Metal‐Helix Frameworks from Short Hybrid Peptide Foldamers

The potential of structured peptides has not been explored much in the design of metal‐organic frameworks (MOFs). This is partly due to the difficulties in obtaining stable secondary structures from the short α‐peptide sequences. Here we report the design, crystal conformations, coordination site dependent different silver coordinated frameworks of short α,γ‐hybrid peptide 12‐helices consisting of terminal pyridyl moieties and the utility of metal‐helix frameworks in the adsorption of CO₂. Upon silver ion coordination the 12‐helix terminated by the 3‐pyridyl derivatives adopted a 2:2 macrocyclic structure, while the 12‐helix terminated by the 4‐pyridyl derivatives displayed remarkable porous metal‐helix frameworks. Both head‐to‐tail intermolecular H‐bonds of the 12‐helix and metal ion coordination have played an important role in stabilizing the ordered metal‐helix frameworks. The studies described here open the door to design a new class of metal‐organic‐frameworks from peptide foldamers.     

Porous Peptide Complexes by a Folding‐and‐Assembly Strategy

Concerted folding and assembly processes are necessary for protein self‐assembly, yet such a concerted strategy has rarely been attempted by synthetic chemists. In this work, we have created a new porous peptide structure through a coordination‐driven folding‐and‐assembly strategy. A porous framework with 1.5 nm‐sized pores and a P_II helical peptide scaffold was successfully obtained by complexation of AgNTf₂ and tripeptide ligands containing the Gly‐Pro‐Pro sequence. The pores were modified in various ways with retention of the latent P_II helical conformation of the peptide ligand.     

Lyotropic Supramolecular Helical Columnar Phases Formed by C3‐Symmetric and Unsymmetric Rigid Molecules

Unlike thermotropic liquid‐crystalline C₃‐symmetric molecules with flexible chains, the herein‐designed fully rigid three‐armed molecules (C₃‐symmetric and unsymmetric) create a fancy architecture for the formation of lyotropic liquid crystals in water. First, hollow columns with triple‐stranded helices, analogous to helical rosette nanotubes, are spontaneously constructed by self‐organization of the rigid three‐armed molecules. Then, the helical nanotubes arrange into hexagonal liquid‐crystalline phases, which show macroscopic chirality as a result of supramolecular chiral symmetry breaking. Interestingly, the helical nanotubes constructed by the fully rigid molecules are robust and stable over a wide concentration range in water. They are hardly affected by ionic defects at the molecular periphery, that is, further decoration of functional groups on the molecular arms can presumably be realized without changing the helical conformation. In addition, the formed columnar phases can be aligned macroscopically by simple shear and show anisotropic ionic conductivity, which suggests promising applications for low‐dimensional ion‐conductive materials.     

Resonance‐Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

Resonance‐assisted hydrogen bonding (RAHB), a concept introduced by Gilli and co‐workers in 1989, concerns a kind of intramolecular H‐bonding strengthened by a conjugated π‐system, usually in 6‐, 8‐, or 10‐membered rings. This Review highlights the involvement of RAHB as a driving force in the synthesis of organic, coordination, and organometallic compounds, as a handy tool in the activation of covalent bonds, and in starting moieties for synthetic transformations. The unique roles of RAHB in molecular recognition and switches, E/Z isomeric resolution, racemization and epimerization of amino acids and chiral amino alcohols, solvatochromism, liquid‐crystalline compounds, and in synthons for crystal engineering and polymer materials are also discussed. The Review can provide practical guidance for synthetic chemists that are interested in exploring and further developing RAHB‐assisted synthesis and design of materials.     

Helical Nano‐crystallite (HNC) Phases: Chirality Synchronization of Achiral Bent‐Core Mesogens in a New Type of Dark Conglomerates

Spontaneous generation of macroscopic homochirality in soft matter systems by self‐assembly of exclusively achiral molecules under achiral conditions is a challenging task with relevance for fundamental scientific research and technological applications. Dark conglomerate phases (DC phases), being optically isotropic mesophases composed of conglomerates of macroscopic chiral domains and formed by some non‐chiral bent‐core mesogens, represent such a case. Here we report two new series of non‐symmetric bent‐core molecules capable of forming a new type of mirror symmetry broken DC phases. In the synthesized molecules, a bent 4‐bromoresorcinol core is connected to a phenyl benzoate wing and an azobenzene wing with or without additional peripheral fluorine substitution. The self‐assembly was investigated by DSC, polarizing microscopy, electro‐optical studies and XRD. Chiral and apparently achiral DC phases were observed besides distinct types of lamellar liquid crystalline phases with different degree of polar order, allowing the investigation of the transition from smectic to DC phases. This indicates a process in which increased packing density at first gives rise to restricted rotation and thus to growing polar order, which then leads to chirality synchronization, layer frustration and nano‐scale crystallization. Topological constraints arising from the twisted packing of helical conformers in lamellar crystals is proposed to lead to amorphous solids composed of helical nano‐crystallites with short coherence length (HNC phases). This is considered as a third major type of DC phases, distinct from the previously known liquid crystalline sponge phases and the helical nano‐filament phases (HNF phases). Guidelines for the molecular design of new materials capable of self‐assembly into these three types of DC phases are proposed.     

Herringbone and Helical Self‐Assembly of π‐Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self‐organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self‐assembly in π‐conjugated molecules based on custom‐designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single‐crystal X‐ray structures were resolved for these OPV synthons and the existence of long‐range multiple‐arm CH/π interactions was revealed in the crystal lattices. Alignment of these π‐conjugated OPVs in the solid state was found to be crucial in producing either right‐handed herringbone packing in the crystal or left‐handed helices in the liquid‐crystalline mesophase. Pitch‐ and roll‐angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self‐assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π‐conjugated materials.     

Crystalline poly(4‐methyl‐1‐pentene): Structure and solubility of gas molecules

Monte Carlo (MC) simulations have been used to study the crystal structure of isotactic poly(4‐methyl‐1‐pentene). Four different tetragonal packing models, each one containing two right‐handed and two left‐handed 7/2 helices, have been considered in an investigation of the up‐and‐down chain statistical disorder proposed on the basis of X‐ray data. Simulations have been performed with the isotropic united‐atom parameterization of the AMBER force field. The influence on the more stable packing models of the force‐field parameterization has been investigated with respect to the anisotropic united‐atom and all‐atom models. Results reveal that packing consisting of two upward and two downward helices arranged at random is more stable than packing with three or four helices with the same sense. Furthermore, the fiber period length for the 7/2 helix is predicted to be 0.56 Å larger than that experimentally determined. The microstructures generated from MC simulations have been employed to study the solubility of gas molecules (He, H₂, Ar, O₂, CH₄, and CO₂) with Widom's test‐particle insertion method. Special attention has been paid to the solubility of CH₄ and CO₂ because experimental data are available for these penetrants, the latter being described by both spherical and explicit models. The results are in good agreement with experimental measures only when a suitable model is used for the penetrant. The solubility of gas molecules in crystalline poly(4‐methyl‐1‐pentene) has been correlated with that measured for the crystal phases of other helical polymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2037–2049, 2002     

Organization of nucleobase‐functionalized β‐peptides investigated by soft electrospray ionization mass spectrometry

The development and validation of analytical methods is a key to succeed in investigating noncovalent interactions between biomolecules or between small molecules and biomolecules. Electrospray ionization mass spectrometry (ESI‐MS) was applied with a Fourier transform ion cyclotron resonance mass spectrometer (FTICR‐MS) as well as a quadrupole/time‐of‐flight tandem mass spectrometer (QqToF‐MS) for a systematic investigation of noncovalent complexes based on nucleobase pairing in an artificial and noncharged backbone topology. Synthetical β‐peptide helices covalently modified with nucleobases were organized by recognition of a sequence of four nucleobases. Specific duplexes of β‐peptide helices were obtained on the basis of hydrogen bonding base pair complementarity. Oligomer interactions were detected with defined stoichiometry and sensitivity for the respective duplex stability. FTICR‐MS and QqToF‐MS were used equally well to indicate double strand stabilities in agreement with the dissociation data determined by UV spectroscopy. Furthermore, the dissociation energies of gas phase ions of the noncovalent complexes were analyzed with collision induced dissociation (CID)‐MS/MS and infrared multiphoton dissociation (IRMPD)‐MS/MS. The CID conditions turned out to be too harsh for a differentiation of the duplex stabilities, whereas IRMPD might be developed as a technique to detect even small interaction energy differences. Copyright © 2009 John Wiley & Sons, Ltd.     

Self‐Assembly of Coronene Bisimides: Mechanistic Insight and Chiral Amplification

The study of the organization of small π‐conjugated molecules is imperative to understanding and controlling its properties for various applications. Coronene bisimides (CBIs) are potential candidates for novel liquid‐crystalline materials and active n‐type semiconductor molecules in organic electronics. To understand the self‐assembly of this seldom‐studied chromophore, we have designed two derivatives of CBIs bearing chiral and achiral 3,4,5‐trialkoxyphenyl groups at the imide position, named as CBI‐GCH and CBI‐GACH , respectively. CBI‐GCH self‐assembles mainly through π‐stacking and van der Waals interactions in nonpolar methylcyclohexane to result in long 1D fibrillar stacks. The mechanism of supramolecular polymerization was probed by using chiroptical studies, which showed an isodesmic pathway for CBI‐GCH . The thermodynamic parameters that govern the self‐assembly are detailed. CBI‐GACH also shows similar self‐assembly behavior as its chiral counterpart. X‐ray diffraction studies of both molecules reveals a 2D hexagonal columnar arrangement. The coassembly of CBI‐GCH and CBI‐GACH shows chiral amplification (sergeant and soldiers experiment) with saturation at 30–50 % of the chiral derivative, which was further used to study the dynamics of the assembly. Thus, this study presents a rare report of chiral amplification in an isodesmic system.     

A Nanohelicoidal Nematic Liquid Crystal Formed by a Non‐Linear Duplexed Hexamer

The twist‐bend modulated nematic liquid‐crystal phase exhibits formation of a nanometre‐scale helical pitch in a fluid and spontaneous breaking of mirror symmetry, leading to a quasi‐fluid state composed of chiral domains despite being composed of achiral materials. This phase was only observed for materials with two or more mesogenic units, the manner of attachment between which is always linear. Non‐linear oligomers with a H‐shaped hexamesogen are now found to exhibit both nematic and twist‐bend modulated nematic phases. This shatters the assumption that a linear sequence of mesogenic units is a prerequisite for this phase, and points to this state of matter being exhibited by a wider range of self‐assembling structures than was previously envisaged. These results support the double helix model of the TB phase as opposed to the simple heliconical model. This new class of materials could act as low‐molecular‐weight surrogates for cross‐linked liquid‐crystalline elastomers.     

Carbon Dots in a Matrix: Energy‐Transfer‐Enhanced Room‐Temperature Red Phosphorescence

High‐efficiency red room‐temperature phosphorescence (RTP) emissions have been achieved by embedding carbon dots (CDs) in crystalline Mn‐containing open‐framework matrices. The rationale of this strategy relies on two factors: 1) the carbon source, which affects the triplet energy levels of the resulting CDs and thus the spectral overlap and 2) the coordination geometry of the Mn atoms in the crystalline frameworks, which determines the crystal‐field splitting and thus the emission spectra. Embedding the carbon dots into a matrix with 6‐coordinate Mn centers resulted in a strong red RTP with a phosphorescence efficiency of up to 9.6 %, which is higher than that of most reported red RTP materials. The composite material has an ultrahigh optical stability in the presence of strong oxidants, various organic solvents, and strong ultraviolet radiation. A green‐yellow RTP composite was also prepared by using a matrix with 4‐coordinate Mn centers and different carbon precursors.     

Site‐Selective Cysteine–Cyclooctyne Conjugation

We report a site‐selective cysteine–cyclooctyne conjugation reaction between a seven‐residue peptide tag (DBCO‐tag, Leu‐Cys‐Tyr‐Pro‐Trp‐Val‐Tyr) at the N or C terminus of a peptide or protein and various aza‐dibenzocyclooctyne (DBCO) reagents. Compared to a cysteine peptide control, the DBCO‐tag increases the rate of the thiol–yne reaction 220‐fold, thereby enabling selective conjugation of DBCO‐tag to DBCO‐linked fluorescent probes, affinity tags, and cytotoxic drug molecules. Fusion of DBCO‐tag with the protein of interest enables regioselective cysteine modification on proteins that contain multiple endogenous cysteines; these examples include green fluorescent protein and the antibody trastuzumab. This study demonstrates that short peptide tags can aid in accelerating bond‐forming reactions that are often slow to non‐existent in water.     

Inversion of Optical Activity in the Synthesis of Mercury Sulfide Nanoparticles: Role of Ligand Coordination

Optical activity in inorganic colloidal materials was controlled through interactions of chiral molecules with the nanoparticle (NP) surface. An inversion of optical activity in the synthesis of mercury sulfide (HgS) NPs was demonstrated with an intrinsically chiral crystalline system in the presence of an identical chiral capping ligand. A continuous decrease in the positive first Cotton effect and an eventual reversal of CD profile were observed upon heating the aqueous solution of HgS NPs capped with N‐acetyl‐l ‐cysteine (Ac‐l ‐Cys) at 80 °C. Ac‐l ‐Cys afforded two bidentate coordination configurations with an almost mirror image of each other using the thiolate and either of carboxylate or acetyl–carbonyl groups on the HgS core. Experiment and calculation suggest that a shift in the distribution of the NP formation with energy in response to the combinations of ligand coordination structure and chiral crystalline surface is responsible for the inversion of optical activity.     

α‐Peptide–Oligourea Chimeras: Stabilization of Short α‐Helices by Non‐Peptide Helical Foldamers

Short α‐peptides with less than 10 residues generally display a low propensity to nucleate stable helical conformations. While various strategies to stabilize peptide helices have been previously reported, the ability of non‐peptide helical foldamers to stabilize α‐helices when fused to short α‐peptide segments has not been investigated. Towards this end, structural investigations into a series of chimeric oligomers obtained by joining aliphatic oligoureas to the C‐ or N‐termini of α‐peptides are described. All chimeras were found to be fully helical, with as few as 2 (or 3) urea units sufficient to propagate an α‐helical conformation in the fused peptide segment. The remarkable compatibility of α‐peptides with oligoureas described here, along with the simplicity of the approach, highlights the potential of interfacing natural and non‐peptide backbones as a means to further control the behavior of α‐peptides.     

β‐Sheet to Helical‐Sheet Evolution Induced by Topochemical Polymerization: Cross‐α‐Amyloid‐like Packing in a Pseudoprotein with Gly‐Phe‐Gly Repeats

Protein‐mimics are of great interest for their structure, stability, and properties. We are interested in the synthesis of protein‐mimics containing triazole linkages as peptide‐bond surrogate by topochemical azide‐alkyne cycloaddition (TAAC) polymerization of azide‐ and alkyne‐modified peptides. The rationally designed dipeptide N₃‐CH₂CO‐Phe‐NHCH₂CCH ( 1 ) crystallized in a parallel β‐sheet arrangement and are head‐to‐tail aligned in a direction perpendicular to the β‐sheet‐direction. Upon heating, crystals of 1 underwent single‐crystal‐to‐single‐crystal polymerization forming a triazole‐linked pseudoprotein with Gly‐Phe‐Gly repeats. During TAAC polymerization, the pseudoprotein evolved as helical chains. These helical chains are laterally assembled by backbone hydrogen bonding in a direction perpendicular to the helical axis to form helical sheets. This interesting helical‐sheet orientation in the crystal resembles the cross‐α‐amyloids, where α‐helices are arranged laterally as sheets.     

An Electrochemiluminesence Chiral Sensor for Propranolol Enantiomers Based on Functionalized Graphite‐like Carbon Nitride Nanosheets

Graphite‐like carbon nitride nanosheets (g‐C₃N₄) have been attracted considerable attention for their applications in catalysis and electrochemiluminesence (ECL) sensor. In this paper, a facile solvothermal method was employed to prepare the functionalized nanocomposites with metal salts cadmium carbonate (CdCO₃) and g‐C₃N₄ hybrids (g‐C₃N₄‐CdCO₃). The prepared materials were characterized by scanning electron microscope (SEM), X‐ray energy dispersive spectroscopy (EDX) and Fourier transform infrared spectra (FTIR). The nanocomposites was used not only as a new type luminophore but also as a chiral selector in this simple and sensitive ECL chiral sensor system for the recognition and detection of propranolol (Pro) enantiomers via quenching effect. The obvious difference of ECL signal of S‐Pro and R‐Pro was obtained. Thus, the sensor had the ability to distinguish Pro enantiomers. The mechanism of recognition was discussed by the theoretical calculation of binding constant and the water contact angle experiments. The sensor for Pro enantiomers was developed at wide concentration range from 0.001 to 1 mmol?L^?1. The sensor with excellent sensitivity, stability and reproducibility provide a new strategy based on ECL for chiral recognition and the ECL chiral sensor.     

Simultaneous Arrangement of up to Three Different Molecules on the Pore Surface of a Metal–Macrocycle Framework: Cooperation and Competition

Porous crystals are excellent materials with potential spatial functions through molecular encapsulation within the pores. Co‐encapsulation of multiple different molecules further expands their usability and designability. Herein we report the simultaneous arrangement of up to three different guest molecules, TTF (tetrathiafulvalene), ferrocene, and fluorene, on the pore surfaces of a porous crystalline metal–macrocycle framework (MMF). The position and orientation of adsorbed molecules arranged in the pore were determined by single‐crystal X‐ray diffraction analysis. The anchoring effect of hydrogen bonds between the hydroxy groups of the guest molecules and inter‐guest cooperation and competition are significant factors in the adsorption behaviors of the guest molecules. This finding would serve as a design basis of multicomponent functionalized nanospaces for elaborate reactions that are realized in enzymes.     

Influence of photoinduced isomerization on the chiral separation of novel liquid crystalline materials with a diazene moiety

The influence of photoinduced isomerization on the enantiomeric separation of two newly synthesized liquid crystalline materials, liquid crystals 1 and 2, was studied by high‐performance liquid chromatography on a chiral stationary phase Chiralpack AD‐3. Both materials have one chiral center and one diazene moiety. The compounds were separated into their E and Z isomeric forms. The conditions and time scale of the ultraviolet‐induced E to Z transition were briefly evaluated. Under the optimized conditions, we were able to baseline separate the S and R enantiomers of both the studied materials in their E isomeric form. The chiral separation of liquid crystal 2 after ultraviolet irradiation was unsuccessful. In contrast, the chiral separation of liquid crystal 1 possessing a similar structure to liquid crystal 2 provided baseline separation in its Z isomeric form as well. Previously, we have shown the influence of photoinduced isomerization and its utilization in the enantioseparation on relatively simple molecules. Here, we demonstrate that (1) much more complex compounds can also be successfully separated despite the bulkiness of the achiral part of the structure and (2) photoinduced isomerization even for such complex molecules still strongly influences their chromatographic properties.     

Water‐Binding‐Mediated Gelation/Crystallization and Thermosensitive Superchirality

Determination of molecular structural parameters of hydrophobic cholesterol–naphthalimide conjugates for water binding capabilities as well as their moisture‐sensitive supramolecular self‐assembly were revealed. Water binding was a key factor in leading trace water‐induced crystallization against gelation in apolar solvent. Ordered water molecules entrapped in self‐assembly arrays revealed by crystal structures behave as hydrogen‐bonding linkers to facilitate three‐dimensional growth into crystals rather than one‐dimensional gel nanofibers. Water binding was also reflected on the supramolecular chirality inversion of vesicle self‐assembly in aqueous media via heating‐induced dehydration. Structural parameters that favor water binding were evaluated in detail, which could help rationally design organic building units for advancing soft materials, crystal engineering, and chiral recognition.     

Thermally Driven Polymorphic Transition Prompting a Naked‐Eye‐Detectable Bending and Straightening Motion of Single Crystals

The amplification of molecular motions so that they can be detected by the naked eye (10⁷‐fold amplification from the ångström to the millimeter scale) is a challenging issue in the development of mechanical molecular devices. In this context, the perfectly ordered molecular alignment of the crystalline phase has advantages, as demonstrated by the macroscale mechanical motions of single crystals upon the photochemical transformation of molecules. In the course of our studies on thermoresponsive amphiphiles containing tetra(ethylene glycol) (TEG) moieties, we serendipitously found that thermal conformational changes of TEG units trigger a single‐crystal‐to‐single‐crystal polymorphic phase transition. The single crystal of the amphiphile undergoes bending and straightening motion during both heating and cooling processes at the phase‐transition temperatures. Thus, the thermally triggered conformational change of PEG units may have the advantage of inducing mechanical motion in bulk materials.