Glyoxalase-based toolbox for the enantioselective synthesis of α-hydroxy carboxylic acids.

We report highly enantioselective synthesis of L-α-hydroxy carboxylic acids (L-αHCAs) via enzymatic intramolecular Cannizzaro reaction of (hetero)aryl glyoxals in the presence of glutathione-independent human glyoxalase DJ-1. Combined with the optimized synthesis of D-αHCAs using glyoxalases I and II, this approach offers a general, scalable and operationally simple access to both enantiomers of α-hydroxy acids in moderate to excellent yields with uniformly high enantioselectivity.

Unsymmetric Chiral Ligands for Large Metallo-Macrocycles: Selectivity of Orientational Self-Sorting.

Nature uses various chiral and unsymmetric building blocks to form substantial and complex supramolecular assemblies. In contrary, majority of organic ligands used in metallosupramolecular chemistry are symmetric and achiral. Here we extend on the group of unsymmetric chiral bile acids used as a scaffold for organic bispyridyl ligands employing the chenodeoxycholic acid (CDCA), epimer of previously used ursodeoxycholic acid (UDCA). Ligands' epimerism, flexibility, and bulkiness leads to large structural differences of coordination products upon reaction with Pd(NO3)2. The UDCA-bispyridyl ligand self-assembles quantitatively into a single crown-like Pd3L6 complex, whereas the CDCA-ligand provides a mixture of coordination complexes of general formula PdnL2n, i.e., Pd2L4, Pd3L6, Pd4L8, Pd5L10, and even Pd6L12 containing impressive 120 chiral centers. The coordination products were studied by a combination of analytical methods, where the ion mobility-mass spectrometry (IM-MS) provided valuable details on their structure and allowed an effective separation of m/z 1461 to individual signals according to arrival time distribution, revealing four different ions of [Pd3L6(NO3)3]3+, [Pd4L8(NO3)4]4+, [Pd5L10(NO3)5]5+, and [Pd6L12(NO3)6]6+. The structures of all complexes were modelled using DFT calculations. Finally, challenges and conclusions in determination of specific structural identity of these unsymmetric species are discussed.

Anion binding and transport with meso-alkyl substituted two-armed calix[4]pyrroles bearing urea and hydroxyl groups.

Calix[4]pyrroles bearing hydroxyl (1) or urea (3) groups attached to the meso-positions with propyl linkers were synthesized as cis- and trans-isomers. The anion binding properties of cis-1 and cis-3 were screened with ion-mobility mass spectrometry, where cis-1 formed complexes with Cl-, Br- and H2PO4-, whereas cis-3 formed complexes with most of the investigated anions, including Cl-, Br-, I-, NO3-, ClO4-, OTf-, SCN- and PF6-. The structures of the chloride complexes were further elucidated with density functional theory calculations and a crystal structure obtained for cis-1. In solution, chloride and dihydrogenphosphate anion binding with cis-1 and cis-3 were compared using 1H NMR titrations. To assess the suitability of two-armed calix[4]pyrroles as anion transporters, chloride transport studies of cis-1, cis-3 and trans-3 were performed using large unilamellar vesicles. The results revealed that cis-3 had the highest activity among the investigated calix[4]pyrroles, which was related to the improved affinity and isolation of chloride inside the binding cavity of cis-3 in comparison to cis-1. The results indicate that appending calix[4]pyrroles with two hydrogen bonding arms is a feasible strategy to obtain anion transporters and receptors with high anion affinity.

Thermodynamically driven self-assembly of pyridinearene to hexameric capsules.

Pyridinearene macrocycles have previously shown unique host-guest properties in their capsular dimers including endo complexation of neutral molecules and exo complexation of anions. Here, we demonstrate for the first time the formation of hydrogen bonded hexamer of tetraisobutyl-octahydroxypyridinearene in all three states of matter - gas phase, solution and solid-state. Cationic tris(bipyridine)ruthenium(ii) template was found to stabilize the hexamer in gas phase, whereas solvent molecules do this in condensed phases. In solution, the capsular hexamer was found to be the thermodynamically favoured self-assembly product and transition from dimer to hexamer occurred in course of time. The crystal structure of hexamer revealed 24 N-HO direct intermolecular hydrogen bonds between the six pyridinearene macrocycles without any bridging solvent molecules. Hydrogen bond patterns correlate well with DFT computed structures. Thus, all structural chemistry methods (IM-MS, DOSY NMR, DFT, X-ray crystallography) support the same structure of the hexameric capsule that has a diameter of ca. 3 nm and volume of 1160 Å3.

Anion-driven encapsulation of cationic guests inside pyridine[4]arene dimers.

Pyridine[4]arenes have previously been considered as anion binding hosts due to the electron-poor nature of the pyridine ring. Herein, we demonstrate the encapsulation of Me4N+ cations inside a dimeric hydrogen-bonded pyridine[4]arene capsule, which contradicts with earlier assumptions. The complexation of a cationic guest inside the pyridine[4]arene dimer has been detected and studied by multiple gas-phase techniques, ESI-QTOF-MS, IRMPD, and DT-IMMS experiments, as well as DFT calculations. The comparison of classical resorcinarenes with pyridinearenes by MS and NMR experiments reveals clear differences in their host-guest chemistry and implies that cation encapsulation in pyridine[4]arene is an anion-driven process.

Simultaneous endo and exo†Complex Formation of Pyridine[4]arene Dimers with Neutral and Anionic Guests.

The formation of complexes between hexafluorophosphate (PF6- ) and tetraisobutyloctahydroxypyridine[4]arene has been thoroughly studied in the gas phase (ESI-QTOF-MS, IM-MS, DFT calculations), in the solid state (X-ray crystallography), and in chloroform solution (1 H, 19 F, and DOSY NMR spectroscopy). In all states of matter, simultaneous endo complexation of solvent molecules and exo complexation of a PF6- anion within a pyridine[4]arene dimer was observed. While similar ternary complexes are often observed in the solid state, this is a unique example of such behavior in the gas phase.

Ion-Pair Complexation with Dibenzo[21]Crown-7 and Dibenzo[24]Crown-8 bis-Urea Receptors.

Synthesis and ion-pair complexation properties of novel ditopic bis-urea receptors based on dibenzo[21]crown-7 (R(1) ) and dibenzo[24]crown-8 (R(2) ) scaffolds have been studied in the solid state, solution, and gas phase. In a 4:1 CDCl3 /[D6 ]DMSO solution, both receptors clearly show positive heterotropic cooperativity toward halide anions when complexed with Rb(+) or Cs(+) , with the halide affinity increasing in order I(-)

Sulfate-induced large amplitude conformational change in a Solomon link.

A doubly-interlocked [2]catenane - or Solomon link - undergoes a complex conformational change upon addition of sulfate in methanol. This transformation generates a single pocket where two SO42- anions bind through multiple hydrogen bonds and electrostatic interactions. Despite the close proximity of the two anions, binding is highly cooperative.

Selective gas adsorption by calixarene-based porous octahedral M32 coordination cages.

Giant octahedral M32 coordination cages were prepared via self-assembly of sulfonylcalix[4]arene-supported tetranuclear M(II) clusters (M = Co, Ni) with hybrid linker based on tris(dipyrrinato)cobalt(III) complexes appended with peripherical carboxylic groups. Due to intrinsic and extrinsic porosity, the obtained solid-state supramolecular architectures demonstrated good performance as adsorbents for the separation of industrially important gases mixtures.

Chiral hemicucurbit[8]uril as an anion receptor: selectivity to size, shape and charge distribution.

A novel eight-membered macrocycle of the hemicucurbit[n]uril family, chiral (all-R)-cyclohexanohemicucurbit[8]uril (cycHC[8]) ‡The name cyclohexylhemicucurbituril, previously used for these macrocycles, is changed in accordance with the IUPAC nomenclature for fused cycles, as the cyclohexane substituents are fused with the parent hemicucurbituril. binds anions in a purely protic solvent with remarkable selectivity. The cycHC[8] portals open and close to fully encapsulate anions in a 1 : 1 ratio, resembling a molecular Pac-Man™. Comprehensive gas, solution and solid phase studies prove that the binding is governed by the size, shape and charge distribution of the bound anion. Gas phase studies show an order of SbF6- ≈ PF6- > ReO4- > ClO4- > SCN- > BF4- > HSO4- > CF3SO3- for anion complexation strength. An extensive crystallographic study reveals the preferred orientations of the anions within the octahedral cavity of cycHC[8] and highlights the importance of the size- and shape-matching between the anion and the receptor cavity. The solution studies show the strongest binding of the ideally fitting SbF6- anion, with an association constant of 2.5 × 105 M-1 in pure methanol. The symmetric, receptor cavity-matching charge distribution of the anions results in drastically stronger binding than in the case of anions with asymmetric charge distribution. Isothermal titration calorimetry (ITC) reveals the complexation to be exothermic and enthalpy-driven. The DFT calculations and VT-NMR studies confirmed that the complexation proceeds through a pre-complex formation while the exchange of methanol solvent with the anion is the rate-limiting step. The octameric cycHC[8] offers a unique example of template-controlled design of an electroneutral host for binding large anions in a competitive polar solvent.

Structural studies of five novel bile acid-4-aminopyridine conjugates.

Synthesis and solid-state structural characterization of five bile acid amides of 4-aminopyridine (4-AP) are reported. Systematic crystallization experiments revealed a number of structural modifications and/or solvate/hydrate systems for these conjugates. Particularly, cholic acid conjugate exhibited five distinct structure modifications, including one anhydrous form, mono- and dihydrates, as well as ethanol and 2-butanol solvates. The obtained crystal forms were examined extensively with various analytical methods, including solid-state NMR, Raman, and IR spectroscopies, powder and single crystal X-ray diffraction methods, thermogravimetry, and differential scanning calorimetry. After releasing their crystal solvent molecules, the resulted non-solvated structure forms showed 50-75°C higher melting points than corresponding bile acids, and thermal degradation occurred for all conjugates at about 300-330°C. Moreover, the single crystal X-ray structure of the ursodeoxycholic acid-4-aminopyridine conjugate is reported.