Günter Wulff (1935 -2023): Father of Molecular Imprinting.

Günter Wulff, internationally well known for his invention of Molecular Imprinting, passed away on December 11, 2023 in Erkrath-Hochdahl, Germany, not far from the University of Düsseldorf, where he made his greatest discoveries. A passionate researcher and deep conceptual thinker, he greatly advanced our understanding of polymer chemistry.

Synthetic Receptors Based on Abiotic Cyclo(pseudo)peptides.

Work on the use of cyclic peptides or pseudopeptides as synthetic receptors started even before the field of supramolecular chemistry was firmly established. Research initially focused on the development of synthetic ionophores and involved the use of macrocycles with a repeating sequence of subunits along the ring to facilitate the correlation between structure, conformation, and binding properties. Later, nonnatural amino acids as building blocks were also considered. With growing research in this area, cyclopeptides and related macrocycles developed into an important and structurally diverse receptor family. This review provides an overview of these developments, starting from the early years. The presented systems are classified according to characteristic structural elements present along the ring. Wherever possible, structural aspects are correlated with binding properties to illustrate how natural or nonnatural amino acids affect binding properties.

Selective sensing of adenosine monophosphate (AMP) over adenosine diphosphate (ADP), adenosine triphosphate (ATP), and inorganic phosphates with zinc(II)-dipicolylamine-containing gold nanoparticles.

Mixed monolayer-protected gold nanoparticles containing surface-bound triethylene glycol and dipicolylamine groups aggregated in water/methanol, 1 : 2 (v/v) in the presence of nucleotides, if the solution also contained zinc(ii) nitrate to convert the dipicolylamine units into the corresponding zinc complexes. Nanoparticle aggregation could be followed with the naked eye by the colour change of the solution from red to purple followed by nanoparticle precipitation. The sensitivity was highest for adenosine triphosphate (ATP), which could be detected at concentrations >10 µM, and decreased over adenosine diphosphate (ADP) to adenosine monophosphate (AMP), consistent with the typically higher affinity of zinc(ii)-dipicolylamine-derived receptors for higher charged nucleotides. Inorganic sodium diphosphate and triphosphate interfered in the assay by also inducing nanoparticle aggregation. However, while the nucleotide-induced aggregates persisted even at higher analyte concentrations, the nanoparticles that were precipitated with inorganic salts redissolved again when the salt concentration was increased. The thus resulting solutions retained their ability to respond to nucleotides, but they now preferentially responded to AMP. Accordingly, AMP could be sensed selectively at concentrations ≥50 µM in an aqueous environment, even in the presence of other nucleotides and inorganic anions. This work thus introduces a novel approach for the sensing of a nucleotide that is often the most difficult analyte to detect with other assays.

Influence of cyclic and acyclic cucurbiturils on the degradation pathways of the chemical warfare agent VX.

The highly toxic nerve agent VX is a methylphosphonothioate that degrades via three pathways in aqueous solution, namely through the hydrolysis of the P-O or P-S bonds, or the cleavage of the C-S bond at the 2-aminoethyl residue. In the latter case, an aziridinium ion and a phosphonothioate is formed. Here it is shown that acyclic or cyclic cucurbiturils inhibit these reactions in phosphate buffer at physiological pH and thus stabilise the nerve agent. When using unbuffered basic solutions as the reaction medium, however, in which the P-S or P-O bonds are normally hydrolysed preferentially, cucurbiturils turned out to strongly shift VX degradation towards the cleavage of the C-S bond. Cucurbit[7]uril, in particular, has a so pronounced effect under suitable conditions that it almost completely suppresses the formation of products resulting from the other degradation pathways. Investigations involving VX analogues in combination with computational methods suggest that one reason for the reaction control exerted by the cucurbiturils is the preorganisation of VX for aziridinium ion formation. In addition, cucurbit[7]uril also lowers the transition state of the reaction by stabilising the positive charge developing on the way to the product. Cucurbiturils thus have a marked effect on the reactivity of a highly toxic nerve agent, which potentially allows using them for decontamination purposes.

Optical detection of di- and triphosphate anions with mixed monolayer-protected gold nanoparticles containing zinc(II)-dipicolylamine complexes.

Gold nanoparticles covered with a mixture of ligands of which one type contains solubilizing triethylene glycol residues and the other peripheral zinc(II)-dipicolylamine (DPA) complexes allowed the optical detection of hydrogenphosphate, diphosphate, and triphosphate anions in water/methanol 1:2 (v/v). These anions caused the bright red solutions of the nanoparticles to change their color because of nanoparticle aggregation followed by precipitation, whereas halides or oxoanions such as sulfate, nitrate, or carbonate produced no effect. The sensitivity of phosphate sensing depended on the nature of the anion, with diphosphate and triphosphate inducing visual changes at significantly lower concentrations than hydrogenphosphate. In addition, the sensing sensitivity was also affected by the ratio of the ligands on the nanoparticle surface, decreasing as the number of immobilized zinc(II)-dipicolylamine groups increased. A nanoparticle containing a 9:1 ratio of the solubilizing and the anion-binding ligand showed a color change at diphosphate and triphosphate concentrations as low as 10 µmol/L, for example, and precipitated at slightly higher concentrations. Hydrogenphosphate induced a nanoparticle precipitation only at a concentration of ca. 400 µmol/L, at which the precipitates formed in the presence of diphosphates and triphosphates redissolved. A nanoparticle containing fewer binding sites was more sensitive, while increasing the relative number of zinc(II)-dipicolylamine complexes beyond 25% had a negative impact on the limit of detection and the optical response. Transmission electron microscopy provided evidence that the changes of the nanoparticle properties observed in the presence of the phosphates were due to a nanoparticle crosslinking, consistent with the preferred binding mode of zinc(II)-dipicolylamine complexes with phosphate anions which involves binding of the anion between two metal centers. This work thus provided information on how the behavior of mixed monolayer-protected gold nanoparticles is affected by multivalent interactions, at the same time introducing a method to assess whether certain biologically relevant anions are present in an aqueous solution within a specific concentration range.

Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors.

Anion receptors often rely on coordinative or multiple ionic interactions to be active in water. In the absence of such strong interactions, anion binding in water can also be efficient, however, as demonstrated by a number of anion receptors developed in recent years. The cyclopeptide-derived receptors comprising an alternating sequence of l-proline and 6-aminopicolinic acid subunits are an example. These cyclopeptides are neutral and, at first sight, can only engage in hydrogen-bond formation with an anionic substrate. Nevertheless, they even interact with strongly solvated sulfate anions in water. The intrinsic anion affinity of these cyclopeptides can be related to structural aspects of their highly preorganized concave binding site, which comprises a wall of hydrophobic proline units arranged around the peptide NH groups at the cavity base. When anions are incorporated into this cavity they can engage in hydrogen-bonding interactions to the NH groups, and complex formation also benefits from cavity dehydration. Formation of 1:1 complexes, in which an anion binds to a single cyclopeptide ring, is associated with only small stability constants, however, whereas significantly more stable complexes are formed if the anion is buried between two cyclopeptide molecules. A major contribution to the formation of these sandwich complexes derives from the addition of the second ring to the initially formed 1:1 cyclopeptide-anion complex. This step brings the apolar proline residues of both cyclopeptides in close proximity, which causes the resulting structure to be stabilized to a large extent by hydrophobic effects. Solvent dependent binding studies provided an estimate to which degree these solvent effects contribute to the overall complex stability. In these studies, bis(cyclopeptides) were used, featuring two cyclopeptide rings covalently connected via linkers that enable both rings to simultaneously interact with the anion. Bis(cyclopeptides) with additional solubilizing groups allowed binding studies in a wide range of solvents, including in water. The systematic analysis of the solvent dependence of anion affinity yielded a quantitative correlation between complex stability and parameters relating to the solvation of the anions and solvent properties, confirming that solvent effects contribute to anion binding. Interestingly, the thermodynamic signature of complex formation in water mirrors that of sulfate binding to a protein complex but is opposite to that of other recently described anion receptors, which also do not engage in ionic or coordinative interactions with the substrate. These receptors not only differ in terms of the thermodynamics of binding from the cyclopeptides but also possess a characteristically different anion selectivity in that they prefer to bind weakly coordinating anions but fail to bind sulfate. Solvent effects likely control the anion binding of both receptors types but their impact on complex formation and anion selectivity seems to be profoundly different. Future work in the area of anion coordination chemistry will benefit from the deeper understanding of these effects and how they can be controlled.

Transmembrane Fluoride Transport: Direct Measurement and Selectivity Studies.

Fluoride has been overlooked as a target in the development of synthetic anion transporters despite natural fluoride transport channels being recently discovered. In this paper we report the direct measurement of fluoride transport across lipid bilayers facilitated by a series of strapped calix[4]pyrroles and show that these compounds facilitate transport via an electrogenic mechanism (determined using valinomycin and monensin coupled transport assays and an additional osmotic response assay). An HPTS transport assay was used to quantify this electrogenic process and assess the interference of naturally occurring fatty acids with the transport process and Cl- over H+/OH- transport selectivity.

Oxoanion binding to a cyclic pseudopeptide containing 1,4-disubstituted 1,2,3-triazole moieties.

A macrocyclic pseudopeptide 3 is described featuring three amide groups and three 1,4-disubstituted 1,2,3-triazole units along the ring. This pseudopeptide was designed such that the amide NH groups and the triazole CH groups converge toward the cavity, thus creating an environment well suited for anion recognition. Conformational studies in solution combined with X-ray crystallography confirmed this preorganisation. Solubility of 3 restricted binding studies to organic media such as 5 vol% DMSO/acetone or DMSO/water mixtures with a water content up to 5 vol%. These binding studies demonstrated that 3 binds to a variety of inorganic anions in DMSO/acetone including chloride, nitrate, sulfate, and dihydrogenphosphate anions. In the more competitive DMSO/water mixtures, only affinity to the more strongly coordinating oxoanions is retained. Quantitative binding studies showed that dihydrogen phosphate complexation in DMSO/water involves the dimer of the H2PO4- anion. By contrast, sulfate and hydrogenpyrophosphate complexation involves a stepwise process comprising formation of a 1 : 1 complex followed by a 2R : 1A complex in which two molecules of 3 (R) bind to a single anion (A). While the second binding equilibrium is associated with a much smaller stability constant in comparison with the first one in the case of sulfate complexation, the two binding constants are of similar magnitude in the case of the hydrogenpyrophosphate anion. Formation of the 2R : 1A complex was attributed to the fact that the cavity size and rigidity of 3 prevents saturation of all hydrogen acceptor sites on the anionic guests.

Detoxification of VX and Other V-Type Nerve Agents in Water at 37 °C and pH†7.4 by Substituted Sulfonatocalix[4]arenes.

Sulfonatocalix[4]arenes with an appended hydroxamic acid residue can detoxify VX and related V-type neurotoxic organophosphonates with half-lives down to 3 min in aqueous buffer at 37 °C and pH 7.4. The detoxification activity is attributed to the millimolar affinity of the calixarene moiety for the positively charged organophosphonates in combination with the correct arrangement of the hydroxamic acid group. The reaction involves phosphonylation of the hydroxamic acid followed by a Lossen rearrangement, thus rendering the mode of action stoichiometric rather than catalytic. Nevertheless, these calixarenes are currently the most efficient low-molecular-weight compounds for detoxifying persistent V-type nerve agents under mild conditions. They thus represent lead structures for novel antidotes that allow treatment of poisonings by these highly toxic chemicals.

Anion binding of a neutral bis(cyclopeptide) in water-methanol mixtures containing up to 95% water.

Anion receptor 2b was designed and synthesized, which was structurally derived from a previously described bis(cyclopeptide) 2a comprising two covalently linked cyclic hexapeptide rings with alternating L-proline and 6-aminopicolinic acid subunits. Solubilizing groups attached to the aromatic cyclopeptide subunits of 2b cause a substantial improvement of water solubility with respect to 2a, but have negligible effects on anion binding properties. Thus, anion affinity of 2b could be evaluated in aqueous solvent mixtures in which 2a is not sufficiently soluble, namely in water­methanol with a water content of up to 95 vol%. The solvent-dependent characterization of anion binding showed that the logKa values of the iodide and sulfate complexes of 2b decrease linearly with increasing water content while the individual contributions of complexation enthalpy and entropy correlate with the solvent composition in a more complex manner. The obtained results provide insight into the factors that control anion affinity and selectivity of these neutral receptors in aqueous media. In addition, they show that substantial anion affinity can be expected even in 100% water.

Molecular cages and capsules with functionalized inner surfaces.

Molecular containers enclose a well defined cavity in which an appropriate guest molecule can be included. The corresponding complexes are generally characterized by high kinetic stability. Thermodynamic stability can be rather low, however, because attractive interactions are largely missing between host and guest causing binding to be mainly due to entropic factors. This situation can be improved by distributing appropriate binding sites across the inner surface of a molecular container to which an included guest can bind. This approach, while being conceptually simple, is not straightforward since the incorporation of converging binding sites into a concave surface is difficult and usually requires receptors architectures that differ from those of conventional covalently assembled molecular containers. Therefore, the term molecular cage rather than molecular container is often more appropriate for such types of receptors. In this overview, a selection of cage-type receptors is presented whose inner cavity is functionalized with groups that can engage in directed interactions with an included guest. These receptors, classified according to the type of interaction responsible for guest binding, were chosen to illustrate effects of the inwardly directed binding sites on receptor affinity, selectivity, or other binding properties.

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.

Structural analysis of an isolated cyclic tetrapeptide and its monohydrate by combined IR/UV spectroscopy.

Cyclopeptides are an important class of substances in nature, and their physiological effects are frequently based on the tendency to form bioactive conformations. Therefore the investigation of their structure yields an understanding of their functionalities. Mass-selective combined IR/UV spectroscopy in molecular beam experiments represents an ideal tool for structural analyses on isolated molecules in the gas phase, such as the investigated cyclo[L-Tyr(Me)-D-Pro](2) peptide and its complexes with water. Using the chosen spectroscopic method in combination with DFT calculations, an assignment of a structure with two intramolecular hydrogen bonds for the naked cyclopeptide is possible. For the monohydrated cluster two isomers have to be discussed: in one of them the water molecule is simply attached to the assigned monomer structure as hydrogen donor, whereas the second isomer can be characterized by a water molecule that is inserted into one of the intramolecular hydrogen bonds.

Anion-binding properties of a cyclic pseudohexapeptide containing 1,5-disubstituted 1,2,3-triazole subunits.

A C(3) symmetric cyclic pseudohexapeptide containing 2-aminopicoline-derived subunits and 1,5-disubstituted 1,2,3-triazole rings is introduced as a potent anion receptor. This macrocycle was designed to mimic both the conformation and the receptor properties of a previously described cyclic hexapeptide containing alternating L-proline and 6-aminopicolinic acid subunits. Conformational analyses demonstrate that the cyclic peptide and the cyclic pseudopeptide are structurally closely related. Most importantly, both exhibit a converging arrangement of the NH groups, hence a good preorganization for anion binding. As a consequence, the pseudopeptide also very efficiently interacts with halide and sulfate ions, and this is the case even in competitive aqueous solvent mixtures. However, there are clear differences in the structures of both compounds, which translate into characteristic differences in receptor properties. Specifically, (i) the pseudopeptide possesses an anion affinity intrinsically higher than that of the cyclopeptide, (ii) the pseudopeptide is well preorganized for anion binding in a wider range of solvents from aprotic to protic, (iii) anion affinity in aprotic solvents is very high and associated with complexation equilibria that are slow on the NMR time-scale, (iv) the propensity of the pseudopeptide to form sandwich-type 2:1 complexes with two receptor molecules surrounding one anion is significantly lower than that of the cyclopeptide. A solvent-dependent calorimetric characterization of the binding equilibria of both compounds provided clear evidence for the stabilizing effect of hydrophobic interactions between the receptor subunits in such 2:1 complexes. The pseudopeptide thus represents the first member of a new family of anion receptors whose properties may be fine-tuned by varying the side chains in the periphery of the cavity.

Anion recognition in water.

Anion recognition by synthetic receptors in water is not a new field, indeed the first receptors that were shown to interact with anionic species exhibited high affinity in aqueous solutions. Anion recognition in aqueous solution was, however, for a long time the domain of receptors containing multiple positive charges and/or metal ions while interactions of neutral receptors with anions were believed to be too weak to be efficient in water. Independent work in several groups has recently shown that this assumption is not necessarily correct. As a consequence, a much wider range of receptors is now available with which anion recognition in competitive aqueous media can be achieved. This tutorial review presents selected examples of synthetic anion receptors active in aqueous solutions and guidelines to achieve anion recognition in water.

Highly efficient cyclosarin degradation mediated by a ß-cyclodextrin derivative containing an oxime-derived substituent.

The potential of appropriately substituted cyclodextrins to act as scavengers for neurotoxic organophosphonates under physiological conditions was evaluated. To this end, a series of derivatives containing substituents with an aldoxime or a ketoxime moiety along the narrow opening of the ß-cyclodextrin cavity was synthesized, and the ability of these compounds to reduce the inhibitory effect of the neurotoxic organophosphonate cyclosarin on its key target, acetylcholinesterase, was assessed in vitro. All compounds exhibited a larger effect than native ß-cyclodextrin, and characteristic differences were noted. These differences in activity were correlated with the structural and electronic parameters of the substituents. In addition, the relatively strong effect of the cyclodextrin derivatives on cyclosarin degradation and, in particular, the observed enantioselectivity are good indications that noncovalent interactions between the cyclodextrin ring and the substrate, presumably involving the inclusion of the cyclohexyl moiety of cyclosarin into the cyclodextrin cavity, contribute to the mode of action. Among the nine compounds investigated, one exhibited remarkable activity, completely preventing acetylcholinesterase inhibition by the (-)-enantiomer of cyclosarin within seconds under the conditions of the assay. Thus, these investigations demonstrate that decoration of cyclodextrins with appropriate substituents represents a promising approach for the development of scavengers able to detoxify highly toxic nerve agents.

A cyclopeptide-derived molecular cage for sulfate ions that closes with a click.

The 2:1 sandwich-type complexes formed between a cyclopeptide with alternating L-proline and 6-aminopicolinic acid subunits and inorganic anions can be stabilized by covalently linking a tris-alkyne and a tris-azide derivative of this peptide through copper-catalyzed azide-alkyne cycloaddition. The resulting triply linked bis-cyclopeptide can interact with anions such as sulfate ions in aqueous solution by including them into the cavity between the two cyclopeptide rings, where they can form hydrogen bonds to amide NH groups, distributed along the inner surface. The binding kinetics of this system differ significantly from those of a bis-cyclopeptide that contains only one linker because the rate of guest exchange is considerably slower. Thermodynamically, the stability of the sulfate complex of the triply linked bis-cyclopeptide approaches a log K(a) value of 6 in H(2)O/CH(3)OH 1:1 (v/v) which is, however, only approximately one order of magnitude larger than affinity of the more flexible monolinked analogue. Titration calorimetry revealed that this behavior is mainly due to the change in the binding enthalpy from exothermic to endothermic upon increasing the number of linkers. Results from NMR spectroscopy and X-ray crystallography indicate that the mono- and triply linked bis-cyclopeptides adopt similar conformations in their complexes with sulfate ions, but the complex formation is enthalpically unfavorable for the cage. The substantial entropic contribution to sulfate complexation of this receptor more than compensates for this disadvantage, so that the overall sulfate affinity of both bis-cyclopeptides ends up in the same range. These investigations provide important insight into the structure-property relationships of such receptors, thus leading the way to further structural improvement.

Anion Binding of a Cyclopeptide-Derived Molecular Cage in Aqueous Solvent Mixtures.

A molecular cage consisting of two cyclic hexapeptides with an alternating sequence of (2S,4S)-4-aminoproline and 6-aminopicolinic acid subunits, covalently linked via three diglycolic acid subunits, interacts with a variety of inorganic anions in acetonitrile/water. In the respective complexes, the anion resides in a cavity between the two cyclopeptide rings where it interacts with six converging NH groups. The cage binds sulfate anions in acetonitrile/water, 2 : 1 (v/v) with a log Ka of 6.7, ca. 2.5 orders of magnitude stronger than an analogous bis(cyclopeptide) with only one linker whose sulfate affinity log Ka amounts to 4.3. The preorganization induced by the three linking units is thus beneficial for sulfate binding. In addition, these linkers cause the dissociation of the sulfate complex to have a substantial Gibbs free energy of activation ΔG≠ of 68.9 kJ mol-1 and they also seem to affect anion selectivity as illustrated by the different effects some anions produce on the 1 H NMR spectra of the triply and singly-linked bis(cyclopeptides). Such anion binding cages represent promising scaffolds to mimic natural anion receptors such as the sulfate-binding protein.

Selective sensing of sulfate anions in water with cyclopeptide-decorated gold nanoparticles.

The interaction of cyclopeptides bound to the surface of mixed monolayer-protected gold nanoparticles with sulfate anions causes the crosslinking and concomitant precipitation of the nanoparticles from aqueous solutions even in presence of an excess of competing anions, thus allowing the naked eye detection of sulfate in water.