Leonard J. Prins

Complete asymmetric chirality in a hydrogen-bonded assembly

Chirality at the supramolecular level involves the non-symmetric arrangement of molecular components in a non-covalent assembly1,2. Supramolecular chirality is abundant in biology, for example in the DNA double helix3, the triple helix of collagen4 and the α-helical coiled coil of myosin5. These structures are stabilized by inter-strand hydrogen bonds, and their handedness is determined by the configuration of chiral centres in the nucleotide or peptide backbone. Synthetic hydrogen-bonded assemblies have been reported that display supramolecular chirality in solution6,7,8 or in the solid state9,10,11,12. Complete asymmetric induction of supramolecular chirality—the formation of assemblies of a single handedness—has been widely studied in polymeric superstructures13,14. It has so far been achieved in inorganic metal-coordinated systems15,16,17, but not in organic hydrogen-bonded assemblies18,19,20. Here we describe the diastereoselective assembly of enantio-pure calix[4]arene dimelamines and 5,5-diethylbarbituric acid (DEB) into chiral hydrogen-bonded structures of one handedness. The system displays complete enantioselective self-resolution: the mixing of homomeric assemblies (composed of homochiral units) with opposite handedness does not lead to the formation of heteromeric assemblies. The non-covalent character of the chiral assemblies, the structural simplicity of the constituent building blocks and the ability to control the assembly process by means of peripheral chiral centres makes this system promising for the development of a wide range of homochiral supramolecular materials or enantioselective catalysts.Chirality at the supramolecular level involves the non-symmetric arrangement of molecular components in a non-covalent assembly,. Supramolecular chirality is abundant in biology, for example in the DNA double helix, the triple helix of collagen and the α-helical coiled coil of myosin. These structures are stabilized by inter-strand hydrogen bonds, and their handedness is determined by the configuration of chiral centres in the nucleotide or peptide backbone. Synthetic hydrogen-bonded assemblies have been reported that display supramolecular chirality in solution or in the solid state. Complete asymmetric induction of supramolecular chirality—the formation of assemblies of a single handedness—has been widely studied in polymeric superstructures,. It has so far been achieved in inorganic metal-coordinated systems, but not in organic hydrogen-bonded assemblies. Here we describe the diastereoselective assembly of enantio-pure calix[4]arene dimelamines and 5,5-diethylbarbituric acid (DEB) into chiral hydrogen-bonded structures of one handedness. The system displays complete enantioselective self-resolution: the mixing of homomeric assemblies (composed of homochiral units) with opposite handedness does not lead to the formation of heteromeric assemblies. The non-covalent character of the chiral assemblies, the structural simplicity of the constituent building blocks and the ability to control the assembly process by means of peripheral chiral centres makes this system promising for the development of a wide range of homochiral supramolecular materials or enantioselective catalysts.

Nichtkovalente Synthese mit Wasserstoffbrücken

Wasserstoffbrucken ahneln Menschen insofern, als dass sie zur Gruppenbildung neigen. Einzeln sind sie schwach, leicht zu brechen und manchmal schwer zu finden. Handeln sie jedoch gemeinsam, so werden sie viel starker und unterstutzen einander. Bei diesem, unter dem Begriff Kooperativitat bekannten Phanomen ist 1+1 mehr als 2. Unter Nutzung dieses Prinzips haben Chemiker eine grose Vielzahl chemisch stabiler Strukturen entwickelt, deren Bildung auf der reversiblen Knupfung mehrerer Wasserstoffbrucken beruht. Mehr als 20 Jahre des Studiums dieser Phanomene haben zur Entwicklung einer neuen Disziplin innerhalb der organischen Synthese gefuhrt, die man heute als nichtkovalente Synthese bezeichnet. Dieser Aufsatz beschreibt die nichtkovalente Synthese auf der Basis der reversiblen Bildung mehrerer Wasserstoffbrucken. Er beginnt mit einer eingehenden Beschreibung des Wesens der Wasserstoffbrucke, fuhrt dann durch eine Vielzahl von bimolekularen Aggregaten und Assoziaten hoherer Ordnung und erlautert die allgemeinen Prinzipien, die deren Stabilitat zugrunde liegen. Besondere Aufmerksamkeit wird reversibel uber Wasserstoffbrucken gebildeten Kapseln gewidmet, die interessante Einschlussphanomene aufweisen. Weiterhin wird die Rolle von Wasserstoffbrucken in Selbstreplikationsprozessen diskutiert, und abschliesend werden neue Materialien (Nanorohren, Flussigkristalle, Polymere usw.) und Prinzipien (dynamische Bibliotheken) vorgestellt, die in jungster Zeit aus diesem faszinierenden Forschungsgebiet hervorgegangen sind.

An enantiomerically pure hydrogen-bonded assembly

Chiral molecules have asymmetric arrangements of atoms, forming structures that are non-superposable mirror images of each other. Specific mirror images (‘enantiomers’) may be obtained either from enantiomerically pure precursor compounds, through enantioselective synthesis, or by resolution of so-called racemic mixtures of opposite enantiomers, provided that racemization (the spontaneous interconversion of enantiomers) is sufficiently slow. Non-covalent assemblies can similarly adopt chiral supramolecular structures, and if they are held together by relatively strong interactions, such as metal coordination, methods analogous to those used to obtain chiral molecules yield enantiomerically pure non-covalent products. But the resolution of assemblies formed through weak interactions, such as hydrogen-bonding, remains challenging, reflecting their lower stability and significantly higher susceptibility to racemization. Here we report the design of supramolecular structures from achiral calix[4]arene dimelamines and cyanurates, which form multiple cooperative hydrogen bonds that together provide sufficient stability to allow the isolation of enantiomerically pure assemblies. Our design strategy is based on a non-covalent ‘chiral memory’ concept, whereby we first use chiral barbiturates to induce the supramolecular chirality in a hydrogen-bonded assembly, and then substitute them by achiral cyanurates. The stability of the resultant chiral assemblies in benzene, a non-polar solvent not competing for hydrogen bonds, is manifested by a half-life to racemization of more than four days at room temperature.

Self-assembly of a catalytic multivalent peptide-nanoparticle complex.

Catalytically active peptide-nanoparticle complexes were obtained by assembling small peptide sequences on the surface of cationic self-assembled monolayers on gold nanoparticles. When bound to the surface, the peptides accelerate the transesterification of the p-nitrophenyl ester of N-carboxybenzylphenylalanine by more than 2 orders of magnitude. The gold nanoparticle serves as a multivalent scaffold for bringing the catalyst and substrate into close proximity but also creates a local microenvironment that further enhances the catalysis. The supramolecular nature of the ensemble permits the catalytic activity of the system to be modulated in situ.

Dissipative self-assembly of vesicular nanoreactors

Dissipative self-assembly is exploited by nature to control important biological functions, such as cell division, motility and signal transduction. The ability to construct synthetic supramolecular assemblies that require the continuous consumption of energy to remain in the functional state is an essential premise for the design of synthetic systems with lifelike properties. Here, we show a new strategy for the dissipative self-assembly of functional supramolecular structures with high structural complexity. It relies on the transient stabilization of vesicles through noncovalent interactions between the surfactants and adenosine triphosphate (ATP), which acts as the chemical fuel. It is shown that the lifetime of the vesicles can be regulated by controlling the hydrolysis rate of ATP. The vesicles sustain a chemical reaction but only as long as chemical fuel is present to keep the system in the out-of-equilibrium state. The lifetime of the vesicles determines the amount of reaction product produced by the system.

Detection of Enzyme Activity through Catalytic Signal Amplification with Functionalized Gold Nanoparticles

The detection of low levels of proteins and other biomarkers is of crucial importance for the early diagnosis of diseases. The development of chemical-sensing methodologies as an alternative to biological assays is of strong current interest, because such methods involve simple detection protocols. In addition, such systems can be adapted through straightforward structural modifications for use with a wide variety of targets. Nevertheless, a common feature of these assays is that the amount of generated signal is proportional to the amount of substrate converted by the enzyme. The sensitivity of such assays would be significantly increased if the enzymatic conversion of a single substrate molecule led to the formation of a multitude of reporter molecules through a cascade of chemical events, each of which magnified the previous event. Examples of chemical systems able to amplify originally weak input signals have been reported. Herein, we report the application of a catalytic amplification process for the detection of proteases. The strategy relies on a cascade of two catalytic events for signal generation, whereby a gold nanoparticle covered with a catalytic self-assembled organic monolayer (Au-MPC) has a crucial central role. In the first event, an enzyme hydrolyzes a peptide substrate, which acts as an inhibitor for the catalytic monolayer (Figure 1). Upon hydrolysis, the catalytic activity of the monolayer is restored, and large quantities of a yellow reporter molecule are produced. The Au nanoparticles are important for two reasons. First, they enable the facile, spontaneous formation of dinuclear catalytic sites on the periphery of the monolayer. Second, their multivalent nature permits the occurrence of multipoint interactions with (biological) targets. The latter aspect, together with the intrinsic physical and chemical properties of the nanoparticles and the ease of their preparation and functionalization, has led to extensive development of assays based on Au nanoparticles that also occasionally rely on various forms of signal amplification. Previously, we showed that Au-MPC 1 catalyzes the transphosphorylation of 2-hydroxypropyl-4-nitrophenyl phosphate (HPNPP) highly efficiently. HPNPP is an activated RNA model substrate. Detailed kinetic studies revealed that catalysis results from the cooperative action of two triazacyclononane·Zn (TACN·Zn) complexes localized on the surface of the monolayer. 22] Au-MPC 1, which is fully covered with the TACN·Zn complex, displays enzymelike saturation behavior with the “overall” values kcat = 6.7 10 3 s 1 and KM = 0.31 mm at pH 7.5 in H2O. [23] This system is intriguing for the following reasons: a) under the experimental conditions, there is practically no background reaction, since kuncat under the same conditions is of the order of 10 7 s ; b) the reaction can be monitored visibly by measuring the absorbance of the p-nitrophenol product at 400 nm; c) a surprisingly high affinity is observed for the binding of HPNPP to 1. Since Au-MPC 1 has a multitude of positively charged TACN·Zn complexes on its surface, we anticipated that the system would have a high affinity for oligoanions owing to multivalent interactions. This hypothesis was also supported by the contributions by Hamachi and co-workers, who demonstrated that oligophosphates and oligoaspartates bind a bis(zinc(II) dipicolylamine) complex with very high affinity. In our system, such oligoanions would act as competitive inhibitors for HPNPP and thus turn off the catalytic activity of the system. To verify whether we could use the catalytic production of p-nitrophenol as a tool to detect binding events on the AuMPC surface, we studied two series of biologically important oligoanions (peptides and phosphates) with negative charges increasing from 1 to 4. The peptide series comprised BocNHGly-OH (1 ), AcNH-Asp-OH (2 ), AcNH-Asp-Asp-OH (3 ), and AcNH-Asp-Asp-Asp-OH (4 ), and the phosphate series cAMP (1 ), AMP (2 ), ADP (3 ) and ATP (4 ; Figure 1). Increasing amounts of each compound were added to a solution of Au-MPC 1 in H2O buffered at pH 7.0 at 40 8C with the concentration of TACN·Zn headgroups equal to 5 mm. This value implies a Au-MPC 1 concentration of around 100 nm, on the basis of the knowledge that a 1.6 nm sized nanoparticle contains roughly 50 thiols. A kinetic Zn titration confirmed that at these concentrations, the Zn ions are quantitatively bound to the TACN ligand (see the Supporting Information). Subsequently, HPNPP was added to give an initial concentration of 1 mm in the mixture, and the initial rate of cleavage, ninit, was measured for 30 min by monitoring the increase in absorbance at 400 nm. A plot of the initial rate (n1), normalized with respect to the initial rate in the absence of an inhibitor (n0), as a function of the concentration of added inhibitor, gave the inhibition curves depicted in Figure 2 for the peptides (the data for the [*] Dr. R. Bonomi, A. Cazzolaro, Dr. A. Sansone, Prof. Dr. P. Scrimin, Dr. L. J. Prins Department of Chemical Sciences University of Padova Via Marzolo 1, 35131 Padova (Italy) Fax: (+ 39)049-827-5239 E-mail: paolo.scrimin@unipd.it leonard.prins@unipd.it Homepage: http://www.chimica.unipd.it/leonard.prins/pubblica/

Catalytic self-assembled monolayers on Au nanoparticles: the source of catalysis of a transphosphorylation reaction.

The catalytic activity of a series of Au monolayer protected colloids (Au MPCs) containing different ratios of the catalytic unit triazacyclononane⋅Zn(II) (TACN⋅Zn(II) ) and an inert triethyleneglycol (TEG) unit was measured. The catalytic self-assembled monolayers (SAMs) are highly efficient in the transphosphorylation of 2-hydroxy propyl 4-nitrophenyl phosphate (HPNPP), an RNA model substrate, exhibiting maximum values for the Michaelis-Menten parameters k(cat) and K(M) of 6.7×10(-3) s(-1) and 3.1×10(-4) M, respectively, normalized per catalytic unit. Despite the structural simplicity of the catalytic units, this renders these nanoparticles among the most active catalysts known for this substrate. Both k(cat) and K(M) parameters were determined as a function of the mole fraction of catalytic unit (x(1)) in the SAM. Within this nanoparticle (NP) series, k(cat) increases up till x(1) ≈0.4, after which it remains constant and K(M) decreases exponentially over the range studied. A theoretical analysis demonstrated that these trends are an intrinsic property of catalytic SAMs, in which catalysis originates from the cooperative effect between two neighboring catalytic units. The multivalency of the system causes an increase of the number of potential dimeric catalytic sites composed of two catalytic units as a function of the x(1) , which causes an apparent increase in binding affinity (decrease in K(M)). Simultaneously, the k(cat) value is determined by the number of substrate molecules bound at saturation. For values of x(1) >0.4, isolated catalytic units are no longer present and all catalytic units are involved in catalysis at saturation. Importantly, the observed trends are indicative of a random distribution of the thiols in the SAM. As indicated by the theoretical analysis, and confirmed by a control experiment, in case of clustering both k(cat) and K(M) values remain constant over the entire range of x(1) .

Exploiting Neighboring‐Group Interactions for the Self‐Selection of a Catalytic Unit

Dynamic combinatorial chemistry (DCC) is based on the principle that the thermodynamic composition of a dynamic library of molecules, that is, a library of which the components are held together either by noncovalent bonds or reversible covalent bonds, spontaneously changes upon the input of an external stimulus. This can be either the addition of a target molecule, but also an alteration of the environment (pH, light, etc.). Ideally, the composition of the library changes in favor of the component that is the most stable under the changed conditions. In the past decade, DCC has emerged as a powerful tool for the discovery of, sometimes very surprising, molecular receptors and novel materials. In principle, DCC could be applied to the selection of a catalyst by shifting the equilibrium of the library with amplification of a molecular receptor for a transition state of a given reaction. By decreasing the energy of the transition state by formation of a complex with this molecular receptor (that is, a catalyst), the reaction rate is obviously accelerated. This concept was first developed by Pauling, and applied to catalyst discovery with catalytic antibodies and imprinted polymers. As a transition state is an elusive species, a stable analogue is required possessing similar features in terms of shape and charge distribution. However, despite the success of DCC, its use for catalyst discovery is significantly lagging, as evidenced by a very limited number of publications and, generally, very modest rate accelerations. This fact suggests that the endeavor is very challenging. In analogy with enzyme catalysis, an ideal catalyst should first bind to the substrate and subsequently transform it to product. Accordingly, the catalyst should both recognize the substrate and the transition state, although the thermodynamic stabilization of the latter must be much higher. It is not surprising that in enzymes the substrate and transition state recognition loci are quite often different because of the different tasks they have to accomplish. Herein we present the dynamic self-selection of a functional group which induces a 60-fold rate enhancement in the basic hydrolysis of a neighboring carboxylic ester; that is, the selection of a catalytic unit on the way to the selection of a fully-fledged catalyst. Recently, the “tethering” strategy has emerged as a powerful tool for the detection of weak, noncovalent interactions between substrates and a target. This approach implies that the target molecule is covalently linked to a scaffold molecule which has the additional ability to interact in a reversible manner with library members (Scheme 1). In this way, the recognition event between target and library component becomes intramolecular, which, for entropic reasons, significantly enhances the sensitivity of the screening process. In fact, Houk has recently pointed out that among the most efficient enzymes are those that covalently bind the substrate before its transformation into products. During studies on hydrazone-based libraries, we recently observed that the presence of a phosphonate group in 1 resulted in the preferential incorporation of hydrazide B with respect to A, owing to an intramolecular, electrostatic interaction between the oppositely charged phosphonate and ammonium groups. The phosphonate group was chosen as a target because it is a model for the transition state of a carboxylic ester hydrolysis. Following the above concept that stabilization of the transition state should lead to an increased rate of hydrolysis, we argued that the phosphonate group in 1 could be used to self-select hydrazides that would enhance the cleavage rates of the corresponding carboxylic ester. Thus, we have screened a library of nine hydrazides, and present herein compelling data showing the existence of a correlation between thermodynamic amplification in the dynamic screening and the efficiency in assisting in intramolecular catalysis. The nine components of the library were chosen from commercially available hydrazides, of which eight could potentially interact with a phosphonate moiety, either by electrostatic interactions (B, C) or the formation of one or more hydrogen-bonds (D–I) (Scheme 1). Hydrazide A was not expected to interact with the target and was used as an internal standard. We also screened aldehyde 2, which contains a neutral methoxy group: the resulting library served as a neutral reference to determine the intrinsic stabilities of the hydrazones in the absence of the target. Any shift in the library composition using scaffold 1with respect to that obtained using scaffold 2 can then be ascribed to an intramolecular stabilization between the hydrazide and the phosphonate target. Library equilibration studies were performed by adding either scaffold 1 or 2 (5 mm) to a mixture of hydrazides A–I (each 1.5 equivalent) in CD3OD. The mixtures were kept at 50 8C until the thermodynamic equilibrium was reached, which was detected by the absence of any further change in [*] G. Gasparini, Dr. L. J. Prins, Prof. Dr. P. Scrimin Department of Chemical Sciences, University of Padova and CNR ITM, Padova Section Via Marzolo 1, 35131 Padova (Italy) Fax: (+39)049-827-5239 E-mail: leonard.prins@unipd.it paolo.scrimin@unipd.it

Multivalent Interactions Regulate Signal Transduction in a Self-Assembled Hg2+ Sensor

A self-assembled sensing system able to detect Hg(2+) at low nanomolar concentrations is reported that operates through a signal transduction pathway involving multivalent interactions. The analyte causes dimerization of low-affinity ligands, resulting in a complex with a high affinity for a multivalent monolayer-protected gold nanoparticle (AuNP). This complex displaces a quenched fluorescent reporter from the AuNP, resulting in a turn ON of fluorescence. It is shown that the strength of the output signal can be regulated by tuning the multivalent interactions between the complex and the NP. Finally, it is shown that multivalent interactions drive the self-selection of a high-affinity complex from a mixture of low-affinity ligands.

Ti(IV)-amino triphenolate complexes as effective catalysts for sulfoxidation

C(3)-symmetric Ti(IV) amino triphenolate complexes efficiently catalyze, without previous activation and in excellent yields, the oxidation of sulfides at room temperature, using both CHP and the more environment friendly aqueous hydrogen peroxide as terminal oxidants, with catalyst loadings down to 0.01%. The Ti(IV) catalysts and the intermediate Ti(IV)-peroxo complexes have been characterized in solution by (1)H NMR and ESI-MS techniques and via density functional studies.