Stackable molecular chairs

Molecular chairs, carrying three amino acids or peptides, stack in an antiparallel fashion to give hexavalent assemblies for bottom-up construction of novel soft materials and therapeutics.

The synthesis of new fluorescent bichromophoric compounds as ratiometric pH probes for intracellular measurements

Three different bichromophoric compounds (1–3) containing an aminomethyl anthracene moiety linked to a second chromophore have been prepared and their fluorescent properties studied.

Synthesis of a covalent monolayer sheet by photochemical anthracene dimerization at the air/water interface and its mechanical characterization by AFM indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Room Temperature Synthesis of a Covalent Monolayer Sheet at Air/Water Interface Using a Shape-Persistent Photoreactive Amphiphilic Monomer

The shape-persistent monomer 3 with its three 1,8-diazaanthracene (DAA) units is spread and compressed at the air/water interface and the layer then converted into a 1.5 nm thick covalent monolayer sheet by photoirradiation under ambient conditions. The sheet obtained under these extremely mild conditions is mechanically stable to carry its own weight when spanned over TEM grids. While its molecular structure cannot be given yet with certainty, it is likely to be the result of [4 + 4]-cycloaddition dimerizations between the DAA units of neighboring monomers. Evidence is based on the wavelength of the monomer fluorescence emission, the kinetics of this emission's intensity decay with irradiation time, and the mechanical sheet stability that suggests a surpassing of percolation threshold. Finally, the thermal stability of the sheet is investigated.

Aromatic interactions by molecular tweezers and clips in chemical and biological systems

Noncovalent interactions involving aromatic rings, such as π-stacking and CH-π, occur throughout a range of fundamental processes including self-assembly and (bio)catalysis. Molecular clips and tweezers possess a central parallel or torus-shaped cavity with a surrounding belt of convergent aromatic rings; hence these structures exploit multiple aromatic interactions in a positively cooperative manner. Both clips and tweezers demonstrate selective binding of cationic or neutral guests that bear acceptor groups. The electrostatic surface potentials (ESP) explain this unexpected behavior: calculated ESPs were highly negative inside the tweezer or clip cavity, providing complementary profiles to the positive ESP plots of their preferred guest molecules. This Account presents more complex systems that use aromatic clips and tweezers to alter the reactivities of included guest species, to distinguish between guest enantiomers, and to interfere with biological processes such as enzymatic activity and protein aggregation. Napthalene tweezers show potential applications in organocatalysis. When pyridinium moieties are bound within the spacious cavity of naphthyl-spaced tweezers, the resulting complex significantly influences the first step of single-electron reductions of (bi)pyridinium salts. In addition, the environment within the tweezer cavity strongly accelerates the Menshutkin reaction (the alkylation of pyridine derivatives). Introduction of phosphonate, phosphate, or sulfate anions into the central aromatic bridge renders clips and tweezers water-soluble. Larger systems form extremely tight intertwined dimers that rely on the nonclassical hydrophobic effect for their stability. Smaller clips and tweezers with a simple benzene bridge remain monomeric in buffered aqueous solution and display a complementary binding profile. While the clips with parallel sidewalls prefer flat aromatic cations such as pyridinium salts, the torus-shaped tweezers bind to basic amino acids lysine and arginine via a threading process. These mutually exclusive binding modes make water-soluble clips and tweezers valuable tools for probing critical biological interactions with positively charged amino acid side chains and cofactors. Molecular clips and tweezers can be employed for the complete inhibition of dehydrogenases. The clip extracts NAD(+) from its Rossman fold, while the tweezer complexes access strategic lysine residues around the active site. Our new enzyme inhibitors recognize the protein surface and thus offer additional targets for medicinal chemistry. Finally, the ability of molecular tweezers to cap critical lysine residues can be used to interfere with the pathology of protein misfolding diseases such as Alzheimer's disease, because many of them involve noncovalent interactions with these critical residues during their early stages. When the key protein produces a β-sheet-rich nucleus, this structure undergoes spontaneous polymerization into highly toxic oligomers, ultimately leading to mature fibrils. The benzene-spaced phosphate tweezer forms a specific complex with lysine residues 16 and 28 in Aβ42 and thus prevents the formation of misfolded oligomers rich in β-sheets. This entirely new process-specific mechanism that prevents pathologic protein aggregation also operates in many other related amyloidogenic proteins.

A molecular clip throws new light on the complexes formed by a family of cyclam-cored dendrimers with Zn(II) ions. Efficient energy transfer in the heteroleptic complexes

Complexation of Zn(II) ions by cyclam cored dendrimers appended with four (G0), eight (G1) and 16 naphthyl chromophores (G2) at the periphery have been investigated in CH₃CN-CH₂Cl₂ 1 : 1 (v/v) solution by absorption and emission, ESI-mass and ¹H NMR spectroscopy. The results obtained can be interpreted by the formation of complexes of 2 : 1 dendrimer to metal stoichiometry, at low metal ion concentration, and 1 : 1 complexes upon further addition of Zn(II) ions, for all the dendrimer generations. Upon addition of a molecular clip C²⁻ consisting of two anthracene sidewalls bridged by a benzene group with two sulfate substituents in the para positions, heteroleptic complexes of general formula [GnZnC] are formed. Interestingly, in these complexes, a very efficient quenching (practically 100%) of the dendrimer naphthyl luminescence and sensitization (ca. 90%) of the clip anthracene emission take place. The complex [G2ZnC] exhibits a very high molar absorption coefficient in the UV spectral region owing to the 16 naphthyl chromophores of the dendrimer and the two anthracene units of the clip (ε = 1.7 × 10⁵ M⁻¹ cm⁻¹ at 263 nm). Furthermore, the excitation energy absorbed by the naphthyl chromophores is efficiently funneled to the two anthracene units of the clip, which emits in the blue spectral region.

Supramolecular aggregation/deaggregation in amphiphilic dipolar Schiff-base zinc(II) complexes

The synthesis, characterization, optical, and fluorescent properties of an amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complex are reported. Detailed (1)H nuclear magnetic resonance (NMR), diffusion-ordered spectroscopy (DOSY) NMR, optical absorption, and fluorescence spectroscopy studies indicate the existence of aggregate species in noncoordinating solvents. The degree and type of aggregation are related to the concentration and the polarity of the noncoordinating solvent. Dilute solutions are likely characterized by the presence of defined dimers, whereas larger oligomeric aggregates are conceivably formed at higher concentrations. The concentration needed to observe the formation of larger species depends upon solvent polarity. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the system occurs, because of the axial coordination to the Zn(II) ion, accompanied by considerable changes of (1)H NMR and optical absorption spectra. A dramatic enhancement of fluorescence emission is observed in dichloromethane solutions upon deaggregation with a coordinating agent. The formation of a defined 2:1 supramolecular structure is demonstrated in the case of a ditopic ligand as coordinating species. Overall, these complexes are promising systems for the development of new supramolecular architectures and supramolecular fluorescent probes.