carbohydrate, etc.). Due to the modest selectivity of chemical methods for protein conjugation, there are increased efforts to develop biocatalysts that confer regioselectivity for site-specific modification, thereby complementing the existing toolbox of chemical conjugation strategies. This review summarizes key advances in the use of enzymes to functionalize proteins with commercial relevance. The examples put forth have demonstrated value at the industrial level or show promising industrial potential in the laboratory. [ABSTRACT FROM AUTHOR]

Selective inhibitors for each serine/threonine phosphatase (PPP) are essential to investigate the biological actions of PPPs and to guide drug development. Biologically diverse organisms (e.g., cyanobacteria, dinoflagellates, beetles) produce structurally distinct toxins that are catalytic inhibitors of PPPs. However, most toxins exhibit little selectivity, typically inhibiting multiple family members with similar potencies. Thus, the use of these toxins as chemical tools to study the relationship between individual PPPs and their biological substrates, and how disruptions in these relationships contributes to human disease, is severely limited. Here, we show that tautomycetin (TTN) is highly selective for a single PPP, protein phosphatase 1 (PP1/PPP1C). Our structure of the PP1:TTN complex reveals that PP1 selectivity is defined by a covalent bond between TTN and a PP1-specific cysteine residue, Cys127. Together, these data provide key molecular insights needed for the development of novel probes targeting single PPPs, especially PP1. [ABSTRACT FROM AUTHOR]

This work addresses the continuing disagreement between two schools of thought concerning the mechanism of covalent bonding. According to Hellmann, Ruedenberg, and Kutzelnigg, covalent bonding is a quantum mechanical phenomenon whereby lowering of the kinetic energy associated with electron sharing, i.e., delocalization, is the key stabilization mechanism. The opposing view of Slater, Feynman, and Bader has maintained that the source of stabilization is electrostatic potential energy lowering due to electron density redistribution to binding regions between nuclei. Following our study of H2+ we present an analogous detailed study of H2 where bonding involves an electron pair with repulsion and correlation playing a significant role in its properties. We use a range of different computational approaches to study and reveal the relevant contributions to bonding as seen in the electron density and corresponding kinetic and potential energy distributions. The energetics associated with the more complex electronic structure of H2, when examined in detail, clearly agrees with the analysis of Ruedenberg; i.e., covalent bonding is due to a decrease in the interatomic kinetic energy resulting from electronic delocalization. Our results support the view that covalent bonding is a quantum dynamical phenomenon requiring a properly quantized kinetic energy to be used in its description. [ABSTRACT FROM AUTHOR]

Osmotic power generated by pressure-retarded osmosis (PRO) has attracted global attention as a clean, abundant and renewable energy resource. However, the substrates of PRO membranes are particularly prone tofouling because of their direct contact with various foulants in raw water. This leads to a significant decline in power density and impedes the commercialization of PRO technology. In this work, a facile surface modification method has been developed to obtain a new type of nanoparticle functionalized antifouling PRO membranes. Carbon quantum dots (CQDs), with an average size around 3.2 nm, are fabricated from citric acid via a simple method. Subsequently, they are immobilized onto the polydopamine (PDA) layer grafted on the substrate surface of poly(ether sulfone) (PES) membranes via covalent bonding. The bacteria diffusion tests show that the CQD modified PRO membranes possess much enhanced antibacterial activity and antibiofouling propensity. The continuous PRO operations at 15 bar also confirm that the CQD modified membranes exhibit a much higher power density (11.0 vs 8.8 W/m2) and water recovery after backwash (94 vs 89%) than the unmodified ones. This study may open up a new avenue in the fabrication of nanostructure functionalized polymeric membranes for wastewater treatment and osmotic power generation. [ABSTRACT FROM AUTHOR]

We present a C-H activation protocol for aromatic compounds that overcomes the current limitations of the need for a directing group or covalently bound activating groups, by exploiting the increase in C-H acidity of aromatic compounds on π-coordination to a Ru(II) center. The increased acidity facilitates catalytic concerted metalation-deprotonation and subsequent arylation reactions. We present the development and optimization of the C-H activation protocol and show the applicability of the reaction to a range of aromatic substrates, including the simplest of substrates (benzene). Furthermore, we demonstrate the recyclability of the activating Ru(II) fragment using photolysis and give a mechanistic study, which provides strong evidence that this reaction occurs via a silver-mediated C-H bond activation. This is the first time Ru complexes have been shown to allow C-H activation of arenes by a π-coordination mechanism. [ABSTRACT FROM AUTHOR]

This work unveils the noncovalent interactions of a novel series of finely tuned gemini surfactants (Cm-E2O-Cm, m = 12, 14, and 16) with myoglobin (Mb) using multifaceted spectroscopic/voltammetric and docking techniques. The Mb-binding capacity of these geminis decreased in the order of C14-E2O-C14 > C16-E2O-C16 > C12-E2O-C12, following the 1:2 stoichiometry, as confirmed by the quantitative evaluation of binding constants via intrinsic fluorescence and cyclic voltammetry. The binding-induced microenvironmental and conformational changes of Mb were explored by pyrene/synchronous/three-dimensional (3-D) fluorescence and absorption spectroscopy. Furthermore, far- and near-ultraviolet (UV) circular dichroism spectral results depicted discernible changes in both secondary and tertiary structures of Mb upon complexation with Cm-E2O-Cm. Molecular docking specified the binding site, and aromatic residues involved in the complexation. These investigations provide deeper insight into the structure-property relationships of biomacromolecules, and they will be useful in designing/selecting appropriate surfactants which, in turn, can facilitate the application of protein-surfactant mixtures in pharmaceutical, biological, and industrial fields. [ABSTRACT FROM AUTHOR]

We demonstrate photocatalytic hydrogen evolution using COF photosensitizers with molecular proton reduction catalysts for the first time. With azine-linked N2-COF photosensitizer, chloro(pyridine)cobaloxime co-catalyst, and TEOA donor, H2 evolution rate of 782 µmol h-1 g-1 and TON of 54.4 has been obtained in a water/acetonitrile mixture. PXRD, solid-state spectroscopy, EM analysis, and quantum-chemical calculations suggest an outer sphere electron transfer from the COF to the co-catalyst which subsequently follows a monometallic pathway of H2 generation from the CoIII-hydride and/or CoII-hydride species. [ABSTRACT FROM AUTHOR]

Using semiclassical Boltzmann transport theory and density functional formalism, we have systematically studied the thermoelectric performance of layered GeAs2. The figure of merit, ZT value, of this layered structure is found to be 2.78 along the out-of-plane direction, with optimal carrier concentration at 800 K. Analysis of the charge density difference and phonon transport properties allows us to attribute such exceptional thermoelectric properties to strong interlayer interaction between the adjacent layers where quasicovalent bonding is responsible for the enhanced electrical conductivity, while the layered structure accounts for the suppressed lattice thermal conductivity. This study highlights the potential of layered crystals for highly efficient thermoelectric materials. [ABSTRACT FROM AUTHOR]

Currently, no effective and safe medicines are available to treat diabetic nephropathy (DN). Bardoxolone methyl (CDDO-Me) has displayed promising anti-DN activity as well as serious side effects in clinical trials, probably because the highly reactive α-cyano-α,β-unsaturated ketone (CUK) in ring A of CDDO-Me can covalently bind to thiol functionalities in many biomacromolecules. In this study, we designed and synthesized a γ-glutamyl transpeptidase (GGT)-based and CUK-modified derivative of CDDO-Me (2) to address this issue. 2 can be specifically cleaved by GGT, which is highly expressed in the kidney, to liberate CDDO-Me in situ. It should be noted that 2 exhibited anti-DN efficacy comparable to that of CDDO-Me with much less toxicity in cells and db/db mice, suggesting that its safety is better than CDDO-Me. Our findings not only reveal the therapeutic potential of 2 but also provide a strategy to optimize other synthetic molecules or natural products bearing a pharmacophore like CUK to achieve safer pharmaceutical drugs. [ABSTRACT FROM AUTHOR]

Controlling spin-spin interactions in multispin molecular assemblies is important for developing new approaches to quantum information processing. In this work, a covalent electron donor-acceptor-radical triad is used to probe spin-selective reduction of the stable radical to its diamagnetic anion. The molecule consists of a perylene electron donor chromophore (D) bound to a pyromellitimide acceptor (A), which is, in turn, linked to a stable α,γ-bisdiphenylene-β-phenylallyl radical (R*) to produce D-A-R*. Selective photoexcitation of D within D-A-R* results in ultrafast electron transfer to form the D+*-A-*-R* triradical, where D+*-A-* is a singlet spincorrelated radical pair (SCRP), in which both SCRP spins are uncorrelated relative to the R* spin. Subsequent ultrafast electron transfer within the triradical forms D+*-AR-, but its yield is controlled by spin statistics of the uncorrelated A[sup -*]-R* radical pair, where the initial charge separation yields a 3:1 statistical mixture of D[sup +*]-³(A-*-R*) and D[sup +*]-¹(A-*-R*), and subsequent reduction of R* only occurs in D[sup +*]-¹(A-*-R*). These findings inform the design of multispin systems to transfer spin coherence between molecules targeting quantum information processing using the agency of SCRPs. [ABSTRACT FROM AUTHOR]

Ab initio MP2/aug'-cc-pVTZ calculations were performed to identify equilibrium complexes and molecules and the transition structures that interconvert them, on the potential energy surfaces of a series of seven binary systems that have nitrogen heterocyclic carbenes (NHCs) as the electron-pair donors to CO2. Seven of the NHCs form complexes stabilized by C···C tetrel bonds, and six of these seven are also stabilized by a secondary interaction between an O of CO2 and the adjacent N-H group of the carbene. Six of the seven NHCs also form stable molecules with C-C covalent bonds, and with one exception, these molecules have binding energies that are significantly greater than the binding energies of the complexes. Charge-transfer stabilizes all of the NHC:CO2 complexes and occurs from the C lone pair of the carbene to the CO2 molecule. The six complexes that have secondary stabilizing interactions are also stabilized by back-donation of charge from the O to the adjacent N-H group of the carbene. Transition structures present barriers to the interconversion of complexes and molecules. With one exception, the barrier for converting a molecule to a complex is much greater than the barrier for the reverse reaction. Atoms in Molecules bonding parameters, shifts of IR C-O stretching and O-C-O bending frequencies, changes in NMR 13C chemical shieldings, and changes in C-C and C-O coupling constants as 1tJ(C-C) and J(C-O) for complexes and transition structures become ¹J(C-C) and 2J(C-O) for molecules, are all consistent with the changing nature of the C···C tetrel bond in the complex through the transition state to a covalent C-C bond in the molecule. [ABSTRACT FROM AUTHOR]

Recovery of the compromised antifouling property because of perturbation in the essential chemistry on top of the hierarchical topography of a superhydrophobic coating is commonly achieved through some stimuli (temperature, humidity, pH, etc.)-driven reassociation of the low surface energy molecules. However, self-healing of superhydrophobicity in physically damaged materials having inappropriate topography is difficult to achieve--and extremely important for the practical utility of this bioinspired property. Recently, very few materials have been introduced--that are capable of recovering the hierarchical features--but only after the application of appropriate external stimuli. Further, the optimization of appropriate stimuli is likely to be a challenging problem in practical scenarios. Here, we have strategically exploited a simple and robust 1,4-conjugate addition reaction between aliphatic primary amine and aliphatic acrylate groups for appropriate and covalent integration of a modified-graphene oxide nanosheet--which is well recognized for its exceptional mechanical properties. The synthesized material exhibited a remarkable ability to protect the antifouling property from various harsh physical insults, including physical erosion of the top surface of the polymeric coating and various physical manipulations etc. However, after application of pressure on the same polymeric coating, the bioinspired, nonadhesive (contact angle hysteresis <5°) superhydrophobicity was compromised, and the physically damaged polymeric coating became highly adhesive (contact angle hysteresis ~50°) and superhydrophobic. But, after releasing the pressure, the native nonadhesive (contact angle hysteresis <5°) extreme wettability was self-restored in the polymeric coating through recovery of the essential hierarchical topography--without requiring any external stimulus. This unique material, having impeccable durability and absolute self-healing capability, was further explored in (i) developing rewritable aqueous patterns on the extremely water-repellent surface and (ii) selective impregnation of water-soluble agents on the surface of polymeric coating--without any permanent change in the extreme water repellency property. The unique self-healing process eventually provided a superhydrophobic print--that was made out of hydrophilic small molecules. This printing was performed directly from an aqueous medium, which is extremely hard to achieve using the conventional superhydrophobic materials. Such multifunctional interfaces could be an important avenue for various smart applications, including delivery of hydrophilic small molecules, catalysis, self-assembly of colloids, reusable chemical sensing, etc. [ABSTRACT FROM AUTHOR]

The surfaces of materials fabricated from stainless steel are challenging to functionalize with antimicrobial moieties. This work demonstrates that stainless steel surfaces can be modified with an N-halamine-based copolymer in order to obtain antimicrobial activity. In this regard, a copolymer (HACM) of 2-acrylamido-2-methyl-1-(5-methylhydantoinyl)propane and 3-cloro-2-hydroxypropyl methacrylate was synthesized and grafted onto a stainless steel surface via covalent attachment. Synthesized monomers and copolymers were characterized by NMR, FTIR, and XPS spectral analyses. Upon treatment with dilute bleach, the stainless steel surfaces were rendered antimicrobial, possessing a sufficient amount of chlorine content and excellent stability and durability. The modified stainless steel samples inactivated 6 logs of Staphylococcus aureus and Escherichia coli O157:H7 bacteria within 15 min of contact time. Stabilities of the coatings toward washing and UVA exposure were also studied. The stainless steel samples showed superior washing stabilities and regenerabilities. After 5 cycles of washing, there was a very minimal change in the initial chlorine contents, and the chlorine content could be recharged to its initial number of Cl+ atoms/cm2. However, only a moderate stability of the coatings was observed after UVA irradiation. These results indicate that N-halamine precursor polymers can be facilely applied to stainless steel surfaces by covalent bonding and that robust, regenerable antimicrobial stainless steel surfaces could be prepared via the N-halamine technology. This technology exhibits potential for use in food processing, prevention of biofilm formation, and biomedical and health-care industries to support the prevention and reduction of cross-contamination and health-care related infections. [ABSTRACT FROM AUTHOR]

Calculations of real-space bonding indicators (RSBI) derived from Atoms-In-Molecules (AIM), Electron Localizability Indicator (ELI-D), Non-Covalent Interactions index (NCI), and Density Overlap Regions Indicator (DORI) toolkits for a set of 36 donor-acceptor complexes X3BNY3 (1, 1a-1h), X3AlNY3 (2, 2a-2h), X3BPY3 (3, 3a-3h), and X3AlPY3 (4, 4a-4h) reveal that the donor-acceptor bonds comprise covalent and ionic interactions in varying extents (X = Y = H for 1-4; X = H, Y = Me for 1a-4a; X = H, Y = Cl for 1b-4b; X = Me, Y = H for 1c-4c; X, Y = Me for 1d-4d; X = Me, Y = Cl for 1e-4e; X = Cl, Y = H for 1f-4f; X = Cl, Y = Me for 1g-4g; X, Y = Cl for 1h-4h). The phosphinoboranes X3BPY3 (3, 3a-3h) in general and Cl3BPMe3 (3f) in particular show the largest covalent contributions and the least ionic contributions. The aminoalanes X3AlNY3 (2, 2a-2h) in general and Me3AlNCl3 (2e) in particular show the least covalent contributions and the largest ionic contributions. The aminoboranes X3BNY3 (1, 1a-1h) and the phosphinoalanes X3AlPY3 (4, 4a-4h) are midway between phosphinoboranes and aminoalanes. The degree of covalency and ionicity correlates with the electronegativity difference BP (ΔEN = 0.15) < AlP (ΔEN = 0.58) < BN (ΔEN = 1.00) < AlN (ΔEN = 1.43) and a previously published energy decomposition analysis (EDA). To illustrate the importance of both contributions in Lewis formula representations, two resonance formulas should be given for all compounds, namely, the canonical form with formal charges denoting covalency and the arrow notation pointing from the donor to the acceptor atom to emphasis ionicity. If the Lewis formula mainly serves to show the atomic connectivity, the most significant should be shown. Thus, it is legitimate to present aminoalanes using arrows; however, for phosphinoboranes the canonical form with formal charges is more appropriate. [ABSTRACT FROM AUTHOR]