Molecular recognition of oxoanions based on guanidinium receptors

An Atomistic View on the Mechanism of Diatom Peptide-Guided Biomimetic Silica Formation

Deciphering nature's remarkable way of encoding functions in its biominerals holds the potential to enable the rational development of nature-inspired materials with tailored properties. However, the complex processes that convert solution-state precursors into solid biomaterials remain largely unknown. In this study, an unconventional approach is presented to characterize these precursors for the diatom-derived peptides R5 and synthetic Silaffin-1A1 (synSil-1A1). These molecules can form defined supramolecular assemblies in solution, which act as templates for solid silica structures. Using a tailored structural biology toolbox, the structure-function relationships of these self-assemblies are unveiled. NMR-derived constraints are employed to enable a recently developed fractal-cluster formalism and then reveal the architecture of the peptide assemblies in atomistic detail. Finally, by monitoring the self-assembly activities during silica formation at simultaneous high temporal and residue resolution using real-time spectroscopy, the mechanism is elucidated underlying template-driven silica formation. Thus, it is demonstrated how to exercise morphology control over bioinorganic solids by manipulating the template architectures. It is found that the morphology of the templates is translated into the shape of bioinorganic particles via a mechanism that includes silica nucleation on the solution-state complexes' surfaces followed by complete surface coating and particle precipitation.

Cationic Polysaccharides Bind to the Endothelial Cell Surface Extracellular Matrix Involving Heparan Sulfate

Cationic polysaccharides have been extensively studied for drug delivery via the bloodstream, yet few have progressed to clinical use. Endothelial cells lining the blood vessel wall are coated in an anionic extracellular matrix called the glycocalyx. However, we do not fully comprehend the charged polysaccharide interactions with the glycocalyx. We reveal that the cationic polysaccharide poly(acetyl, arginyl) glucosamine (PAAG) exhibits the highest association with the endothelial glycocalyx, followed by dextran (neutral) and hyaluronan (anionic). Furthermore, we demonstrate that PAAG binds heparan sulfate (HS) within the glycocalyx, leading to intracellular accumulation. Using an in vitro glycocalyx model, we demonstrate a charge-based extent of association of polysaccharides with HS. Mechanistically, we observe that PAAG binding to HS occurs via a condensation reaction and functionally protects HS from degradation. Together, this study reveals the interplay between polysaccharide charge properties and interactions with the endothelial cell glycocalyx toward improved delivery system design and application.

Fluorescence "turn-on" sensing for five PDE5 inhibitors in functional food based on bimetallic nanoclusters

Some phosphodiesterase type-5 (PDE5) inhibitors are active ingredients of prescription drugs that are widely used in the treatment of erectile dysfunction (ED). Recently, a large number of substances with this activity have been developed. Illegal addition of PDE5 inhibitors to foods could lead to cardiovascular diseases and even death, which poses a serious threat to food safety, therefore an on-site rapid screening method is urgently needed. Herein, a host functionalized bimetallic nanoclusters, CD/Au Ag NCs, were synthesized through self-assembly of 6-Aza-2-thiothymine gold nanoclusters (ATT-Au NCs), Arginine silver nanoclusters (Arg-Ag NCs) and carboxymethyl β-cyclodextrin (β-CMCD). The introduction of Rhodamine 6G (R6G) could quench the fluorescence of CD/Au Ag NCs based on the inner filter effect (IFE) and fluorescence resonance energy transfer effect (FRET). Importantly, it was discovered that several PDE5 inhibitors exhibited a higher binding affinity to β-CMCD and could displace R6G binding with CD cavity, which disrupted the fluorescence quenching effects and resulted in the fluorescence recovery of CD/Au Ag NCs. This fluorescence turn-on signal could be utilized for the detection of PDE5 inhibitors. At present, emerging PDE5 inhibitor analogues pose a great challenge to food safety due to their unknown efficacy and safety. The proposed method holds the advantages of high sensitivity, simple probe synthesis, easy operation, and simultaneous detection of multiple PDE5 inhibitors. Meanwhile, the successful application in functional food sample demonstrated its high application potential in multiple PDE5 inhibitors screening.

Hydroxyfluorooxoborate (NH4)[C(NH2)3][B3O3F4(OH)] for exploring the effects of cation substitution on structure and optical properties

Cation substitution is a straightforward but effective technique for improving the structure and properties; however, controlling directed substitution still poses significant difficulties. Herein, a metal-free hydroxyfluorooxoborate (NH4)[C(NH2)3][B3O3F4(OH)] has been synthesized using the strategy of heterologous substitution based on the template of A2[B3O3F4(OH)]. Tunable structure and optical properties have been achieved via varied A-site cation substitution. The intrinsic mechanism for this tunability was established by crystallography and theoretical research.

Silk/Polyamidoamine Membranes for Removing Chromium VI from Water

Polyamidoamine hydrogels prepared by the radical post-polymerization of α,ω-bisacrylamide-terminated M-AGM oligomers, in turn obtained by the polyaddition of 4-aminobutylguanidine with N,N'-methylenebisacrylamide, were reinforced with raw silk fibers, which can establish covalent bonds with the polyamidoamine matrix via reaction of the amine groups in the lysine residues with the acrylamide terminals of the M-AGM oligomer. Silk/M-AGM membranes were prepared by impregnating silk mats with M-AGM aqueous solutions and subsequent crosslinking by UV irradiation. The guanidine pendants of the M-AGM units imparted the ability to form strong but reversible interactions with oxyanions, including the highly toxic chromate ions. The potential of the silk/M-AGM membranes to purify Cr(VI)-contaminated water down to the drinkability level, that is, below 50 ppb, was tested by performing sorption experiments both in static (Cr(VI) concentration 20-2.5 ppm) and flow conditions (Cr(VI) concentration 10-1 ppm). After static sorption experiments, the Cr(VI)-loaded silk/M-AGM membranes could easily be regenerated via treatment with a 1 M sodium hydroxide solution. Dynamic tests performed using two stacked membranes and a 1 ppm Cr(VI) aqueous solution reduced Cr(VI) concentration down to 4 ppb. Remarkably, the use of renewable sources, the environmentally friendly preparation process, and the goal achieved meet eco-design requirements.

Structural (XRD) Characterization and an Analysis of H-Bonding Motifs in Some Tetrahydroxidohexaoxidopentaborate(1-) Salts of N-Substituted Guanidinium Cations

The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: [C(NH₂)₂(NHMe)][B₅O₆(OH)₄]·H₂O (1), [C(NH₂)₂(NH{NH₂})][B₅O₆(OH)₄] (2), [C(NH₂)₂(NMe₂)][B₅O₆(OH)₄] (3), [C(NH₂)(NMe₂)₂][B₅O₆(OH)₄] (4), [C(NHMe)(NMe₂)₂][B₅O₆(OH)₄]·B(OH)₃ (5), and [TBDH][B₅O₆(OH)₄] (6) (TBD = 1,5,7-triazabicyclo [4.4.0]dec-5-ene). Compounds 1-6 were prepared as crystalline salts from basic aqueous solution via self-assembly processes from B(OH)₃ and the appropriate substituted cation. Compounds 1-6 were characterized by spectroscopic (NMR and IR) and by single-crystal XRD studies. A thermal (TGA) analysis on compounds 1-3 and 6 demonstrated that they thermally decomposed via a multistage process to B₂O₃ at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H₂O. Reactant stoichiometry, solid-state packing, and H-bonding interactions are all important in assembling these structures. An analysis of H-bonding motifs in known unsubstituted guanidinium salts [C(NH₂)₃]₂[B₄O₅(OH)₄]·2H₂O, [C(NH₂)₃][B₅O₆(OH)₄]·H₂O, and [C(NH₂)₃]₃[B₉O₁₂(OH)₆] and in compounds 1-6 revealed that two important H-bonding R₂²(8) motifs competed to stabilize the observed structures. The guanidinium cation formed charge-assisted pincer cation-anion H-bonded rings as a major motif in [C(NH₂)₃]₂[B₄O₅(OH)₄]·2H₂O and [C(NH₂)₃]₃[B₉O₁₂(OH)₆], whereas the anion-anion ring motif was dominant in [C(NH₂)₃][B₅O₆(OH)₄]·H₂O and in compounds 1-6. This behaviour was consistent with the stoichiometry of the salt and packing effects also strongly influencing their solid-state structures.

Guanidine-Functionalized Fluorescent sp2 Carbon-Conjugated Covalent Organic Framework for Sensing and Capture of Pd(II) and Cr(VI) Ions

Palladium is a key element in fuel cells, electronic industries, and organic catalysis. At the same time, chromium is essential in leather, electroplating, and metallurgical industries. However, their unpremeditated leakage into aquatic systems has caused human health and environmental apprehensions. Herein, we reported the development of an sp² carbon-conjugated fluorescent covalent organic framework with a guanidine moiety (sp² c-gCOF) that showed excellent thermal and chemical stability. The sp² c-gCOF showed effective sensing, capture, and recovery/removal of Pd(II) and Cr(VI) ions, which could be due to the highly accessible pore walls decorated with guanidine moieties. The fluorescent sp² c-gCOF showed higher selectivity for Pd(II) and Cr(VI) ions, with an ultra-low detection limit of 2.7 and 3.2 nM, respectively. The analysis of the adsorption properties with a pseudo-second-order kinetic model showed that sp² c-gCOF could successfully and selectively remove both Pd(II) and Cr(VI) ions from aqueous solutions. The polymer also showed excellent capture efficacy even after seven consecutive adsorption-desorption cycles. Hence, this study reveals the potential of fluorescent sp² c-gCOF for detecting, removing, and recovering valuable metals and hazardous ions from wastewater, which would be useful for economic benefit, environmental safety, human health, and sustainability. The post-synthetic modification of sp² c-COF with suitable functionalities could also be useful for sensing and extracting other water pollutants and valuable materials from an aqueous system.

Guanidinylated Polymyxins as Outer Membrane Permeabilizers Capable of Potentiating Rifampicin, Erythromycin, Ceftazidime and Aztreonam against Gram-Negative Bacteria

Polymyxins are considered a last-line treatment against infections caused by multidrug-resistant (MDR) Gram-negative bacteria. In addition to their use as a potent antibiotic, polymyxins have also been utilized as outer membrane (OM) permeabilizers, capable of augmenting the activity of a partner antibiotic. Several polymyxin derivatives have been developed accordingly, with the objective of mitigating associated nephrotoxicity. The conversion of polymyxins to guanidinylated derivatives, whereby the L-γ-diaminobutyric acid (Dab) amines are substituted with guanidines, are described herein. The resulting guanidinylated colistin and polymyxin B (PMB) exhibited reduced antibacterial activity but preserved OM permeabilizing properties that allowed potentiation of several antibiotic classes. Rifampicin, erythromycin, ceftazidime and aztreonam were particularly potentiated against clinically relevant MDR Gram-negative bacteria. The potentiating effects of guanidinylated polymyxins with ceftazidime or aztreonam were further enhanced by adding the β-lactamase inhibitor avibactam.

Evolution of Artificial Arginine Analogues—Fluorescent Guanidiniocarbonyl-Indoles as Efficient Oxo-Anion Binders

In this article, we present fluorescent guanidiniocarbonyl-indoles as versatile oxo-anion binders. Herein, the guanidiniocarbonyl-indole (GCI) and methoxy-guanidiniocarbonyl-indole (MGCI) were investigated as ethylamides and compared with the well-known guanidiniocarbonyl-pyrrole (GCP) concerning their photophysical properties as well as their binding behavior towards oxo-anions. Hence, a variety of anionic species, such as carboxylates, phosphonates and sulfonates, have been studied regarding their binding properties with GCP, GCI and MGCI using UV-Vis titrations, in combination with the determination of the complex stoichiometry using the Job method. The emission properties were studied in relation to the pH value using fluorescence spectroscopy as well as the determination of the photoluminescence quantum yields (PLQY). Density functional theory (DFT) calculations were undertaken to obtain a better understanding of the ground-lying electronic properties of the investigated oxo-anion binders. Additionally, X-ray diffraction of GCP and GCI was conducted. We found that GCI and MGCI efficiently bind carboxylates, phosphonates and sulfonates in buffered aqueous solution and in a similar range as GCP (Kass ≈ 1000–18,000 M−1, in bis-tris buffer, pH = 6); thus, they could be regarded as promising emissive oxo-anion binders. They also exhibit a visible fluorescence with a sufficient PLQY. Additionally, the excitation and emission wavelength of MGCI was successfully shifted closer to the visible region of the electromagnetic spectrum by introducing a methoxy-group into the core structure, which makes them interesting for biological applications.

A guanidino-γ-cyclodextrin superdimer generates a twin receptor for phosphate dimers assembled by anti-electrostatic hydrogen bonds

Octakis-6-guadinidino-γ-cyclodextrin (gguan) hydrochloride in the presence of phosphates crystallises from aqueous solution in the unprecedented form of a superdimer (dimer-within-a-dimer). The self-assembly exposes four circular octa-guanidinium regions that bind and stabilise discrete H-bonded phosphate anion dimers. The small (∼2 nm) gguan-phosphate assembly is preorganised and stable in aqueous solution, as demonstrated by DLS and NMR experiments.

When Molecules Meet in Water-Recent Contributions of Supramolecular Chemistry to the Understanding of Molecular Recognition Processes in Water

Molecular recognition processes in water differ from those in organic solvents in that they are mediated to a much greater extent by solvent effects. The hydrophobic effect, for example, causes molecules that only weakly interact in organic solvents to stay together in water. Such water‐mediated interactions can be very efficient as demonstrated by many of the synthetic receptors discussed in this review, some of which have substrate affinities matching or even surpassing those of natural binders. However, in spite of considerable success in designing such receptors, not all factors determining their binding properties in water are fully understood. Existing concepts still provide plausible explanations why the reorganization of water molecules often causes receptor‐substrate interactions in water to be strongly exothermic rather than entropically favored as predicted by the classical view of the hydrophobic effect.

Red-Emitting Polymerizable Guanidinium Dyes as Fluorescent Probes in Molecularly Imprinted Polymers for Glyphosate Detection

The development of methodologies to sense glyphosate has gained momentum due to its toxicological and ecotoxicological effects. In this work, a red-emitting and polymerizable guanidinium benzoxadiazole probe was developed for the fluorescence detection of glyphosate. The interaction of the fluorescent probe and the tetrabutylammonium salt of glyphosate was studied via UV/vis absorption and fluorescence spectroscopy in chloroform and acetonitrile. The selective recognition of glyphosate was achieved by preparing molecularly imprinted polymers, able to discriminate against other common herbicides such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 3,6-dichloro-2-methoxybenzoic acid (dicamba), as thin layers on submicron silica particles. The limits of detection of 4.8 µM and 0.6 µM were obtained for the sensing of glyphosate in chloroform and acetonitrile, respectively. The reported system shows promise for future application in the sensing of glyphosate through further optimization of the dye and the implementation of a biphasic assay with water/organic solvent mixtures for sensing in aqueous environmental samples.

Anion Recognition by a Pincer-Type Host Constructed from Two Polyamide Macrocyclic Frameworks Jointed by a Photo-Addressable Azobenzene Switch

A sterically crowded light-responsive host 1 was synthetized with a 93% yield by applying a post-functionalization protocol utilizing the double amidation of 4,4'-azodibenzoyl dichloride with a readily available 26-membered macrocyclic amine. X-ray structures of two hydrates of trans-1 demonstrate a very different alignment of the azobenzene linkage, which is involved in T-shape or parallel-displaced π⋯π stacking interactions with the pyridine-2,6-dicarboxamide moieties from the macrocyclic backbone. Despite the rigidity of the macrocyclic framework, which generates a large steric hindrance around the azobenzene chromophore, the host 1 retains the ability to undergo a reversible cis⟷trans isomerization upon irradiation with UVA (368 nm) and blue (410 nm) light. Moreover, thermal cis→trans back-isomerization (ΔG0 = 106.5 kJ∙mol-1, t½ = 141 h) is markedly slowed down as compared to the non-macrocyclic analog. 1H NMR titration experiments in DMSO-d6/0.5% water solution reveal that trans-1 exhibits a strong preference for dihydrogenphosphate (H2PO4-) over other anions (Cl-, MeCO2-, and PhCO2-), whereas the photogenerated metastable cis-1 shows lower affinity for the H2PO4- anion.

Insights into the binding of arginine to adenosine phosphate from mimetic complexes

The recognition of adenosine monophosphate (AMP) by arginine-rich proteins is conditioned by the competitive binding of the guanidinium side group with metal cations, as derived from vibrational spectroscopy and modelling of mimetic complexes.

Porous polyelectrolyte frameworks: synthesis, post-ionization and advanced applications

Porous organic polymers (POPs), which feature high surface areas, robust skeletons, tunable pores, adjustable functionality and versatile applicability, have constituted a designable platform to develop advanced organic materials. Endowing polyelectrolytes with the distinct characteristics of POPs will attract mounting interest as the structural diversity of polyelectrolytes will bring the new hope of intriguing applications and potential benefits. In this review, the striking progress in ionized POPs (i-POPs) has been systematically summarized with regard to their synthetic strategies and applications. In the synthesis of i-POPs, we illustrate the representative ionic building blocks and charged functional groups capable of constructing the polyelectrolyte frameworks. The synthetic methods, including direct synthesis and post-modification, are detailed for the i-POPs with amorphous or crystalline structures, respectively. Subsequently, we outline the distinctive performances of i-POPs in adsorption, separation, catalysis, sensing, ion conduction and biomedical applications. The survey concerns the interplay between the surface chemistry, ionic interaction and pore confinement that cooperatively promote the performance of i-POPs. Finally, we conclude with the remaining challenges and promising opportunities for the on-going development of i-POPs.

Elucidation of Charge Contribution in Iridium-Chelated Hydrogen-Bonding Systems

We present two iridium complexes 1H⁺ and 2H⁺ that contain cationic ligands to extend the knowledge of charge-assisted hydrogen bonding (CAHB), which counts among the strongest non-covalent bonding interactions. Upon protonation, both complexes were converted into new hydrogen-bonding arrays with various selectivity for respective H-bonding partners. This study compares the association strengths of four hydrogen-bonding co-systems, emphasizing the roles of CAHB in supramolecular systems. We determined that the cationic charge in these systems contributed up to 2.7 kJ mol⁻¹ in the H-bonding complexation processes.

Turn-On Fluorescent Probe Based on a Dansyl Triarginine Peptide for Ganglioside Imaging

Gangliosides play pivotal biological roles in the animal cell membranes, and it is vital to develop fluorescent probes for imaging them. To date, various artificial receptors for ganglioside imaging have been developed; however, turn-on fluorescence imaging for gangliosides with high contrast has not been achieved. We developed a simple fluorescent probe on the basis of a dansyl triarginine peptide for turn-on ganglioside imaging on the liposome membrane. The probe bound to monosialyl gangliosides and other anionic lipids with association constants was 10⁵ M^-1, which enhanced from 6-fold to 7-fold the fluorescence intensity. Upon binding to monosialyl ganglioside-containing giant liposomes, the turn-on probe selectively enhanced the fluorescence intensity compared with the other anionic lipids. This simple peptide probe for turn-on fluorescence imaging of gangliosides would provide a novel molecular tool for chemical biology.

Quantum chemical study in exploring the role of donor→acceptor interactions in 1,3-bis carbene-stabilized guanidinium cations

Guanidinium species are highly basic and hence mostly exist in cationic state. Because these cations carry electron-deficient centers, they can be stabilized with the help of electron-donating ligands like N-heterocyclic carbenes. A few novel guanidinium cationic species stabilized by electron-donating ligands were designed and quantum chemically evaluated. It was shown that strong hydrogen bonds and tautomerism are the important characteristics of these species. Further, the possibility of donor→acceptor coordination interactions in these species have been explored between the electron-donating carbenes and the central guanidinium unit. The results suggest that the title compounds can be considered as ligand-stabilized guanidinium cations similar to the ligand-stabilized N⁺ and N₃⁺ centers.

Molecular Recognition in Water Using Macrocyclic Synthetic Receptors

Molecular recognition in water using macrocyclic synthetic receptors constitutes a vibrant and timely research area of supramolecular chemistry. Pioneering examples on the topic date back to the 1980s. The investigated model systems and the results derived from them are key for furthering our understanding of the remarkable properties exhibited by proteins: high binding affinity, superior binding selectivity, and extreme catalytic performance. Dissecting the different effects contributing to the proteins' properties is severely limited owing to its complex nature. Molecular recognition in water is also involved in other appreciated areas such as self-assembly, drug discovery, and supramolecular catalysis. The development of all these research areas entails a deep understanding of the molecular recognition events occurring in aqueous media. In this review, we cover the past three decades of molecular recognition studies of neutral and charged, polar and nonpolar organic substrates and ions using selected artificial receptors soluble in water. We briefly discuss the intermolecular forces involved in the reversible binding of the substrates, as well as the hydrophobic and Hofmeister effects operating in aqueous solution. We examine, from an interdisciplinary perspective, the design and development of effective water-soluble synthetic receptors based on cyclic, oligo-cyclic, and concave-shaped architectures. We also include selected examples of self-assembled water-soluble synthetic receptors. The catalytic performance of some of the presented receptors is also described. The latter process also deals with molecular recognition and energetic stabilization, but instead of binding ground-state species, the targets become elusive counterparts: transition states and other high-energy intermediates.

Half-sandwich complexes of osmium containing guanidine-derived ligands

Pyridinyl- and phosphano-guanidino complexes of formula [(η6-p-cymene)OsCl(H2L)][SbF6] (cymene = MeC6H4iPr; H2L = N,N'-bis(p-Tolyl)-N''-(2-pyridinylmethyl)guanidine, H2L1 (1) and N,N'-bis(p-Tolyl)-N''-(2-diphenylphosphanoethyl)guanidine, H2L2 (2)) have been prepared from the dimer [{(η6-p-cymene)OsCl}2(μ-Cl)2] and H2L in the presence of NaSbF6. Treatment of complex 2 with HCl renders the phosphano-guanidinium complex [(η6-p-cymene)OsCl2(H3L2)][SbF6] (3). Compounds 1 and 2 react with AgSbF6 rendering the cationic aqua complexes [(η6-p-cymene)Os(H2L)(OH2)][SbF6]2 (H2L = H2L1 (4), H2L2 (5)). Addition of monodentate ligands L to compound 4 affords complexes of formula [(η6-p-cymene)Os(H2L1)L][SbF6]2 (L = py (6), 4-(NHMe)py (7), CO (8), P(OMe)3 (9)). Treatment of complexes 4 and 5 with NaHCO3 renders the monocationic complexes [(η6-p-cymene)Os(κ3N,N',N''-HL1)][SbF6] (10) and [(η6-p-cymene)Os(κ3N,N',P-HL2)][SbF6] (11), respectively, in which the HL ligand adopts a fac-κ3 coordination mode. The new complexes have been characterised by analytical and spectroscopic means, including the determination of the crystal structures of the compounds 1-4, 6, 8, and 11, by X-ray diffractometric methods. The phosphano-guanidino complexes 2 and 5 exhibit a temperature dependent fluxional process in solution. The new 18 electron complexes 1, 2, 6, and 8-10 are active catalysts for the Friedel-Crafts reaction between trans-β-nitrostyrene and N-methyl-2-methylindole. Conversions greater than 90% were obtained. Proton NMR studies support a mechanism involving the Brønsted-acid activation of trans-β-nitrostyrene through the NH functionalities of the coordinated guanidine ligands.

Iminoguanidines: from anion recognition and separation to carbon capture

Iminoguanidines, first reported in 1898, have received renewed attention in the last 5 years due to their ability to recognize and separate anions from competitive aqueous environments. Iminoguanidines display high recognition abilities towards hydrophilic oxyanions (e.g., sulfate, chromate, carbonate) through strong and complementary hydrogen bonding from the guanidinium groups. This feature article reviews the fundamental anion recognition chemistry of iminoguanidines, as well as real-world applications including sulfate removal from seawater and CO₂ capture for climate change mitigations.

Automated high-content imaging for cellular uptake, from the Schmuck cation to the latest cyclic oligochalcogenides

Recent progress with chemistry tools to deliver into living cells has seen a shift of attention from counterion-mediated uptake of cell-penetrating peptides (CPPs) and their mimics, particularly the Schmuck cation, toward thiol-mediated uptake with cell-penetrating poly(disulfide)s (CPDs) and cyclic oligochalcogenides (COCs), here exemplified by asparagusic acid. A persistent challenge in this evolution is the simultaneous and quantitative detection of cytosolic delivery and cytotoxicity in a high-throughput format. Here, we show that the combination of the HaloTag-based chloroalkane penetration assay (CAPA) with automated high-content (HC) microscopy can satisfy this need. The automated imaging of thousands of cells per condition in multiwell plates allows us to obtain quantitative data on not only the fluorescence intensity but also on the localization in a very short time. Quantitative and statistically relevant results can be obtained from dose-response curves of the targeted delivery to selected cells and the cytotoxicity in the same experiment, even with poorly optimized cellular systems.

Lithium and Dilithium Guanidinates, a Starter Kit for Metal Complexes Containing Various Mono- and Dianionic Ligands

Comparative studies of the synthesis of lithium guanidinates via nucleophilic addition of lithium amides to carbodiimides were performed. Four combinations of small or sterically crowded carbodiimide and sterically crowded lithium amide or lithium amide containing an adjacent amino donor group give ten different types of complexes. In particular, 2,6-[(CH₃)₂CH]₂C₆H₃NHLi (DipNHLi, 1) reacts with (CH₃)₂CHN═C═NCH(CH₃)₂ upon the formation of the dissymmetric dimeric complex 2 with four-coordinate Li atoms. In contrast, 1 with DipN═C═NDip gives the mononuclear lithium guanidinate 3 with two-coordinate lithium by κ¹-guanidinate, solvent molecule, and additional interaction with a π-electron cloud of one of the Dip groups. Analogous reactions of 2-[(CH₃)₂NCH₂]C₆H₄NHLi (7) yield complexes 8 and 9, where the adjacent amino donors are always coordinated. Further deprotonation of 2, 3, 8, and 9 leads to dilithium guanidinates(2-)-4, 5, 10, and 11, among which only 5, containing three Dip groups, is monomeric with contacts to two π-electron systems of Dip groups. The rest of the complexes are tetranuclear with different structural patterns. In the central parts of molecules, toward which the nitrogen atoms of the guanidinates are oriented, lithium atoms are usually pseudotetrahedral, but trigonal in peripheral parts. Adjacent solvent molecules, chelating amino groups, and π-electron systems of Dip groups are coordinated in order to complete coordination polyhedra. Complexes 4 and 5 deoligomerize in solution upon the formation of fluxional monomeric dilithium species. Conversely, 11 is a dimer in solution due to the strong donation of an amino group. The silylated lithium amide {2-[(CH₃)₂NCH₂]C₆H₄}[(Si(CH₃)₃]NLi (12) reacts with both carbodiimides to give dinuclear 13 obtained from diisopropylcarbodiimide and monomeric 14 from the second carbodiimide. Complexes 13 and 14 structurally resemble 8 and 9, with the highest degree of the localization of π-electron density within the N₃C guanidinate system, η³-contact to the Dip ring, and a lack of the solvent molecule in 14.

Functionalized Chiral Bambusurils: Synthesis and Host-Guest Interactions with Chiral Carboxylates

Bambusurils are a class of macrocyclic anion receptors that exhibit notable anion recognition properties, able to bind various inorganic anions as well the carboxylates or sulfonates. Recently, we reported enantioselective recognition of chiral carboxylates using non-functionalized chiral bambusuril derivatives. Herein, we report the synthesis and host-guest properties of two new representatives of chiral bambusuril macrocycles bearing ester functional groups, differing by the substituents attached to their portals. Their supramolecular properties in terms of carboxylate binding were studied by means of NMR in DMSO-d₆ . The reported bambusurils bind selected chiral carboxylates with enantioselectivity factors up to 3.1. The results indicated that the selectivity towards different carboxylates is governed by the steric constraint of the substituents surrounding bambusuril portals. No clear trend in the binding affinities and their enantioselectivities was found.

From Interaction to Function in DNA-Templated Supramolecular Self-Assemblies

DNA-templated self-assembly represents a rich and growing subset of supramolecular chemistry where functional self-assemblies are programmed in a versatile manner using nucleic acids as readily-available and readily-tunable templates. In this review, we summarize the different DNA recognition modes and the basic supramolecular interactions at play in this context. We discuss the recent results that report the DNA-templated self-assembly of small molecules into complex yet precise nanoarrays, going from 1D to 3D architectures. Finally, we show their emerging functions as photonic/electronic nanowires, sensors, gene delivery vectors, and supramolecular catalysts, and their growing applications in a wide range of area from materials to biological sciences.

Functional Disruption of the Cancer-Relevant Interaction between Survivin and Histone H3 with a Guanidiniocarbonyl Pyrrole Ligand

The protein Survivin is highly upregulated in most cancers and considered to be a key player in carcinogenesis. We explored a supramolecular approach to address Survivin as a drug target by inhibiting the protein-protein interaction of Survivin and its functionally relevant binding partner Histone H3. Ligand L1 is based on the guanidiniocarbonyl pyrrole cation and serves as a highly specific anion binder in order to target the interaction between Survivin and Histone H3. NMR titration confirmed binding of L1 to Survivin's Histone H3 binding site. The inhibition of the Survivin-Histone H3 interaction and consequently a reduction of cancer cell proliferation were demonstrated by microscopic and cellular assays.

Proton-Triggered Fluorescence Switching in Self-Exfoliated Ionic Covalent Organic Nanosheets for Applications in Selective Detection of Anions

The exfoliation of covalent organic frameworks into covalent organic nanosheets (CONs) not only helps to reduce fluorescence turn-off phenomena but also provides well-exposed active sites for fast response and recovery for various applications. The present work is an example of rational designing of a structure constructed by condensing triaminoguanidinium chloride (TG_Cl), an intrinsic ionic linker, with a fluorophore, 2, 5-dimethoxyterephthalaldehyde (DA), to produce highly fluorescent self-exfoliable ionic CONs (DATG_Cl-iCONs). These fluorescent iCONs are able to sense fluoride ions selectively down to the ppb level via the fluorescence turn-off mechanism. A closer look at the quenching mechanism via NMR, zeta potential measurement, lifetime measurement, and density functional theory calculations reveals unique proton-triggered fluorescence switching behavior of newly synthesized DATG_Cl-iCONs.

Selective Phosphate Removal from Water and Wastewater using Sorption: Process Fundamentals and Removal Mechanisms

Eutrophication of water bodies is a serious and widespread environmental problem. Achieving low levels of phosphate concentration to prevent eutrophication is one of the important goals of the wastewater engineering and surface water management. Meeting the increasingly stringent standards is feasible in using a phosphate-selective sorption system. This critical review discusses the most fundamental aspects of selective phosphate removal processes and highlights gains from the latest developments of phosphate-selective sorbents. Selective sorption of phosphate over other competing anions can be achieved based on their differences in acid-base properties, geometric shapes, and metal complexing abilities. Correspondingly, interaction mechanisms between the phosphate and sorbent are categorized as hydrogen bonding, shape complementarity, and inner-sphere complexation, and their representative sorbents are organic-functionalized materials, molecularly imprinted polymers, and metal-based materials, respectively. Dominating factors affecting the phosphate sorption performance of these sorbents are critically examined, along with a discussion of some overlooked facts regarding the development of high-performance sorbents for selective phosphate removal from water and wastewater.

Supramolecular Self-Assembly Driven Selective Sensing of Phosphates

A new bis-heteroleptic Ru^II complex (1[PF₆]₂) with iodotriazole as the anion binding group along with the attached pyrene moiety is developed to investigate anion sensing properties and the origin of its selectivity toward a particular class of anions. Selective sensing of phosphates over other anions in both the solution and solid states by 1[PF₆]₂ is clearly evident from the perturbation of the absorption band and a large degree of amplification of ³MLCT emission band in the presence of phosphates. Importantly, macroscopic investigation such as Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) indicated the formation of supramolecular architecture in the presence of dihydrogen phosphate via halogen bonding interaction and π-π stacking of pyrene moieties. Such macroscopic property is further corroborated by solution and solid state spectroscopic studies, e.g., ¹H-DOSY NMR, single crystal X-ray crystallography, and solid state photoluminescence (PL) spectroscopy.

Precise Cross-Dimensional Regulation of the Structure of a Photoreversible DNA Nanoswitch

In this study, an accurately and digitally regulated allosteric nanoswitch based on the conformational control of two DNA hairpins was developed. By switching between UV irradiation and blue light conditions, the second molecular beacon (H#2) would bind/separate with a repression sequence (RES) via the introduced PTG molecules (a photosensitive azobenzene derivative), resulting in the target aptamer sequence in the first molecular beacon (H#1) not being able/being able to hold the stem-loop configuration, hence losing/regaining the ability to bind with the target. Importantly, we successfully monitor conformation changes of the nanoswitch by an elegant mathematical model for connecting K_i (the dissociation constant between RES and H#2) with K_d (the overall equilibrium constant of the nanoswitch binding the target), hence realizing "observing" DNA structure across dimensions from "structural visualization" to digitization and, accurately, digitally regulating DNA structure from digitization to "structural visualization".

Insights into the Recognition of Phosphate Groups by Peptidic Arginine from Action Spectroscopy and Quantum Chemical Computations

The side group of the amino acid arginine is typically in its guanidinium protonated form under physiological conditions and participates in a broad range of ligand binding and charge transfer processes of proteins. The recognition of phosphate moieties by guanidinium plays a particularly key role in the interactions of proteins with ATP and nucleic acids. Moreover, it has been recently identified as the driving force for the inhibition of kinase phosphorilation activity by guanidinium derivatives devised as potential anticancer agents. We report on a fundamental investigation of the interactions and coordination arrangements formed by guanidinium with phosphoric, phosphate, and pyrophosphate groups. Action vibrational spectroscopy and ab initio quantum chemical computations are employed to characterize the conformations of benchmark positively charged complexes isolated in an ion trap. The multidentate structure of guanidinium and of the phosphate groups gives rise to a rich conformational landscape with a particular relevance of tweezer-like configurations, where phosphate is effectively trapped by two guanidinium cations. The pyrophosphate complex incorporates a Na⁺ cation, which serves to compare the interactions associated with the localized versus diffuse charge distributions of the alkali cation and guanidinium, respectively, within a common supramolecular framework.

Interactions of Amino Acids and Aminoxazole Derivatives: Cocrystal Formation and Prebiotic Implications Enabled by Computational Analysis

In line with the postulated intermediacy of aminoxazoles derived from small sugars toward the direct assembly of nucleoside precursors, we show here a potential prebiotic scenario where aminoxazolines might have also played further roles as complexing and/or sequestering agents of other primeval blocks, namely amino acids. To this end, a bis-aminoxazoline derivative, generated from dihydroxyacetone and cyanamide, gives rise to stable co-crystal forms with dicarboxylic amino acids (Asp and Glu), while ionic interactions owing to proton transfer are inferred from spectroscopic data in aqueous solution. The structure of a 1:2 aminoxazoline: aspartic acid complex, discussed in detail, was elucidated by X-ray diffractometry. Optimized geometries of such ionic structures with bulk aqueous solvation were assessed by DFT calculations, which disclose preferential arrangements that validate the experimental data. Peripherally, we were able to detect in a few cases amino acid dimerization (i.e. dipeptide formation) after prolonged incubation with the bis-aminoxazole derivative. A mechanistic simulation aided by computation provides some predictive conclusions for future explorations and catalytic design.

Diverse Properties of Guanidiniocarbonyl Pyrrole-Based Molecules: Artificial Analogues of Arginine

The guanidinium moiety, which is present in active sites of many enzymes, plays an important role in the binding of anionic substrates. In addition, it was also found to be an excellent binding motif for supramolecular chemistry. Inspired by Nature, scientists have developed artificial receptors containing guanidinium scaffolds that bind to a variety of oxoanions through hydrogen bonding and charge pairing interactions. However, the majority of binding studies is restricted to organic solvents. Polyguanidinium based molecules can form efficient complexes in aqueous solvents due to strong electrostatic interactions. However, they only have moderate association constants, which are significantly decreased in the presence of competing anions and salts. Hence, to improve the binding affinity of the guanidinium moiety, our group developed the cationic guanidiniocarbonyl pyrrole (GCP) moiety. This rigid planar analogue binds efficiently to oxoanions, like carboxylates even in aqueous solvents. The lower p K_a value (7-8) of GCP compared to guanidinium derivatives (p K_a 13) favors the formation of strong, hydrogen bonded ion pairs. In addition, carboxylate binding is further enhanced by additional amide hydrogen bond donors located at the five position of the pyrrole core. Moreover, the design has allowed for introducing secondary interactions between receptor side chains and guest molecules, which allows for optimizing binding specificity and selectivity. The spectroscopic data confirmed stabilization of guanidiniocarbonyl pyrrole/oxoanion complexes through a combination of ion pairing and multiple hydrogen bonding interactions. The key role of the ionic interaction in a polar solvent, is demonstrated by a zwitterion derivative of the guanidiniocarbonyl pyrrole, which self-assembles in both dimethyl sulfoxide and pure water with association constants of K > 10¹⁰ M^-1 and K = 170 M^-1, respectively. In this Account, we discuss strategies for making GCP functionalized compounds, in order to boost their ability to bind oxoanions. Then we explore how these building blocks have been incorporated into different synthetic molecules and peptide sequences, highlighting examples that demonstrated the versatility of this binding scaffold. For instance, the high oxoanion binding property of GCP-based compounds was exploited to generate a detectable signal for sensing applications, thus improving selectivity and sensitivity in aqueous solution. Moreover, peptides and molecules containing GCP have shown excellent gene transfections properties. Furthermore, the self-assembly and zwitterionic behavior of zwitterionic GCP analogues was used to develop variety of supramolecular architectures such as stable supramolecular β-helix structure, linear supramolecular oligomers, one-dimensional rods or two-dimension sheets, fibers, vesicles, soft nanospheres, as well as stimuli responsive supramolecular gels.