Saccharide Recognition and Helix Formation in Water with an Amphiphilic Pyridine-Phenol Alternating Oligomer

Cycloalkyl Groups as Building Blocks of Artificial Carbohydrate Receptors: Studies with Macrocycles Bearing Flexible Side-Arms

The cyclopentyl group was expected to act as a building block for artificial carbohydrate receptors and to participate in van der Waals contacts with the carbohydrate substrate in a similar way as observed for the pyrrolidine ring of proline in the crystal structures of protein-carbohydrate complexes. Systematic binding studies with a series of 1,3,5-trisubstituted 2,4,6-triethylbenzenes bearing various cycloalkyl groups as recognition units provided indications of the involvement of these groups in the complexation process and showed the influence of the ring size on the receptor efficiency. Representatives of compounds that exhibit a macrocyclic backbone and flexible side arms were now chosen as further model systems to investigate whether the previously observed effects represent a general trend. Binding studies with these macrocycles towards β-D-glucopyranoside, an all-equatorial substituted carbohydrate substrate, included 1H NMR spectroscopic titrations and microcalorimetric investigations. The performed studies confirmed the previously observed tendency and showed that the compound bearing cyclohexyl groups displays the best binding properties.

PCage: Fluorescent Molecular Temples for Binding Sugars in Water

The development of synthetic receptors that recognize carbohydrates in water with high selectivity and specificity is challenging on account of their structural complexity and strong hydrophilicity. Here, we report on the design and synthesis of two pyrene-based, temple-shaped receptors for the recognition of a range of common sugars in water. These receptors rely on the use of two parallel pyrene panels, which serve as roofs and floors, capable of forming multiple [C-H···π] interactions with the axially oriented C-H bonds on glycopyranosyl rings in the carbohydrate-based substrates. In addition, eight polarized pyridinium C-H bonds, projecting from the roofs and floors of the temple receptors toward the binding cavities, form [C-H···O] hydrogen bonds, with the equatorially oriented OH groups on the sugars located inside the hydrophobic cavities. Four para-xylylene pillars play a crucial role in controlling the distance between the roof and floor. These temple receptors are highly selective for the binding of glucose and its derivatives. Furthermore, they show enhanced fluorescence upon binding with glucose in water, a property which is useful for glucose-sensing in aqueous solution.

Arylethynyl Helices Supported by π-Stacking and Halogen Bonding

Co-crystallization of a pyridyl-containing arylethynyl (AE) moiety with 1,4-diiodotetrafluorobenzene leads to unique, figure-eight shaped helical motifs within the crystal lattice. A slight twist in the AE backbone allows each AE unit to simultaneously interact with haloarene units that are stacked on top of one another. Left-handed (M) and right-handed (P) helices are interspersed in a regular pattern throughout the crystal. The major driving forces for assembly are 1) halogen bonding between the pyridyl nitrogen atoms and the iodine substituents of the haloarene, with N⋅⋅⋅I distances between 2.81 and 2.84 Å, and 2) π-π stacking of the haloarenes, with distances of approximately 3.57 Å between centroids. Halogen bonding and π-π stacking not only work in concert, but also seem to mutually enhance one another. Calculations suggest that the presence of π-π stacking modestly intensifies the halogen bonding interaction by <0.2 kcal/mol; likewise, halogen bonding to the haloarene enhances the π-π stacking interaction by 0.59 kcal/mol.

Unprotected Galactosamine as a Dynamic Key for a Cyclochiral Lock

The discrimination of d-galactosamine (G), representative of the amino-sugar class of compounds, has been probed through nano-ESI-FT-ICR mass spectrometry by isolating the relevant [C·H·G]+ proton-bound complexes with the enantiomers of the cyclochiral resorcin[4]arene C and allowing them to react toward three primary amines (B = EtNH2, iPrNH2, and (R)- and (S)-sBuNH2). The system under investigation presents several features that help to unveil the behavior of unprotected G in such a supramolecular architecture: (i) the hydrophobic derivatization of the C convex side forces the polar guest G to be coordinated by the cyclochiral concave region; (ii) protonated d-galactosamine exists as an anomeric mixture, dynamically interconverting throughout the experimental time-window; and (iii) different basicities of B allow the experiment to subtly tune the reactivity of the [C·H·G]+ complexes. Three [C·H·G]+ aggregate-types were found to exist, differing in both their origin and reactivity. The most reactive adducts ([C·H·G]ESI+), generated in the electrospray environment, undergo a G-to-B ligand exchange in competition with a partial isomerization to the unreactive [C·H·G]GAS+-type complexes. Finally, the poorly reactive [C·H·G]SOL+ aggregates are formed in solution over an hours-long time scale. A cyclochirality effect on the reactivity was found to depend on the considered [C·H·G]+ aggregate-type.

A Pyridine-Acetylene-Aniline Oligomer: Saccharide Recognition and Influence of this Recognition Array on the Activity as Acylation Catalyst

In order to create new functions of foldamer-type hosts, various kinds of recognition arrays are expected to be developed. Here, a pyridine-acetylene-aniline unit is presented as a new class of a saccharide recognition array. The conformational stabilities of this array were analyzed by DFT calculation, and suggested that a pyridine-acetylene-aniline oligomer tends to form a helical structure. An oligomer of this array was synthesized, and its association for octyl β-D-glucopyranoside was confirmed by ¹ H NMR measurements. UV/Vis, circular dichroism, and fluorescence titration experiments revealed its high affinity for octyl glycosides in apolar solvents (K_a =10⁴ to 10⁵  M^-1 ). This oligomer was relatively stable under basic conditions, and therefore this array was expected to be applied to the derivatization of saccharides. A 4-(dialkylamino)pyridine attached pyridine-acetylene-aniline oligomer proved to catalyze the acylation of the octyl glucoside.

Biomimetic carbohydrate recognition

Binding saccharides with non-covalent interactions is challenging, especially in the natural medium of water, but synthetic carbohydrate receptors can be surprisingly effective.

Hexaphenolic Rigid Cages Prepared by Self-Organization of C3 v Tridentates

Coordination cages were composed by self-organization of rigid C_{3 v}-symmetric heptaarene tridentates and Pd(II) precursors. The heptaarene framework involves one mesitylene, three phenol, and three pyridine moieties, which were connected by Suzuki coupling reactions. The treatment of the tridentates with Pd(dppp)(OTf)₂ or Pd(en)(NO₃)₂ in a 2:3 molar ratio furnished coordination cages, which was ascertained by crystallography, ¹H NMR and DOSY measurements, and ESI-TOFMS and UV-vis spectra. The cages have six phenolic hydroxy groups inside and were expected to incorporate hydrogen-bonding guest molecules such as saccharides. CD and DOSY measurements showed that octyl hexoside guests could be incorporated into the cage.

Maltodextrin recognition by a macrocyclic synthetic lectin

This carbohydrate receptor achieves high affinities in water and shows an unusual preference for α-linked maltodextrins.