Molecular recognition of oxoanions based on guanidinium receptors

Self-Assembly of Bilayer Vesicles Made of Saturated Long Chain Fatty Acids

Saturated long chain fatty acids (sLCFA, e.g., C14:0, C16:0, and C18:0) are potentially the greenest and cheapest surfactants naturally available. However, because aqueous sodium soaps of sLCFA are known to crystallize, the self-assembly of stable bilayer vesicles has not been reported yet. Here, by using such soaps in combination with guanidine hydrochloride (GuHCl), which has been shown recently to prevent crystallization, we were capable of producing stable bilayer vesicles made of sLCFA. The phase diagrams were established for a variety of systems showing that vesicles can form in a broad range of composition and pH. Both solid state NMR and small-angle neutron scattering allowed demonstrating that in such vesicles sLCFA are arranged in a bilayer structure which exhibits similar dynamic and structural properties as those of phospholipid membranes. We expect these vesicles to be of interest as model systems of protocells and minimal cells but also for various applications since fatty acids are potentially substitutes to phospholipids, synthetic surfactants, and polymers.

Photodependent Melting of Unmodified DNA Using a Photosensitive Intercalator: A New and Generic Tool for Photoreversible Assembly of DNA Nanostructures at Constant Temperature

External control of DNA melting and hybridization, a key step in bio- and DNA nanotechnology, is commonly achieved with temperature. The use of light to direct this process is a challenging alternative, which has been only possible with a DNA modification, such as covalent grafting or mismatch introduction, so far. Here we describe the first photocontrol of DNA melting that relies on the addition of a molecule that noncovalently interacts with unmodified DNA and affects its melting properties in a photoreversible and highly robust manner, without any prerequisite in the length or sequence of the target DNA molecule. We synthesize azobenzene-containing guanidinium derivatives and show that a bivalent molecule with a conformation-dependent binding mode, AzoDiGua, strongly increases the melting temperature (Tm) of DNA under dark conditions because its trans isomer intercalates in the DNA double helix. Upon UV irradiation at 365 nm, the trans-cis isomerization induced the ejection of AzoDiGua from the intercalation binding sites, resulting in a decrease in Tm up to 18 °C. This illumination-dependent Tm shift is observed on many types of DNA, from self-complementary single-stranded or double-stranded oligonucleotides to long genomic duplex DNA molecules. Finally, we show that simply adding AzoDiGua allows us to photoreversibly control the assembly/disassembly of a DNA nanostructure at constant temperature, as demonstrated here with a self-hybridized DNA hairpin. We anticipate that this strategy will be the key ingredient in a new and generic way of placing DNA-based bio- and nanotechnologies under dynamic control by light.

Ionic hydrogen bond donor organocatalyst for fast living ring-opening polymerization

A positive charge enhanced H-bond donor combined with H-bond acceptor as a bifunctional organocatalyst enables fast living ring-opening polymerization of lactide.

Anion recognition by oligo-(thio)urea-based receptors

Recent progress in the construction of metal-coordination-assisted and covalently connected oligourea receptors and their anion coordination chemistry is presented.

Synthesis of tricyclic quinazolinones via intramolecular cyclization of 3-(2-aminoalkyl)-2-(phenylamino)quinazolin-4(3H)-ones

Bioactive tricyclic quinazolines class of 3,4-dihydro-1H-pyrimido[2,1-b]quinazolin-6(2H)-ones I and 2,3-dihydroimidazo[2,1-b]quinazolin-5(1H)-ones II were synthesized by the formic acid-catalyzed intramolecular cyclization of 3-(2-aminoalkyl)-2-(phenylamino)quinazolin-4(3H)-ones 1 in high yields. A plausible mechanism of the cyclization step is proposed.

Development of the aza-crown ether metal complexes as artificial hydrolase

Hydrolases play a crucial role in the biochemical process, which can catalyze the hydrolysis of various compounds like carboxylic esters, phosphoesters, amides, nucleic acids, peptides, and so on. The design of artificial hydrolases has attracted extensive attention due to their scientific significance and potential applications in the field of gene medicine and molecular biology. Numerous macrocyclic metal complexes have been used as artificial hydrolase in the catalytic hydrolysis of the organic substrate. Aza-crown ether for this comment is a special class of the macrocyclic ligand containing both the nitrogen atoms and oxygen atoms in the ring. The studies showed that the aza-crown complexes exhibited high activity of hydrolytic enzyme. However, the aza-crown ether metal complex as artificial hydrolase is still very limited because of its difficulty in synthesis. This review summarizes the development of the aza-crown ether metal complexes as the artificial hydrolase, including the synthesis and catalysis of the transition metal complexes and lanthanide metal complexes of aza-crown ethers. The purpose of this review is to highlight: (1) the relationship between the structure and hydrolytic activity of synthetic hydrolase; (2) the synergistic effect of metal sites and ligands in the course of organic compound hydrolysis; and (3) the design strategies of the aza-crown ethers as hydrolase.

Chiral Metallo-Supramolecular Complex Directed Enantioselective Self-Assembly of β-Sheet Breaker Peptide for Amyloid Inhibition

Chiral recognition plays an important role for biomacromolecules involved self-assembly and further affects their biological functions. Herein, it is demonstrated that two chiral metal complexes can enantioselectively bind with Aβ15-20, leading to the formation of different self-assembled nanostructures. With the ability of both metal complexes and Aβ15-20 to inhibit Aβ1-40 aggregation, the NiM@P hybrid particles can act as bifunctional Aβ inhibitors.

Tailoring of the desired selectivity and the turn-on detection range in a self-assembly-based fluorescence sensory system

This study demonstrates how to control the selectivity and the turn-on detection range toward the tailoring of an assembly-based fluorescence (FL) sensory system. Assembly-based FL chemosensors composed of oligophenylenevinylene with a varied number of guanidinium receptors (G2, G4 and G6) were newly developed, and their FL response to nucleotides (AMP, ADP and ATP) was investigated. Indeed, G6 exhibited FL emission via self-assembly with ATP. More importantly, the FL response of G6 showed markedly improved selectivity for ATP over ADP and a broadly extended detection range of ATP concentration under adjusted salt conditions. The salt effect on the FL response revealed the competitive binding interactions affecting the subsequent self-assembly process. These studies have unveiled the pivotal binding mechanisms operating in the self-assembly process, which tailor the performance level of the assembly-based sensory system. We believe that this study offers a new design principle of an assembly-based FL chemosensor with high selectivity and the appropriate detection range, being different from the conventional key-and-lock system.

The guanidinium unit in the catalysis of phosphoryl transfer reactions: from molecular spacers to nanostructured supports

Examples of guanidinium-based artificial phosphodiesterases are illustrated in this review article. A wide set of collected catalytic systems are presented, from the early examples to the most recent developments of the use of this unit in the design of supramolecular catalysts. Special attention is dedicated to illustrate the operating catalytic mechanism and the role of guanidine/ium units in the catalysis. One or more of these units can act by themselves or in conjunction with other active units. The analogy with the mechanism of enzymatic systems is presented and discussed. In the last part of this overview, recent examples of guanidinophosphodiesterases based on nanostructured supports are reported, namely gold-monolayer-protected clusters and polymer brushes grafted to silica nanoparticles. The issue of the dependence of the catalytic performance on the preorganization of the spacer is tackled and discussed in terms of effective molarity, a parameter that can be taken as a quantitative measurement of this preorganization for both conventional molecular linker and nanosized supports.

Chiral bifunctional guanidine-catalyzed enantioselective Aza-Henry reaction of isatin-derived ketimines

An efficient asymmetric aza-Henry reaction of isatin-derived ketimines has been achieved by using a chiral guanidine-amide organocatalyst. A series of 3-substituted 3-amino-2-oxindoles was obtained with excellent results (up to 99% yield, 94% ee). Other functionalized derivatives were also conveniently transformed. This metal-free system was convenient, practical, and insensitive to air and moisture. On the basis of the crystal structure of the catalyst and NMR spectra analysis, a bifunctional catalytic model was suggested to explain the origin of the asymmetric process.

Recent development of synthetic preparation methods for guanidines via transition metal catalysis

This article provides an overview of guanidine synthesis via transition-metal-catalyzed reactions including cycloaddition, guanylation and tandem guanylation/cyclization.

A conformationally persistent pseudo-bicyclic guanidinium for anion coordination as stabilized by dual intramolecular hydrogen bonds

The first example of a pseudo-bicyclic guanidinium ligand is reported, and its complexes are evaluated by crystallographic and computational approaches.

A case of oxoanion recognition based on combined cationic and neutral C–H hydrogen bond interactions

A bidentate bis-(benzimidazolium) receptor containing pyrene as fluorescent signaling units recognizes sulphate and hydrogenpyrophosphate in a competitive water–DMSO medium through combinations of cationic and neutral C–H hydrogen bonding.

The construction of supramolecular polymers through anion bridging: from frustrated hydrogen-bonding networks to well-ordered linear arrays

The construction of supramolecular polymers has been realized by converting random hydrogen-bonding networks into well-ordered linear hydrogen-bonding arrays through an anion-bridging strategy.

XPS of guanidinium ionic liquids: a comparison of charge distribution in nitrogenous cations

We report first X-ray photoelectron spectroscopy (XPS) for a range of functionalised guanidinium based systems that find application in the dissolution of biomolecules. Measured binding energies are compared to those of more common ionic liquid families containing other nitrogen based cations.

Luminescent probes for the bioimaging of small anionic species in vitro and in vivo

This comprehensive review examines recent developments in the use of fluorescent/luminescent probes for the bioimaging of anionic species. Images in cover art reproduced with permission from ref. 290 and 306.

Catalytic addition of amines to carbodiimides by bis(β-diketiminate)lanthanide(ii) complexes and mechanistic studies

Bis(β-diketiminate)lanthanide(ii) complexes L₂Ln(THF)_n were found to be excellent pre-catalysts for catalytic addition of amines to carbodiimides and a possible catalytic mechanism was proposed on the basis of the isolation of monoguanidinate Eu(ii) and bis(guanidinate) Yb(iii) species.

Small polyanion recognition of a triazolium cyclodextrin click cluster in water

In order to detect small polyanions (sPAs), which play important roles in many biological systems, a triazolium cyclodextrin click cluster was synthesized and its sPA binding properties were characterized.

Structure and function of the silicifying peptide R5

The first detailed description of the structure and function of the silicifying peptide R5.

Synthesis of bicyclic guanidines via cascade hydroamination/Michael additions of mono-N-acryloylpropargylguanidines

A cascade silver(I)-catalyzed hydroamination/Michael addition sequence has been developed to deliver highly substituted bicyclic guanidines. This transformation gives rise to geometrically and constitutionally stable ene-guanidines and generates a remote stereocenter with moderate to high diastereoselectivity.

Nucleotide sensing with a perylene-based molecular receptor via amplified fluorescence quenching

A competitive fluorescence assay of perylene-based molecular receptors has been established, and selective detection of UTP is achieved through improved aggregation arising from the specific interaction of perylene-tethered guanidinium with uridine and phosphate groups in UTP.

Advances in anion supramolecular chemistry: from recognition to chemical applications

Since the start of this millennium, remarkable progress in the binding and sensing of anions has been taking place, driven in part by discoveries in the use of hydrogen bonding, as well as the previously under-exploited anion-π interactions and halogen bonding. However, anion supramolecular chemistry has developed substantially beyond anion recognition, and now encompasses a diverse range of disciplines. Dramatic advance has been made in the anion-templated synthesis of macrocycles and interlocked molecular architectures, while the study of transmembrane anion transporters has flourished from almost nothing into a rapidly maturing field of research. The supramolecular chemistry of anions has also found real practical use in a variety of applications such as catalysis, ion extraction, and the use of anions as stimuli for responsive chemical systems.

Enhancing molecular recognition in electron donor-acceptor hybrids via cooperativity

Herein, we report the synthesis of guanidinium bis-porphyrin tweezers 1 and fullerene carboxylate 3, their assembly into a novel supramolecular 1@3 electron donor-acceptor hybrid, and its characterization. In solution, the binding constant affording 1@3 is exceptionally high. 1@3, which features a highly confined topography, builds up from a combination of guanidinium-carboxylate hydrogen bonding and π-π stacking/charge-transfer motifs. The latter is governed by interactions between the electron-donating porphyrin and the electron-accepting fullerene. Importantly, positive cooperativity between the applied binding motifs is corroborated by a number of experimental techniques, such as NMR, absorption, fluorescence, etc. In addition, transient absorption experiments shed light onto electron-transfer processes taking place in the ground state and upon photoexcitation. In fact, porphyrin excitation powers an electron transfer to the fullerene yielding charge separated state lifetimes in the nanosecond regime.

Solid-phase synthesis of peptide thioureas and thiazole-containing macrocycles through Ru-catalyzed ring-closing metathesis

N-Terminally modified α-thiourea peptides can selectively be synthesized on solid support under mild reaction conditions using N,N'-di-Boc-thiourea and Mukaiyama's reagent (2-chloro-1-methyl-pyridinium iodide). This N-terminal modification applies to the 20 proteinogenic amino acid residues on three commonly used resins for solid-phase synthesis. Complementary methods for the synthesis of α-guanidino peptides have also been developed. The thiourea products underwent quantitative reactions with α-halo ketones to form thiazoles in excellent purities and yields. When strategically installed between two alkene moieties, said thiazole core was conveniently embedded in peptide macrocycles via Ru-catalyzed ring-closing metathesis reactions. Various 15-17 membered macrocycles were easily accessible in all diastereomeric forms using this methodology. The developed "build/couple/pair" strategy is well suited for the generation of larger and stereochemically complete screening libraries of thiazole-containing peptide macrocycles.

Guanidinium functionalized superparamagnetic silica spheres for selective enrichment of phosphopeptides and intact phosphoproteins from complex mixtures

Capture of phosphopeptides and phosphoproteins from complex mixtures has been demonstrated based on the interaction between phosphate groups and guanidine groups.

Self-assembly of fatty acids: from foams to protocell vesicles

Stable aqueous dispersions of fatty acids can now be obtained and yield multiple applications.