Hydrogen Bonded Foldamers with Axial Chirality: Chiroptical Properties and Applications

Folding phenomenon refers to the formation of a specific conformation widely featured by the intramolecular interactions, which broadly exist in biomacromolecules, and are closely related to their structures and functions. A variety of oligomeric folded molecules have been designed and synthesized, namely "foldamer", exhibiting potentials in pharmaceutical and catalysis. Molecular folding is a promising strategy to transfer chirality from substituents to the whole skeleton, when chirality transfer, amplification, evolution, and other behaviors could be achieved. Investigating chirality using foldamer model deepens the understanding of the structure-function correlation in biomacromolecules and expands the molecular toolbox towards chiroptical and asymmetrical chemistry. Substitutes with abundant hydrogen bonding sites conjugated to a rotatable aryl group afford a parallel β-sheet-like conformation, which enables the emergence and manipulation of axial chirality. This concept aims to give a brief introduction and summary of the hydrogen bonded foldamers with anchored axial chirality, by taking some recent cases as examples. Design principles, control over axial chirality and applications are also reviewed.

Crystal structure of a tripeptide biphenyl hybrid C50H56N6O10·0.5H2O

The synthesis of the peptide biphenyl hybrid compound dimethyl 2,2′-[((2S,2′S)-2,2′-{[(2S,2′S)-1,1′-([1,1′-biphen­yl]-2,2′-dicarbon­yl)bis­(pyrrolidine-1,2-diyl-2-carbon­yl)]bis­(aza­nedi­yl)}bis­(3-phenyl­propano­yl))bis­(aza­nedi­yl)](2S,2′S)-dipropionate) is described. The crystal structure of this compound has a highly ordered supra­molecular structure with extensive inter­molecular hydrogen bonding.

A peptide biphenyl hybrid compound {systematic name: dimethyl 2,2′-[((2S,2′S)-2,2′-{[(2S,2′S)-1,1′-([1,1′-biphen­yl]-2,2′-dicarbon­yl)bis­(pyrrolidine-1,2-diyl-2-carbon­yl)]bis­(aza­nedi­yl)}bis­(3-phenyl­propano­yl))bis­(aza­nedi­yl)](2S,2′S)-dipropionate hemihydrate}, C₅₀H₅₆N₆O₁₀·0.5H₂O, was prepared by coupling of [1,1′-biphen­yl]-2,2′-dicarbonyl dichloride, tri­ethyl­amine and the tripeptide Pro–Phe–Ala in CH₂Cl₂ at 273 K under an N₂ atmosphere. In the crystal, the asymmetric unit contains the peptide biphenyl hybrid accompanied by one-half of a water mol­ecule. A C atom of one of the proline rings is disordered between two positions in a 0.746 (11):0.254 (11) ratio. An important structural aspect of peptide compounds is their capacity to self-associate mediated by inter­molecular and intra­molecular hydrogen bonding. This characteristic can be useful in understanding the inter­actions between peptides and biomacromolecular targets, as well as to explain peptide properties.

Peptide-biphenyl hybrid-capped AuNPs: stability and biocompatibility under cell culture conditions

In this study, we explored the biocompatibility of Au nanoparticles (NPs) capped with peptide-biphenyl hybrid (PBH) ligands containing glycine (Gly), cysteine (Cys), tyrosine (Tyr), tryptophan (Trp) and methionine (Met) amino acids in the human hepatocellular carcinoma cell line Hep G2. Five AuNPs, Au[(Gly-Tyr-Met)2B], Au[(Gly-Trp-Met)2B], Au[(Met)2B], Au[(Gly-Tyr-TrCys)2B] and Au[(TrCys)2B], were synthesised. Physico-chemical and cytotoxic properties were thoroughly studied. Transmission electron micrographs showed isolated near-spherical nanoparticles with diameters of 1.5, 1.6, 2.3, 1.8 and 2.3 nm, respectively. Dynamic light scattering evidenced the high stability of suspensions in Milli-Q water and culture medium, particularly when supplemented with serum, showing in all cases a tendency to form agglomerates with diameters approximately 200 nm. In the cytotoxicity studies, interference caused by AuNPs with some typical cytotoxicity assays was demonstrated; thus, only data obtained from the resazurin based assay were used. After 48-h incubation, only concentrations ≥50 μg/ml exhibited cytotoxicity. Such doses were also responsible for an increase in reactive oxygen species (ROS). Some differences were observed among the studied NPs. Of particular importance is the AuNPs capped with the PBH ligand (Gly-Tyr-TrCys)2B showing remarkable stability in culture medium, even in the absence of serum. Moreover, these AuNPs have unique biological effects on Hep G2 cells while showing low toxicity. The production of ROS along with supporting optical microscopy images suggests cellular interaction/uptake of these particular AuNPs. Future research efforts should further test this hypothesis, as such interaction/uptake is highly relevant in drug delivery systems.

Decabromobiphenyl (PBB-209) activates the aryl hydrocarbon receptor while decachlorobiphenyl (PCB-209) is inactive: experimental evidence and computational rationalization of the different behavior of some halogenated biphenyls

In rat H4IIE cells permanently transfected with a luciferase gene under the control of AhR, incubation with PBB-209 led to a statistically significant increase of luminescence. In this system, PCB-209 only caused a small induction of luciferase activity. In a fish cell line, only PBB-209 was able to provoke an induction of ethoxyresorufin- O-deethylase activity. Ligand binding to the AhR was studied by means of a cell-free in vitro system in which the activation of AhR is very unlikely to occur without ligand binding. None of the biphenyls studied provoked any activation of AhR in this system. To rationalize the results and to get insight into the molecular mechanism of activation of AhR by PBB-209 as compared with PCB-209, a comprehensive computational study was carried out on these congeners as well as on PCB-126 and PCB-169, two potent AhR activators through ligand binding. The calculations include (i) conformational analysis and dipole moments of each conformer, (ii) aromaticity indices, (iii) molecular electrostatic potentials, (iv) quadrupole moments, (v) electronic and reactivity descriptors, and (vi) dissociation energies of C-Cl and C-Br bonds in model aromatic compounds. It was found that some molecular features of PBB-209, such as the electrostatic potential (EP) and EP-derived descriptors (Politzer's parameters), indicate that PBB-209 is more similar to PCB-126 and PCB-169 than to PCB-209, which share quite similar geometries based on the substitution pattern. The similarity between PBB-209, PCB-126, and PCB-169 seems to hint that these three compounds can share, at least partially, similar mechanisms of activation of AhR. It is unquestionable that PCB-126 and PCB-169 directly bind AhR and PBB-209 does not. We hypothesize that there are several simultaneous mechanisms for activation of AhR, and the most active compounds act for more than one mechanism.

Neural Networks as a Tool To Classify Compounds According to Aromaticity Criteria

Aromaticity is a fundamental concept in chemistry, with many theoretical and practical implications. Although most organic compounds can be categorized as aromatic, non-aromatic, or antiaromatic, it is often difficult to classify borderline compounds as well as to quantify this property. Many aromaticity criteria have been proposed, although none of them gives an entirely satisfactory solution. The inability to fully arrange organic compounds according to a single criterion arises from the fact that aromaticity is a multidimensional phenomenon. Neural networks are computational techniques that allow one to treat a large amount of data, thereby reducing the dimensionality of the input set to a bidimensional output. We present the successful applications of Kohonen's self-organizing maps to classify organic compounds according to aromaticity criteria, showing a good correlation between the aromaticity of a compound and its placement in a particular neuron. Although the input data for the training of the network were different aromaticity criteria (stabilization energy, diamagnetic susceptibility, NICS, NICS(1), and HOMA) for five-membered heterocycles, the method can be extended to other organic compounds. Some useful features of this method are: 1) it is very fast, requiring less than one minute of computational time to place a new compound in the map; 2) the placement of the different compounds in the map is conveniently visualized; 3) the position of a compound in the map depends on its aromatic character, thus allowing us to establish a quantitative scale of aromaticity, based on Euclidean distances between neurons, 4) it has predictive power. Overall, the results reported herein constitute a significant contribution to the longstanding debate on the quantitative treatment of aromaticity.