Squalyl Crown Ether Self‐Assembled Conjugates: An Example of Highly Selective Artificial K+ Channels

The natural KcsA K⁺ channel, one of the best‐characterized biological pore structures, conducts K⁺ cations at high rates while excluding Na⁺ cations. The KcsA K⁺ channel is of primordial inspiration for the design of artificial channels. Important progress in improving conduction activity and K⁺/Na⁺ selectivity has been achieved with artificial ion‐channel systems. However, simple artificial systems exhibiting K⁺/Na⁺ selectivity and mimicking the biofunctions of the KcsA K⁺ channel are unknown. Herein, an artificial ion channel formed by H‐bonded stacks of squalyl crown ethers, in which K⁺ conduction is highly preferred to Na⁺ conduction, is reported. The K⁺‐channel behavior is interpreted as arising from discreet stacks of dimers resulting in the formation of oligomeric channels, in which transport of cations occurs through macrocycles mixed with dimeric carriers undergoing dynamic exchange within the bilayer membrane. The present highly K⁺‐selective macrocyclic channel can be regarded as a biomimetic alternative to the KcsA channel.     

Multivalency by Self‐Assembly: Binding of Concanavalin A to Metallosupramolecular Architectures Decorated with Multiple Carbohydrate Groups

Multiplication of functional units through self‐assembly is a powerful way to new properties and functions. In particular, self‐organization of components decorated with recognition groups leads to multivalent entities, amenable to strong and selective binding with multivalent targets, such as protein receptors. Here we describe an efficient, supramolecular, one‐pot valency multiplication process proceeding through self‐organization of monovalent components into well‐defined, grid‐shaped [2×2] tetranuclear complexes bearing eight sugar residues for multivalent interaction with the tetrameric lectin, concanavalin A (Con A). The grids are stable in water under physiological pH at a relatively high concentration, but dissociate readily at slightly more acidic pH or upon dilution below a certain threshold, in a type of on–off behavior. The carbohydrate‐decorated grids interact strongly and selectively with Con A forming triply supramolecular bio‐hybrid polymeric networks, which lead to a highly specific phase‐separation and quasi‐quantitative precipitation of Con A out of solution. Dramatic effects of valency number on agglutination properties were demonstrated by comparison of grids with divalent carbohydrates of covalent and non‐covalent (L ‐shaped, mononuclear zinc complex) scaffolds. The results presented here provide prototypical illustration of the power of multivalency generation by self‐assembly leading to defined arrays of functional groups and binding patterns.     

Metal‐Atom Impact on the Self‐Assembly of Cup‐and‐Ball Metalloporphyrin–Fullerene Conjugates

A fullerene ammonium derivative has been combined with different metalloporphyrin–crown ether receptors to generate very stable supramolecules. The combination of fullerene–porphyrin and ammonium–crown ether interactions leads to a strong chelate effect as evidenced by a high effective molarity (3.16 M ). The different parameters influencing the stability of the supramolecular ensembles, in particular the nature of the metal in the porphyrin moiety, have been rationalized with the help of theoretical calculations thus providing new insights into fullerene–porphyrin interactions.     

p‐Nitrophenyl Picolinate Cleavage by Unsymmetrical Bis‐Schiff Base Manganese(III) and Cobalt(II) Complexes with Aza‐Crown Ether or Morpholino Pendants

The unsymmetrical bis‐Schiff base manganese(III) and cobalt(II) complexes with either benzo‐10‐aza‐crown ether pendants (MnL¹Cl, MnL²Cl) or morpholino pendant (MnL³Cl, CoL³) have been employed as models for hydrolase by studying the kinetics of their hydrolysis reactions with p‐nitrophenyl picolinate (PNPP). A kinetic model of PNPP cleavage catalyzed by these complexes is proposed. The effects of complex structures and reaction temperature on the rate of PNPP hydrolysis have been examined. All four complexes exhibit high catalytic activity and the rate increases with pH under 25°C. The complexes of ligands containing a crown ether group exhibit higher catalytic activities than the non‐crown analogues. The catalytic activity of the complexes follows the order Mn(III)>Co(II) under the same ligands.     

Reactivity of Hydroxy‐ and Aquo(hydroxy)‐λ3‐iodane–Crown Ether Complexes

We have designed a series of hydroxy(aryl)‐λ³‐iodane–[18]crown‐6 complexes, prepared from the corresponding iodosylbenzene derivatives and superacids in the presence of [18]crown‐6, and have investigated their reactivities in aqueous media. These activated iodosylbenzene monomers are all non‐hygroscopic shelf‐storable reagents, but they maintain high oxidizing ability in water. The complexes are effective for the oxidation of phenols, sulfides, olefins, silyl enol ethers, and alkyl(trifluoro)borates under mild conditions. Furthermore, hydroxy‐λ³‐iodane–[18]crown‐6 complexes serve as efficient progenitors for the synthesis of diaryl‐, vinyl‐, and alkynyl‐λ³‐iodanes in water. Other less polar organic solvents, such as methanol, acetonitrile, and dichloromethane, are also usable in some cases.     

From Valleys to Ridges: Exploring the Dynamic Energy Landscape of Single Membrane Proteins

Membrane proteins are involved in essential biological processes such as energy conversion, signal transduction, solute transport and secretion. All biological processes, also those involving membrane proteins, are steered by molecular interactions. Molecular interactions guide the folding and stability of membrane proteins, determine their assembly, switch their functional states or mediate signal transduction. The sequential steps of molecular interactions driving these processes can be described by dynamic energy landscapes. The conceptual energy landscape allows to follow the complex reaction pathways of membrane proteins while its modifications describe why and how pathways are changed. Single‐molecule force spectroscopy (SMFS) detects, quantifies and locates interactions within and between membrane proteins. SMFS helps to determine how these interactions change with temperature, point mutations, oligomerization and the functional states of membrane proteins. Applied in different modes, SMFS explores the co‐existence and population of reaction pathways in the energy landscape of the protein and thus reveals detailed insights into local mechanisms, determining its structural and functional relationships. Here we review how SMFS extracts the defining parameters of an energy landscape such as the barrier position, reaction kinetics and roughness with high precision.     

Ionic Conductivity of β‐Cyclodextrin–Polyethylene‐Oxide/Alkali‐Metal‐Salt Complex

Highly conductive, crystalline, polymer electrolytes, β‐cyclodextrin (β‐CD)–polyethylene oxide (PEO)/LiAsF₆ and β‐CD–PEO/NaAsF₆, were prepared through supramolecular self‐assembly of PEO, β‐CD, and LiAsF₆/NaAsF₆. The assembled β‐CDs form nanochannels in which the PEO/X⁺ (X=Li, Na) complexes are confined. The nanochannels provide a pathway for directional motion of the alkali metal ions and, at the same time, separate the cations and the anions by size exclusion.     

Reversible Unfolding–Refolding of Rubredoxin: A Single‐Molecule Force Spectroscopy Study

In metalloproteins, metal centers serve as active sites for a range of functional purposes and as important structural elements to facilitate protein folding and assembly. It is challenging to observe the reversible unfolding and refolding of metalloproteins because of a loss or decomposition of the metal center. Here, the reversible unfolding–refolding of the iron–sulfur protein rubredoxin was observed directly using single‐molecule force spectroscopy. The results demonstrate that the iron can remain attached to the CXXC motif when rubredoxin is unfolded. Upon relaxation, the unfolded rubredoxin can refold into its native holo state with the reconstituted FeS₄ center. The possible loss of iron from the unfolded protein prevents rubredoxin from refolding into its native holo state. These results demonstrated that unfolding of rubredoxin is reversible, as long as the iron remains attached, and provide experimental evidence for the iron‐priming mechanism for the folding of rubredoxin.     

Comparative analyses of a family of potential self-replicators: the subtle interplay between molecular structure and the efficacy of self-replication

It is envisioned that protocols based on self-replication will emerge as a formidable synthetic apparatus for the production of nanoscale assemblies through molecular structures that are capable of automultiplication with high reaction rates and selectivities. To achieve this goal, a complete understanding of the relationship between molecular structure and replication efficiency is necessary. Rigorous experimental and theoretical analyses of a series of self-complementary scaffolds that are intimately related in a constitutional sense, manufactured through the Diels-Alder reaction of complementary subunits, were undertaken. Experimental and computational methods were employed to map the key determinants that dictate the emergence of self-replicative function, as well as the efficiency, rate and selectivity of the self-replicative processes.     

Two—dimensional Network Crown Ether Complex.Synthesis and Crystal Structure of 18—Crown—6 Complex：[K（18—C—6）]2[Cd—（nmt）2]

The novel complex [K(18-C-6)]2[Cd(mnt)2][18-C-6-18-crown-6,nmt=1,2-dicyanoethene-1,2-dithiolate,C2S2-(CN)2^2-] was synthesized and characterized by elemental analysis,IR spectrum and X-ray diffraction analysis.The complex displays two-dimensional network structure of [K(18-C-6)] complex segments and [Cd(nmt)2] complex segment bridged by S-K-S,S-K-N and N-K-N interactions between adjacent[K(18-C-6)] and [Cd(mnt)2]units.     

Heterometallic Fe2II–UV and Ni2II–UV Exchange‐Coupled Single‐Molecule Magnets: Effect of the 3 d Ion on the Magnetic Properties

Uranium‐based compounds have been put forward as ideal candidates for the design of single‐molecule magnets (SMMs) with improved properties, but to date, only two examples of exchange‐coupled 3d–5f SMM containing uranium have been reported and both are based on the Mn^II ion. Here we have synthesized the first examples of exchange‐coupled uranium SMMs based on Fe^II and Ni^II. The SMM behavior of these complexes containing a quasi linear {M?O?U?O?M} core arises from intramolecular Fe?U and Ni?U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. The measured values of the relaxation barrier (53.9±0.9 K in the UFe₂ complex and of 27.4±0.5 K in the UNi₂ complex) show clearly the dependency on the spin value of the transition metal, providing important new information for the future design of improved uranium‐based SMMs.     

Force Spectroscopy of Individual Stimulus‐Responsive Poly(ferrocenyldimethylsilane) Chains: Towards a Redox‐Driven Macromolecular Motor

Summary: Progress in the development of a redox‐driven macromolecular motor and the characterization of its redox‐mechanical cycle using electrochemical AFM‐based single‐molecule force spectroscopy (SMFS) is described. The elasticities of individual neutral and oxidized poly(ferrocenyldimethylsilane) (PFS) macromolecules were reversibly controlled in situ by adjusting the potential in electrochemical SMFS experiments. For the operating cycle of one individual PFS‐based molecular motor, an output of 3.4 × 10⁻¹⁹ J and an efficiency of 5% have been estimated.

Force‐extension curves of a single‐molecule motor.     

A New Strategy for Storage and Transportation of Sensitive High‐Energy Materials: Guest‐Dependent Energy and Sensitivity of 3D Metal–Organic‐Framework‐Based Energetic Compounds

Reaction of Co(II) with the nitrogen‐rich ligand N,N‐bis(1H‐tetrazole‐5‐yl)‐amine (H₂bta) leads to a mixed‐valence, 3D, porous, metal–organic framework (MOF)‐based, energetic material with the nitrogen content of 51.78%, [Co₉(bta)₁₀(Hbta)₂(H₂O)₁₀]_n?(22 H₂O)_n ( 1 ). Compound 1 was thermohydrated to produce a new, stable, energetic material with the nitrogen content of 59.85% and heat of denotation of 4.537 kcal cm^?3, [Co₉(bta)₁₀(Hbta)₂(H₂O)₁₀]_n ( 2 ). Sensitivity tests show that 2 is more sensitivity to external stimuli than 1 , reflecting guest‐dependent energy and sensitivity of 3D, MOF‐based, energetic materials. Less‐sensitive 1 can be regarded as a more safe form for storage and transformation to sensitive 2 .     

Metal‐Ion‐Promoted Electron Transfer between Tetrathiafulvalene and Quinone Units Within a Calix[4]arene Framework and Tuning through Coordination of the Neighboring Crown Ether with a Sodium Cation

A new calix[4]arene 1 with tetrathiafulvalene (TTF), quinone, and crown ether units in the lower rim was designed and synthesized with the aim to investigate the possibility to tune the metal‐ion promoted electron transfer by coordination of the crown ether unit with additional metal ions. Both absorption and electron spin resonance (ESR) spectroscopic studies clearly indicate that electron transfer occurs efficiently from TTF to the quinone units in the presence of Sc³⁺/Pb²⁺/Zn²⁺. Moreover, the intramolecular electron transfer within 1 induced by Zn²⁺ can be switched off by addition of Na⁺ and further turned on by addition of either Sc³⁺ or Pb²⁺.