Helicity control of a perfluorinated carbon chain within a chiral supramolecular cage monitored by VCD.

Confinement within supramolecular systems is the leading technology to finely tune guest functional properties. In this communication we report the synthesis of a chiral supramolecular cage able to bias the helicity of a perfluorinated carbon chain hosted within the cage. We monitor the phenomenon of chiral induction by Vibrational Circular Dichroism (VCD) experiments complemented by DFT calculations over the possible conformers.

Artificial transmembrane signal transduction mediated by dynamic covalent chemistry.

Reversible formation of covalent adducts between a thiol and a membrane-anchored Michael acceptor has been used to control the activation of a caged enzyme encapsulated inside vesicles. A peptide substrate and papain, caged as the mixed disulfide with methane thiol, were encapsulated inside vesicles, which contained Michael acceptors embedded in the lipid bilayer. In the absence of the Michael acceptor, addition of thiols to the external aqueous solution did not activate the enzyme to any significant extent. In the presence of the Michael acceptor, addition of benzyl thiol led to uncaging of the enzyme and hydrolysis of the peptide substrate to generate a fluorescence output signal. A charged thiol used as the input signal did not activate the enzyme. A Michael acceptor with a polar head group that cannot cross the lipid bilayer was just as effective at delivering benzyl thiol to the inner compartment of the vesicles as a non-polar Michael acceptor that can diffuse across the bilayer. The concentration dependence of the output signal suggests that the mechanism of signal transduction is based on increasing the local concentration of thiol present in the vesicles by the formation of Michael adducts. An interesting feature of this system is that enzyme activation is transient, which means that sequential addition of aliquots of thiol can be used to repeatedly generate an output signal.

Dissection of the Polar and Non-Polar Contributions to Aromatic Stacking Interactions in Solution.

Aromatic stacking interactions have been a matter of study and debate due to their crucial role in chemical and biological systems. The strong dependence on orientation and solvent together with the relatively small interaction energies have made evaluation and rationalization a challenge for experimental and theoretical chemists. We have used a supramolecular cage formed by two tris(pyridylmethyl)amines units to build chemical Double Mutant Cycles (DMC) for the experimental measurement of the free energies of π-stacking interactions. Extrapolating the substituent effects to remove the contribution due to electrostatic interactions reveals that there is a substantial contribution to the measured stacking interaction energies which is due to non-polar interactions (-3 to -6 kJ mol-1 ). The perfectly flat nature of the surface of an aromatic ring gives π-stacking an inherent advantage over non-polar interactions with alkyl groups and accounts for the wide-spread prevalence of stacking interactions in Nature.

Wide range detection of C-Reactive protein with a homogeneous immunofluorimetric assay based on cooperative fluorescence quenching assisted by gold nanoparticles.

Homogeneous sandwich immunofluorimetric assays are valued for the rapid, low-cost and accurate detection of analytes in liquid phase. However, their exploitation with analytes covering a wide range of concentrations is limited by low sensitivity and the hook effect. Here, we describe a homogeneous immunofluorimetric system based on the quenching of fluorescence in a Förster resonance energy transfer (FRET) donor/acceptor couple of antibody functionalized with two different dyes, respectively fluorescein (donor) and eosin (acceptor), which form a sandwich multi-component assembly with antibody-functionalized gold nanoparticles (GNPs) in the presence of the analyte. The resulting cooperative fluorescence quenching is assisted by the GNPs scaffold through the nanomaterial-surface energy transfer (NSET) effect, which gives an extended linear response versus the antigen concentration that is not possible with the bi-component assays. This immunofluorimetric method allows accurate, reproducible and immediate detection of C-reactive protein (CRP) in the wide concentration range of clinical interest (over two orders of magnitude from 3.5 to 455 nM, 0.4-52 mg/L), without the hook effect. Moreover, the method does not require sample treatment or washing steps. The concept of this multi-component FRET/NSET fluorescence quenching system can be extended to any analyte amenable to the detection with homogeneous sandwich assays.

Hetero-Coencapsulation within a Supramolecular Cage: Moving away from the Statistical Distribution of Different Guests.

Beside sensing and delivery, another peculiar property arising from confinement in discrete molecular hosts comes from the possibility to have in close proximity, and in defined position, two different molecules (hetero-coencapsulation). This phenomenon can be tuned considering steric and electronic properties of the guests. In this work, a study on the parameters affecting homo- and hetero-coencapsulation processes within a supramolecular cage is reported. In particular, different benzoate guests were bound within a supramolecular cage containing two metal-binding sites and the experimental binding thermodynamics measured. Unexpectedly, from competition experiments it was observed that the maximum concentration of hetero-coencapsulation is achieved if a weakly binding guest is used to partially displace a strongly binding guest.

Template effects of vesicles in dynamic covalent chemistry.

Vesicle lipid bilayers have been employed as templates to modulate the product distribution in a dynamic covalent library of Michael adducts formed by mixing a Michael acceptor with thiols. In methanol solution, all possible Michael adducts were obtained in similar amounts. Addition of vesicles to the dynamic covalent library led to the formation of a single major product. The equilibrium constants for formation of the Michael adducts are similar for all of the thiols used in this experiment, and the effect of the vesicles on the composition of the library is attributed to the differential partitioning of the library members between the lipid bilayer and the aqueous solution. The results provide a quantitative approach for exploiting dynamic covalent chemistry within lipid bilayers.

A Diastereodynamic Probe Transducing Molecular Length into Chiroptical Readout.

Nature takes advantage of molecular conformational changes to express functions such as signaling across cellular membranes or allosteric protein activation. At the synthetic level, molecular recognition events have been used to induce conformational changes able to trigger functions such as catalysis or sensing. In this context, transduction of stereochemical information has been the leading strategy. In particular, stereodynamic elements have been extensively employed to amplify and/or transduce chiral information. In this article, we report a chiral supramolecular cage with two stereodynamic units, which invert their helicities according to the length of the molecular guest confined within the system. Interestingly, achiral information is transduced by the supramolecular system to different diastereomeric states that have opposite chiroptical absorptions. This is the first example in which it is possible to produce a continuous modulation of the chiroptical output of a system by varying a physical achiral molecular property (viz. molecular dimension). This phenomenon can be exploited for the establishment of novel methods to program conformational control, for the development of innovative sensors and/or for transduction of molecular properties into chiroptical information.

Supramolecular cages as differential sensors for dicarboxylate anions: guest length sensing using principal component analysis of ESI-MS and 1H-NMR raw data.

Dynamic covalent libraries (DCLs) have been widely used in the development of differential sensors. Inspired by recent studies which use supramolecular recognition systems for sensing, we report the use of a tris(-pyridylmethyl)amine (TPMA)-based supramolecular cage as a differential sensor for dicarboxylate anions. In particular, a library of molecular cages constituted by linkers differing in size and flexibility was allowed to equilibrate toward a series of guests differing in molecular size. The differential system was able to discriminate a series of dicarboxylates depending on their chain length. This differentiation was evaluated through the application of the Principal Component Analysis (PCA) method using interpolated and raw data obtained from ESI-MS. Interestingly, while the 1H NMR spectra of the differential system did not allow for the discrimination of the analytes by the naked eye, PCA of the raw data from the spectra revealed information on the chain length of the guest and also on the odd-even character of the carbon chain.

Supramolecular cage encapsulation as a versatile tool for the experimental quantification of aromatic stacking interactions.

The widespread presence of aromatic stacking interactions in chemical and biological systems, combined with their relatively small energetic contribution, have led to a plethora of theoretical and experimental studies for their quantification and rationalization. Typically, π-π aromatic interactions are studied as a function of substituents to gather information about the interaction mechanism. While experiments suggest that aromatic interactions are dominated by local electrostatic contacts between π-electron density and CH groups, theoretical work has raised the possibility that direct electrostatic interactions between local dipoles of the substituents may play a role. We describe a supramolecular cage that binds two aromatic carboxylates in a stacked geometry such that the aromatic substituents are remote in space. Chemical Double Mutant Cycles (DMCs) were used to measure fifteen different aromatic stacking interactions as a function of substituent (NMe2, OMe, Me, Cl and NO2). When both aromatic rings have electron-withdrawing nitro substituents, the interaction is attractive (-2.8 kJ mol-1) due to reduced π-electron repulsion. When both aromatic rings have electron-donating di-methylamino substituents, the interaction is repulsive (+2.0 kJ mol-1) due to increased π-electron repulsion. The results show that aromatic stacking interactions are dominated by short range electrostatic contacts rather than substituent dipole interactions.

Diasteroselective multi-component assemblies from dynamic covalent imine condensation and metal-coordination chemistry: mechanism and narcissistic stereochemistry self-sorting.

Self-assembly of a modified tris(2-pyridylmethyl)amine TPMA ligand, zinc(ii) or cobalt(ii) ions, and amino acids have been used effectively as stereo dynamic optical probes for the determination of the enantiomeric excess of free amino acids either using Electronic or Vibrational Circular Dichroism (CD and VCD). Herein, we report the mechanistic and stereochemical study of the self-assembly process which reveals a complex equilibrium in solution where even small variations in the experimental conditions can profoundly affect the final products of the reaction. In particular, variation on the metal stoichiometry switch give rises to an entirely enantio narcissistic self-assembly of the structure.

Binding Profiles of Self-Assembled Supramolecular Cages from ESI-MS Based Methodology.

Confined molecular environments have peculiar characteristics that make their properties unique in the field of biological and chemical sciences. In recent years, advances in supramolecular capsule and cage synthesis have presented the possibility to interpret the principles behind their self-assembly and functions, which has led to new molecular systems that display outstanding properties in molecular recognition and catalysis. Herein, we report a rapid method based on ESI-MS to determine the binding profiles for linear saturated dicarboxylic acids in a series of different cages. The cages were obtained by self-assembly of modified tris(pyridylmethyl)amine (TPMA) complexes and diamines chosen to explore variations in their size and flexibility. This methodology has provided information on how small changes in the structures of the host and guest can contribute to recognition events. Moreover, it was possible to study molecular systems that contain paramagnetic metals, which are not suitable for classical binding-constant determination by 1 H NMR spectroscopy.

Triggering Assembly and Disassembly of a Supramolecular Cage.

A novel supramolecular cage built from the self-assembly of tris(2-pyridylmethyl)amine zinc complexes through imine condensation chemistry is reported. The cage recognition properties over a variety of structurally related guests, together with the kinetic study of the template assembly and disassembly, have been investigated in detail. This knowledge has been used to selectively modulate the rate of both assembly and disassembly processes. In particular, a novel disassembly method induced by strain release of the guest has been developed.