Towards the Discrimination of Carboxylates by Hydrogen‐Bond Donor Anion Receptors

The binding constants (log K_ass) of small synthetic receptor molecules based on indolocarbazole, carbazole, indole, urea and some others, as well as their combinations were measured for small carboxylate anions of different basicity, hydrophilicity and steric demands, that is, trimethylacetate, acetate, benzoate and lactate, in 0.5 % H₂O/[D₆]DMSO by using the relative NMR‐based measurement method. As a result, four separate binding affinity scales (ladders) including thirty‐eight receptors were obtained with the scales anchored to indolocarbazole. The results indicate that the binding strength is largely, but not fully, determined by the strength of the primary hydrogen‐bonding interaction. The latter in turn is largely determined by the basicity of the anion. The higher is the basicity of the anion the stronger in general is the binding, leading to the approximate order of increasing binding strength, lactate<benzoate<acetate≤trimethylacetate, which holds with all investigated receptors. Nevertheless, there are a number of occasions when the binding order changes with changing of the carboxylate anion, sometimes quite substantially. Principal component analysis (PCA) reveals that this is primarily connected to preferential binding of trimethylacetate, supposedly caused by an additional hydrophobic/solvophobic interaction. These findings enable making better predictions, which receptor framework or cavity is best suited for carboxylate anions in receptor design.     

Recognition of Anions by Synthetic Receptors in Aqueous Solution

Natural anion binding systems achieve high substrate affinity and selectivity most often by arranging converging binding sites inside a cavity or cleft that is well shielded from surrounding solvent molecules by the folded peptide chain. Types of interactions employed for anion recognition are electrostatic interactions, hydrogen-bonding, and coordination to a Lewis-acidic metal center. In this review, successful strategies aimed at the development of synthetic receptors active in water or aqueous solvent mixtures are described. It is shown that considerable progress has been made during recent years in the development of potent anion receptors and that for every type of interaction used in nature for anion binding, corresponding synthetic models exist today. Representative examples of these systems are presented with a special emphasis on synthetic receptors whose characterization involved a detailed thermodynamic analysis of complex formation to demonstrate the important interplay between enthalpy and entropy for anion recognition in water.This revised version was published online in July 2005 with a corrected issue number.     

Selective Pyrophosphate Recognition by Cyclic Peptide Receptors in Physiological Saline

The anion binding ability of a family of bis(Zn^II‐Dpa) functionalized cyclic peptides has been investigated using displacement assays with a fluorescent coumarin indicator in water, saline solution, and Krebs buffer. Non‐binding side‐chain steric bulk, the relative position of binding sites, and the scaffold size were all found to affect the ability of these receptors to discriminate between polyphosphate ions. Most receptors showed some selectivity for pyrophosphate over ATP and ADP in water and saline, and this selectivity was significantly enhanced in the biologically relevant Krebs buffer giving chemosensing ensembles capable of selective recognition of pyrophosphate in the presence of excess ATP.     

Molecular Recognition and Scavenging of Arsenate from Aqueous Solution Using Dimetallic Receptors

A series of copper(II), nickel(II) and zinc(II) dimetallic complexes were prepared and their affinities towards arsenate investigated. Indicator displacement assays (IDAs) were carried out to establish the complexes with best affinities towards arsenate. A di‐zinc complex ( 3 ) was selected and its arsenate‐binding abilities investigated by isothermal titration calorimetry (ITC). The X‐ray crystal structure of this metallo‐receptor bound to arsenate is also reported, which allowed us to establish the binding mode between 3 and this oxyanion. Immobilising 3 onto HypoGel resin yielded a novel adsorbent (Zn–HypoGel) with high affinity for arsenate. Adsorption of arsenate from competitive solutions and natural groundwater was greater than that of the commercially used iron oxide Bayoxide E33. Zn–HypoGel could be efficiently and simply regenerated by washing with sodium acetate solution.     

The G‐Protein‐Coupled Neuropeptide Y Receptor Type 2 is Highly Dynamic in Lipid Membranes as Revealed by Solid‐State NMR Spectroscopy

In spite of the recent success in crystallizing several G‐protein‐coupled receptors (GPCRs), a comprehensive biophysical characterization of these molecules under physiological conditions also requires the study of the molecular dynamics of these proteins. The molecular mobility of the human neuropeptide Y receptor type 2 reconstituted into dimyristoylphosphatidylcholine (DMPC) membranes was investigated by means of solid‐state NMR spectroscopy. Static ¹⁵N NMR spectra show that the receptor performs axially symmetric motions in the membrane, and several residues undergo large amplitude fluctuations. This was confirmed by quantitative measurements of the motional ¹H,¹³C order parameter of the CH, CH₂, and CH₃ groups. In directly polarized ¹³C NMR experiments, these order parameters showed astonishingly low values of S_CH=0.55, S=0.33, and S=0.17, which corresponds to segmental amplitudes of approximately 50° in the backbone and approximately 50–60° in the side chain. At physiological temperature, ²H NMR spectra of the deuterated receptor showed a narrow component that is indicative of molecular order parameters of S≤0.3 superimposed with a very broad spectrum that could stem from the transmembrane α‐helices. These results suggest that the crystal structures of GPCRs only represent a static snapshot of these highly mobile molecules, which undergo significant structural fluctuations with relatively large amplitudes in a liquid‐crystalline membrane at physiological temperature.     

Anion Recognition by Urea Derivatives of Anthraquinone: Dihydrogen Phosphate Ion Selective Neutral Receptors

Three urea derivatives of anthraquinone were synthesized and they showed a high selectivity for dihydrogen phosphate ions.     

Selective Recognition of Amino Acids and Peptides by Small Supramolecular Receptors

To this day, the recognition and high affinity binding of biomolecules in water by synthetic receptors remains challenging, while the necessity for systems for their sensing, transport and modulation persists. This problematic is prevalent for the recognition of peptides, which not only have key roles in many biochemical pathways, as well as having pharmacological and biotechnological applications, but also frequently serve as models for the study of proteins. Taking inspiration in nature and on the interactions that occur between several receptors and peptide sequences, many researchers have developed and applied a variety of different synthetic receptors, as is the case of macrocyclic compounds, molecular imprinted polymers, organometallic cages, among others, to bind amino acids, small peptides and proteins. In this critical review, we present and discuss selected examples of synthetic receptors for amino acids and peptides, with a greater focus on supramolecular receptors, which show great promise for the selective recognition of these biomolecules in physiological conditions. We decided to focus preferentially on small synthetic receptors (leaving out of this review high molecular weight polymeric systems) for which more detailed and accurate molecular level information regarding the main structural and thermodynamic features of the receptor biomolecule assemblies is available.     

Straightforward and Controlled Shape Access to Efficient Macrocyclic Imidazolylboronium Anion Receptors

A straightforward synthesis of air‐ and water‐stable bis‐cationic macrocyclic imidazolylboronium anion receptors is described herein. By taking advantage of the bulky and rigid 9‐borabicyclo[3.3.1]‐nonane (9‐BBN) attaching point and a well‐designed bis‐imidazolylaryl, highly stable dimeric imidazolylboronium macrocycles were synthesized. Additionally, NMR spectroscopy (¹H, DOSY, and HOESY), mass spectrometry (MS), and X‐ray diffraction studies revealed that these macrocyclic scaffolds can bind several monoanions with high association constants in DMSO, and are particularly sensitive for the MS detection of anions (with concentrations in the nm range). This anion/receptor interaction involves eight C?H binding sites, which include C ?H and unusual C_sp3?H hydrogen‐bonding donors.     

Recognition of Anions by Synthetic Receptors in Aqueous Solution

Natural anion binding systems achieve high substrate affinity and selectivity most often by arranging converging binding sites inside a cavity or cleft that is well shielded from surrounding solvent molecules by the folded peptide chain. Types of interactions employed for anion recognition are electrostatic interactions, hydrogen-bonding, and coordination to a Lewis-acidic metal center. In this review, successful strategies aimed at the development of synthetic receptors active in water or aqueous solvent mixtures are described. It is shown that considerable progress has been made during recent years in the development of potent anion receptors and that for every type of interaction used in nature for anion binding, corresponding synthetic models exist today. Representative examples of these systems are presented with a special emphasis on synthetic receptors whose characterization involved a detailed thermodynamic analysis of complex formation to demonstrate the important interplay between enthalpy and entropy for anion recognition in water.     

Molecular Recognition by a Short Partial Peptide of the Adrenergic Receptor: A Bottom‐Up Approach

Receptor–neurotransmitter molecular recognition is key for neurotransmission. Although crystal structures of the receptors are known, the mechanism for recognition is not clear. Reported here is the ultraviolet (UV) and infrared (IR) spectra of complexes between a partial peptide (SIVSF), mimicking the binding motif of a catechol ring in the adrenergic receptor, and various ligands. The UV spectra show that two isomers coexist in the complex of SIVSF with properly recognized ligands, such as protonated adrenaline (adrenalineH⁺). From IR spectra, they are assigned to catechol‐ and amino‐bound structures. The catechol‐bound structure is not observed when the ligand is replaced by nonproper molecules, such as noradrenalineH⁺. The results suggest that SIVSF not only recognizes the catechol ring but can distinguish differences in the amine side chain. The method provides a new possibility for screening molecules as potential therapeutics for activating the receptor.     

Peptide‐Based Carbohydrate Receptors

A broad spectrum of physiological processes is mediated by highly specific noncovalent interactions of carbohydrates and proteins. In a recent communication we identified several cyclic hexapeptides in a dynamic combinatorial library that interact selectively with carbohydrates with high binding constants in water. Herein, we report a detailed investigation of the noncovalent interaction of two cyclic hexapeptides (Cys‐His‐Cys (which we call HisHis) and Cys‐Tyr‐Cys (which we call TyrTyr)) with a selection of monosaccharides and disaccharides in aqueous solution. The parallel and antiparallel isomers of HisHis or TyrTyr were synthesized separately, and their interaction with monosaccharides and disaccharides in aqueous solution was studied by isothermal titration calorimetry, NMR spectroscopic titrations, and circular dichroism spectroscopy. From these measurements, we identified particularly stable complexes (K_a>1000 M ^?1) of the parallel isomer of HisHis with N‐acetylneuraminic acid and with methyl‐α‐D ‐galactopyranoside as well as of both isomers of TyrTyr with trehalose. To gain further insight into the structure of the peptide–carbohydrate complexes, structure prediction was performed using quantum chemical methods. The calculations confirm the selectivity observed in the experiments and indicate the formation of multiple intermolecular hydrogen bonds in the most stable complexes.     

Cross‐Reactive Sensor Arrays for the Detection of Peptides in Aqueous Solution by Fluorescence Spectroscopy

A simple but powerful method for the sensing of peptides in aqueous solution has been developed. The transition‐metal complexes [PdCl₂(en)], [{RhCl₂Cp*}₂], and [{RuCl₂(p‐cymene)}₂] were combined with six different fluorescent dyes to build a cross‐reactive sensor array. The fluorescence response of the individual sensor units was based on competitive complexation reactions between the peptide analytes and the fluorescent dyes. The collective response of the sensor array in a time‐resolved fashion was used as an input for multivariate analyses. A sensor array comprised of only six metal–dye combinations was able to differentiate ten different dipeptides in buffered aqueous solution at a concentration of 50 μM . Furthermore, the cross‐reactive sensor could be used to obtain information about the identity and the quantity of the pharmacologically interesting dipeptides carnosine and homocarnosine in a complex biological matrix, such as deproteinized human blood serum. The sensor array was also able to sense longer peptides, which was demonstrated by differentiating mixtures of the nonapeptide bradykinin and the decapeptide kallidin.     

Exploration of the Chiral Recognition of Sugar‐Based Diindolylmethane Receptors: Anion and Receptor Structures

In this study, we have conducted a systematic investigation of the chiral recognition of carboxylic anions by D ‐glucuronic acid/diindolylmethane receptors. We investigate the influence of the anion structure on chiral recognition in the diindolylmethane/glucuronic acid‐based receptor 1 a . We found that presence of an additional hydrogen‐bond donor at the α position to the carboxylic function is essential for effective chiral differentiation in these systems. Furthermore, we present a synthetic procedure that allows for the synthesis of sugar‐decorated receptors that possess a modified substituent at the anomeric position. Four new receptors 1 b – e have been synthesized, and their chiral‐discrimination ability toward model carboxylates is studied. The obtained results show that the chiral recognition of these receptors can be fine‐tuned by incorporation of a proper substituent into the receptor structure.