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.

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.

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.

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.

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.

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.

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.

Chirality sensing of terpenes, steroids, amino acids, peptides and drugs with acyclic cucurbit[n]urils and molecular tweezers.

Achiral chromophoric hosts, i.e. acyclic cucurbit[n]urils and molecular tweezers, were found to respond with characteristic Circular Dichroism (CD) spectra to the presence of micromolar concentrations of chiral hydrocarbons, terpenes, steroids, amino acids and their derivates, and drugs in water. In favourable cases, this allows for analyte identification or for reaction monitoring.

Palladium(II)-Mediated Assembly of a M2L2 Macrocycle and M3L6 Cage from a Cyclopeptide-Derived Ligand.

A cyclic tetrapeptide L comprising two l-proline and two 3-amino-5-(pyridin-4-yl)benzoic acid subunits assembles into a dimetallic metallamacrocycle Pd2L2 and a trimetallic coordination cage Pd3L6 in the presence of suitable palladium(II) precursors. Pd2L2 recognizes organic anions in aqueous media, forming a particularly stable complex with sodium 2,6-naphthalenedisulfonate, in which two dianionic guest molecules reside in the cavity surrounded by the cyclopeptide ligands.

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.

Efficient stabilisation of a dihydrogenphosphate tetramer and a dihydrogenpyrophosphate dimer by a cyclic pseudopeptide containing 1,4-disubstituted 1,2,3-triazole moieties.

A cyclic pseudooctapeptide 2 is described containing 1,4-disubstituted 1,2,3-triazole moieties. This compound features eight converging hydrogen bond donors along the ring, namely four amide NH and four triazole CH groups, which enable 2 to engage in interactions with anions. While fully deprotonated sulfate anions exhibit only moderate affinity for 2, protonated anions such as dihydrogenpyrophosphate and dihydrogenphosphate anions are strongly bound. Complexation of the phosphate-derived anions involves sandwiching of a dihydrogenpyrophosphate dimer or a dihydrogenphosphate tetramer between two pseudopeptide rings. X-ray crystallography provided structural information, while 1H NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry demonstrated that these complexes are stable in solution (2.5 vol% water/DMSO) and can even be transferred without decomposition into the gas phase. The observed high thermodynamic stabilities are attributed to the mutual reinforcement of the interactions between the individual complex components, namely, hydrogen-bonding between the anions, multiple hydrogen bonding interactions between the anion aggregates and the triazole CH and NH hydrogen bond donors of 2, and potential dispersive interactions between the closely arranged pseudopeptide rings. Pseudopeptide 2 thus represents a promising lead for the construction of phosphate receptors, whose binding selectivity makes use of the unique ability of certain anions to assemble into higher aggregates.

A neutral halogen bonding macrocyclic anion receptor based on a pseudocyclopeptide with three 5-iodo-1,2,3-triazole subunits.

The converging arrangement of iodine atoms along its confined cavity causes a cyclic pseudopeptide with three 5-iodo-1,2,3-triazole subunits to interact with halides, in particular with chloride, in 2.5 vol% water/DMSO.

Effects of Solvent Properties on the Anion Binding of Neutral Water-Soluble Bis(cyclopeptides) in Water and Aqueous Solvent Mixtures.

In this study, the anion-binding bis(cyclopeptide) 2 is introduced, which dissolves freely in water, affording up to 10 mM concentrations, thanks to triethylene glycol-derived substituents in the cyclopeptide subunits and the linker connecting them. Binding studies provided evidence that the anion affinity previously demonstrated for less-soluble analogs of this compound is retained under highly competitive aqueous conditions. The highest affinity in water was observed for iodide, closely followed by sulfate anions, whereas binding of soft and weakly coordinating anions could not be observed. The anion selectivity of 2 thus differs from that of other recently described receptors, which also do not require electrostatic or coordinative interactions for anion binding in water but typically fail to bind strongly coordinating sulfate anions. The ability of 2 to overcome sulfate hydration is attributed to the special mode of binding, combining direct N-H···A- interactions with the release of water molecules from the receptor cavity. The characterization of the anion binding of 2 and a related bis(cyclopeptide) in a variety of different solvents and aqueous solvent mixtures furthermore allowed the correlation of the binding properties with solvent parameters. These analyses provided qualitative and even quantitative insights into the solvent properties and solvation phenomena that mainly affect anion complexation.

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.

Synthesis and Structural Characterization of a Cyclen-Derived Molecular Cage.

Reaction of a tetrafunctionalized cyclen derivative containing four aldehyde groups with an appropriate diamine followed by reduction and demetalation highly efficiently affords a bis(cyclen)-derived molecular cage. Potentiometric investigations show that this compound forms dimetallic complexes with copper(II), with the two metal ions selectively coordinated to the cyclen units. X-ray crystallography indicates that these complexes could give rise to new cascade complexes after incorporation of anions between the metal centers.

Elimination kinetics and molecular reaction mechanisms of cyclosarin (GF) by an oxime substituted ß-cyclodextrin derivative in vitro.

Detoxification mechanisms of the chemical warfare agent cyclosarin (GF) in presence of 6-OxP-CD were investigated in detail in in vitro model systems. Most important finding was the preference of 6-Ox-P-CD to eliminate the more toxic (-)-GF. However, elimination of GF enantiomers was dependent on the 6-OxP-CD:GF ratios showing decreasing stereoselectivity and speed of the reaction with increasing GF concentrations. Formation of covalent mono, bis, tris and tetrakis conjugates ((CHMP)n-6-OxP-CD) and appearance of small molecular fragments (SMF) as possible decomposition products after consumption of 6-OxP-CD could be observed.. Interestingly, the non-toxic metabolite O-cyclohexyl methylphosphonic acid (CHMPA) and covalent mono and bis conjugates of 6-OxP-CD and GF were immediately formed, indicating that GF elimination proceeds along different pathways. These important new insights provide information about the mode of action of 6-Ox-P-CD including the role of the pyridinium aldoxime group on the cyclodextrin ring. They are the basis for further investigations in biological media, which could eventually lead to approval of 6-OxP-CD as a new antidote against nerve agent toxicity.

Dipeptide recognition in water mediated by mixed monolayer protected gold nanoparticles.

Mixed monolayer protected gold nanoparticles were prepared featuring functional groups on their surfaces that can engage in interactions with peptides. DOSY NMR binding studies indicate that nanoparticles containing a combination of three orthogonal functional groups are more efficient in binding to dipeptides than mono or difunctionalised analogues.