Chiral recognition of dipeptides in phosphatidylcholine aggregates

Enantiodiscrimination of ditryptophan enantiomers (l-Trp-l-Trp and d-Trp-d-Trp, l-Trp-d-Trp and d-Trp-l-Trp) was observed in bio-membrane models, such as micellar aggregates of 1,2-diheptanoyl-sn-glycero-3-phospatidylcholine (DHPC) and multilamellar vesicles of either 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dimyristoyl-sn-glycero-3-phospatidylcholine (DMPC) by solution ¹H NMR and HR-MAS ¹H NMR, respectively. The attainment of resolved signals, allowed the first detection of enantiodiscrimination at a molecular level, and the identification of the site of chiral recognition and of the interactions and conformations of homo- and heterochiral dipeptides in large-sized aggregates formed by a common component of bio-membranes.     

Chiral resolution of diphenylalanine by high-performance liquid chromatography on a crown-ether-based chiral stationary phase and by NMR spectroscopy

Summary The enantiomers of diphenylalanine (DPA) were well separated by chiral HPLC and NMR spectroscopy on the chiral stationary phase (CSP) derived from (18-crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TA). The chromatographic parameters such as separation factors and retention times were greatly influenced by the mobile phase conditions. The (+)-18-C-6-TA used in the CSP was also employed as a chiral solvating agent for the enantiodiscrimination of the DPA enantiomers by NMR spectroscopy. The proton of the DPA analyte showing the chemical shift nonequivalences was used in determining the enantiomeric composition of the analyte.     

Spectroscopy, NMR and DFT studies on molecular recognition of crown ether bridged chiral heterotrinuclear salen Zn(II) complex

A barium-containing crown ether bridged chiral heterotrinuclear salen Zn(II) complex BaZn2L(ClO4)2, where L is a folded dinuclear chiral (R,R)-salen ligand, has been synthesized and characterized by elemental analysis, 1H NMR, UV-vis, IR, circular dichroism (CD) spectra, and mass spectra. As a folded dinuclear chiral host, its recognition with achiral guests (imidazole derivatives), rigid bidentate guest (1,4-diazobicyclo[2,2,2]octane, DABCO) and chiral guests (amino acid methyl esters) was investigated by means of UV-vis spectrophotometric titration, CD spectra. The association constants of D-amino acid methyl esters are found to be higher than those of their L-enantiomer. The sandwich-type binding of BaZn2L(ClO4)2-DABCO supramolecular assembly was specially studied via 1H NMR titration and 1H ROESY. To understand the recognition on molecular level, density functional theory (DFT) calculations on B3LYP/LanL2DZ were performed on the minimal energy conformations of host, guests, and host-guest complexes. The minimal energy conformations were obtained by molecular mechanics (MM) optimization and molecular dynamics (MD) simulation. The results of single point energy, HOMO energy, and charges transfer were analyzed. The results of theoretical calculations are in good agreement with the experimental data.     

New chiral solvating agents for carboxylic acids: discrimination of enantiotopic nuclei and binding properties

New chiral hosts for carboxylic acids have been synthesized and their capacity for discrimination of enantiotopic nuclei explored by NMR. In addition, binding properties were examined by ¹H NMR titration experiments and molecular mechanics calculations.     

Synthesis and structure of macrocyclic bis(hydroxynaphthoic amide)s connected by an achiral or chiral diamine

Macrocyclic bis(hydroxynaphthoic amide)s 6, connected by an achiral or chiral diamine, were synthesized by the tandem Claisen rearrangement. CD spectra, X-ray crystallographic analyses, and variable-temperature NMR measurements of the chiral bis(hydroxynaphthoic amide)s revealed that the two hydroxynaphthalene rings in these macrocycles adopt a twisted conformation both in solution and in the crystalline state because of the steric hindrance between the two hydroxynaphthalene rings and that the chirality of the twisted conformation is generated by that of the chiral linker. Theoretical calculations revealed that the chiral linker works effectively to favor energetically one conformer of the diastereomers, although a flipping process was possible and can be observed to occur on the NMR time scale in variable-temperature experiments.     

Dynamic Formation of Noncovalent Calixsalen Aggregates

A procedure for studying “dynamic structural behavior” of large chiral macrocycles is presented. Ion mobility MS, diffusion‐ordered NMR spectroscopy (DOSY NMR), and optical rotation (OR) measurements, supported by calculations, are used together as effective complementary methods to study dynamic formation of noncovalent aggregates. It is shown that the monomer–dimer equilibrium is driven by π–π or CH–π interactions and controlled largely by the substitution pattern of the calixsalen skeleton.     

Solution structures by NMR of a novel antifungal drug: Petriellin A

Petriellin A is a novel cyclic depsipeptide antifungal compound consisting of nine l-configured residues, one d-phenyllactic acid (PhLac) and three unknown chiral centres: two N-methyl-threonines (MeThr1 & MeThr2) and one N-methyl-isoleucine (MeIle). NMR experiments including 2D ROESY, NOESY along with structural and energy calculations predicted that the unknown chiral centres were all l-configured, which was later verified chemically. Simulated annealing, dynamics calculations and minimisation processes showed Petriellin A to have a folded "C-shaped" structure.     

Co-Assembly between Fmoc Diphenylalanine and Diphenylalanine within a 3D Fibrous Viscous Network Confers Atypical Curvature and Branching

Supramolecular polymer co-assembly is a useful approach to modulate peptide nanostructures. However, the co-assembly scenario where one of the peptide building blocks simultaneously forms a hydrogel is yet to be studied. Herein, we investigate the co-assembly formation of diphenylalanine (FF), and Fmoc-diphenylalanine (FmocFF) within the 3D network of FmocFF hydrogel. The overlapping peptide sequence between the two building blocks leads to their co-assembly within the gel state modulating the nature of the FF crystals. We observe the formation of branched microcrystalline aggregates with an atypical curvature, in contrast to the FF assemblies obtained from aqueous solution. Optical microscopy reveal the sigmoidal kinetic growth profile of these aggregates. Microfluidics and ToF-SIMS experiments exhibit the presence of co-assembled structures of FF and FmocFF in the crystalline aggregates. Molecular dynamics simulation was used to decipher the mechanism of co-assembly formation.     

Elucidation of the isomeric domains formed by sodium N-dodecanoyl-L-prolinate

Investigations aimed at clarifying the nature of geometrical domains of amidic sodium N-dodecanoyl-l-prolinate are reported. NMR investigations on the association of aromatic solutes with the aggregates formed by the amidic surfactant in water show the selective binding of some solutes to the Z domains. Fluorescence quenching experiments allowed us to measure the aggregation number of the aggregates. SAXS experiments gave the average size and structural features of the aggregates. A NMR ROESY experiment evidenced the mosaic-like nature of the domains inside the same aggregate.     

Impact of Molecular Size and Shape on the Supramolecular Co-Assembly of Chiral Tricarboxamides: A Comparative Study

A comparative investigation of the chiral amplification features of a series of three families of C₃-symmetric tricarboxamides, 1,3,5-triphenylbenzenetricarboxamides (TPBAs), benzenetricarboxamides (BTAs) and oligo(phenylene ethynylene) tricarboxamides (OPE-TAs), is here reported. As previously observed for BTAs and OPE-TAs, a similar dichroic response is obtained for TPBAs decorated with one, two or three chiral side chains bearing stereogenic centers, thus confirming the efficient transfer of point chirality to the supramolecular helical aggregates. Unlike BTAs and OPE-TAs, the chiral amplification ability of TPBAs in majority rules experiments shows a negligible dependence on the number of chiral centers per monomeric unit, and stands the largest among the series of tricarboxamides. Detailed experimental and theoretical studies demonstrate that the rotation angle between the TPBA units in the helical stack is intermediate to that observed for BTAs and OPE-TAs. This feature strongly conditions the steric interactions between vicinal molecules in the stack and the final chiral amplification outcome. Furthermore, theoretical calculations show that achiral side chains favor the interdigitation of the helical aggregates and thereby the formation of bundle superstructures.     

Synthesis of novel chiral bis-N-substituted-hydrazinecarboxamide receptors and probing their solution-phase recognition to chiral carboxylic guests by ESI-TOF/MS and tandem ESI-MS

Seven novel bis-N-substituted-hydrazinecarboxamide receptors were synthesized in good to excellent yields by reacting chiral dicarbohydrazides, obtained from commercially available tartaric acid, with substituted aromatic isocyanates. The newly synthesized hydrazinecarboxamides formed structurally unique supramolecular aggregates, which have been confirmed by ESI-TOF/MS and tandem ESI-MS. They also showed molecular recognition to a selection of chiral carboxylic guests and oligopeptides, which mimic the backbone structure of the bacterial cell wall. The structures of the novel compounds were verified by various spectroscopic techniques including FTIR, ¹H NMR, ¹³C NMR, ESI-TOF/MS, tandem ESI-MS, 2D ROESY NMR, and CD spectroscopy.     

Di-(R,R)-1-[10-(1-hydroxy-2,2,2-trifluoroethyl)-9-anthryl]-2,2,2-trifluoroethyl muconate: a highly chiral cavity for enantiodiscrimination by NMR

A new chiral molecular tweezer, di-(R,R)-1-[10-(1-hydroxy-2,2,2-trifluoroethyl)-9-anthryl]-2,2,2-trifluoroethyl muconate 2, was synthesized in enantiopure form, and its geometry was studied using NMR and molecular mechanics. The effectiveness of 2 as a chiral solvating agent for determining the enantiomeric composition of chiral compounds using NMR was demonstrated, improving the results obtained with other methods. The stoichiometry and the association constant of the resulting diastereomeric complexes were studied, and their geometry was analyzed by NOE and 1H NMR.     

Use of NMR binding interaction mapping techniques to examine interactions of chiral molecules with molecular micelles

NMR spectroscopy was used to investigate the association of four chiral molecules with the molecular micelle poly(sodium N-undecanoyl-l-leucylvalinate) (poly(SULV)). Adding poly(SULV) to the background electrolyte in electrokinetic chromatography (EKC) allows enantiomeric resolution to be achieved because enantiomers interact differentially with the chiral centers on the micelle headgroups as they both move in the electric field. Pulsed field gradient diffusion experiments were used to measure molecular micelle association constants for enantiomers of each analyte. These association constants were consistent with EKC elution order for the compounds 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNP), 1,1'-bi-2-naphthol (BOH), and Troger's base. In addition, nuclear Overhauser enhancement spectroscopy, nuclear Overhauser effect difference, and intermolecular cross relaxation diffusion experiments were used to generate binding interaction maps for each chiral analyte. These maps showed that BNP and BOH inserted into the surfactant headgroup's major chiral groove and interacted predominately with the leucine chiral center. (+)-Troger's base was also found to insert into the major chiral groove. However, this compound instead interacted with the valine chiral atom. In diffusion experiments with long diffusion times, the linearized diffusion plots for each analyte-molecular micelle mixture showed curvature characteristic of intermolecular cross relaxation. The magnitude of this effect scaled linearly with the analytes' free energies of binding.     

Chiral recognition in aggregates formed by chiral bola-amphiphiles

The aggregates of three new isomeric chiral bola-amphiphiles have been taken into consideration as models for investigating chiral recognition in biological membranes. The recognition capabilities of the aggregates were explored following (by CD and HPLC experiments) the shift in the equilibrium between the inter-converting enantiomers of a biphenylic derivative and bilirubin, used as markers of chirality. The chiral recognition experiments were performed under different pH and concentration conditions. The results were compared with those relative to the aggregates formed by an equimolar mixture of single head/single chain amphiphiles that mimic the structure of the bola surfactant devoid of a covalent link.     

Atropisomeric phosphinines: design and synthesis

The first atropisomeric phosphinine was designed and prepared by introducing substituents into specific positions of the heterocyclic framework; the presence of axial chirality was predicted by means of DFT calculations and experimentally verified by chiral HPLC analysis, derivatization experiments as well as temperature dependent 31P{1H} NMR spectroscopy.     

Conformation and absolute configuration of nocathiacin I determined by NMR spectroscopy and chiral capillary electrophoresis

Nocathiacin I (BMS-249524) is a highly cross-linked thiazolyl peptide that displays potent activity against Gram-positive bacteria, including a number of antibiotic-resistant strains. This natural product contains 10 chiral centers. NMR studies have been performed to characterize the solution structure of nocathiacin I. A uniformly 13C,15N-labeled sample was used to obtain NMR assignments. Restrained simulated annealing calculations were performed by using accurately determined NOE distance restraints. All of the chiral centers were allowed to float during the simulated annealing protocol. Two clusters of structures were obtained that satisfy the NOE restraints very well and that are reasonably consistent with vicinal J-coupling constants. Within each cluster, all 10 chiral centers are uniquely defined. The two clusters are effectively mirror images of each other: all chiral centers that have the R(S) configuration in one cluster have the S(R) configuration in the other. The single threonine residue in nocathiacin I was subsequently determined to be l-threonine by chiral capillary electrophoresis, allowing the absolute configurations of all 10 chiral centers to be defined.     

Additive controlled,stereoselective benzylation of 2-thioxotetrahydropyrimidin-4(1H)-ones via chiral induction from a remote stereocenter

Stereoselective alkylation reactions of 3-aryl-1-alkyl-2-thioxotetrahydropyrimidin-4(1H)-one derivatives were studied. The reaction conditions were optimized to obtain the monobenzylated adduct with improved diastereoselectivity by regulating the reaction kinetics using HMPA as the additive and chiral ethyl lactate as the quencher. The absolute configuration of the product was established by NMR experiments, computational calculations, and single crystal X-ray analysis.     

Discrimination of the enantiomers of biphenylic derivatives in micellar aggregates formed by chiral amidic surfactants

The chirality of micellar aggregates formed by surfactants derived from l-proline was investigated by using two chiral biphenylic derivatives as probes of chirality. The investigation carried out by ¹H NMR, circular dichroism, and HPLC on chiral phase demonstrates that chiral recognition may occur in sites of the aggregates far from the head-group stereogenic centers and it is due to interaction with a whole region of the aggregate rather than with a single monomer in the aggregate. Subtle changes of the structure of the monomer influence the aggregation properties of the surfactants and its expression of chirality.     

Nickel-Catalyzed Asymmetric Hydrogenation of 2-Amidoacrylates

Earth-abundant nickel, coordinated with a suitable chiral bisphosphine ligand, was found to be an efficient catalyst for the asymmetric hydrogenation of 2-amidoacrylates, affording the chiral α-amino acid esters in quantitative yields and excellent enantioselectivity (up to 96 % ee). The active catalyst component was studied by NMR and HRMS, which helped us to realize high catalytic efficiency on a gram scale with a low catalyst loading (S/C=2000). The hydrogenated products could be simply converted into chiral α-amino acids, β-amino alcohols, and their bioactive derivatives. Furthermore, the catalytic mechanism was investigated using deuterium-labeling experiments and computational calculations.     

Nickel‐Catalyzed Asymmetric Hydrogenation of 2‐Amidoacrylates

Earth‐abundant nickel, coordinated with a suitable chiral bisphosphine ligand, was found to be an efficient catalyst for the asymmetric hydrogenation of 2‐amidoacrylates, affording the chiral α‐amino acid esters in quantitative yields and excellent enantioselectivity (up to 96 % ee). The active catalyst component was studied by NMR and HRMS, which helped us to realize high catalytic efficiency on a gram scale with a low catalyst loading (S/C=2000). The hydrogenated products could be simply converted into chiral α‐amino acids, β‐amino alcohols, and their bioactive derivatives. Furthermore, the catalytic mechanism was investigated using deuterium‐labeling experiments and computational calculations.