Benocyclidine (BTCP) as Non‐labelled Reporter Ligand for MS Binding Assays for the PCP Ion Channel Binding Site of the Desensitized Torpedo Nicotinic Acetylcholine Receptor (nAChR)

In this study we present MS Binding Assays for the PCP ion channel binding site of Torpedo californica nicotinic acetylcholine receptor (nAChR) as an alternative to radioligand binding assays. As MS Marker Benocyclidine (BTCP) was employed, found to be more affine (Kd of 84.2 nM) than the radioligands, e. g. [3H]PCP, used so far in respective binding assays. Based on a highly sensitive and fast LC-ESI-MS/MS method for quantification of BTCP samples, BTCP MS Binding Assays for the PCP ion channel binding site of Torpedo nAChR could be established comprising saturation, kinetic and competition experiments. The affinities obtained in competitive BTCP MS Binding Assays for ligands addressing the PCP ion channel binding site of Torpedo nAChR were in excellent accord with those reported from radioligand experiments. Thus, the new BTCP MS Binding Assays represent a potent and reliable alternative to radioligand binding assays used so far for the characterization of ligand binding to the PCP ion channel binding site of the nAChR

Computational Design and Synthesis of a Deeply Red-Shifted and Bistable Azobenzene

We computationally dissected the electronic and geometrical influences of ortho-chlorinated azobenzenes on their photophysical properties. X-ray analysis provided the insight that trans-tetra-ortho-chloro azobenzene is conformationally flexible and thus subject to molecular motions. This allows the photoswitch to adopt a range of red-shifted geometries, which account for the extended n → π* band tails. On the basis of our results, we designed the di-ortho-fluoro di-ortho-chloro (dfdc) azobenzene and provided computational evidence for the superiority of this substitution pattern to tetra-ortho-chloro azobenzene. Thereafter, we synthesized dfdc azobenzene by ortho-chlorination via 2-fold C–H activation and experimentally confirmed its structural and photophysical properties through UV–vis, NMR, and X-ray analyses. The advantages include near-bistable isomers and an increased separation of the n → π* bands between the trans- and cis-conformations, which allows for the generation of unusually high levels of the cis-isomer by irradiation with green/yellow light as well as red light within the biooptical window

Phylloxanthobilins are Abundant Linear Tetrapyrroles from Chlorophyll Breakdown with Activities Against Cancer Cells

The major chlorophyll catabolites are colorless phyllobilins with four deconjugated pyrrole units. Their yellow oxidation products, are getting more and more in the spotlight due to their interesting physicochemical properties and potential bioactivities. Although studies on physiologically relevant activities of phyllobilins are in early stages, it already becomes evident that when it comes to bioactivities, yellow beats colorless

Discrete Stacked Dimers of Aromatic Oligoamide Helices

Tight binding was observed between the C-terminal cross section of aromatic oligoamide helices in aqueous solution, leading to the formation of discrete head-to-head dimers in slow exchange on the NMR timescale with the corresponding monomers. The nature and structure of the dimers was evidenced by 2D NOESY and DOSY spectroscopy, mass spectrometry and X-ray crystallography. The binding interface involves a large hydrophobic aromatic surface and hydrogen bonding. Dimerization requires that helices have the same handedness and the presence of a C-terminal carboxy function. The protonation state of the carboxy group plays a crucial role, resulting in pH dependence of the association. Dimerization is also influenced by neighboring side chains and can be programmed to selectively produce heteromeric aggregates

DNA Mimic Foldamer Recognition of a Chromosomal Protein

Helical aromatic oligoamide foldamers bearing anionic side chains that mimic the overall shape and charge surface distribution of DNA were synthesized. Their interactions with chromosomal protein Sac7d, a non-sequence-selective DNA-binder that kinks and packages DNA, were investigated by Surface Plasmon Resonance (SPR), Circular Dichroism (CD), and Nuclear Magnetic Resonance (NMR) spectroscopies, as well as by single crystal X-ray crystallography. The foldamers were shown to bind to Sac7d better than a DNA duplex of comparable length. The interaction is diastereoselective and takes place at the DNA-binding site. Crystallography revealed that the DNA-mimic foldamers have a binding mode of their own and that they can bind to Sac7d without being kinked

MS Binding Assays for Glycine Transporter 2 (GlyT2) Employing Org25543 as Reporter Ligand

This study describes the first binding assay for glycine transporter 2 (GlyT2) following the concept of MS Binding Assays. The selective GlyT2 inhibitor Org25543 was employed as a reporter ligand and it was quantified with a highly sensitive and rapid LC-ESI-MS/MS method. Binding of Org25543 at GlyT2 was characterized in kinetic and saturation experiments with an off-rate of 7.07x10(-3) s(-1), an on-rate of 1.01x10(6) M-1 s(-1), and an equilibrium dissociation constant of 7.45 nM. Furthermore, the inhibitory constants of 19 GlyT ligands were determined in competition experiments. The validity of the GlyT2 affinities determined with the binding assay was examined by a comparison with published inhibitory potencies from various functional assays. With the capability for affinity determination towards GlyT2 the developed MS Binding Assays provide the first tool for affinity profiling of potential ligands and it represents a valuable new alternative to functional assays addressing GlyT2

Conformational interplay in hybrid peptide–helical aromatic foldamer macrocycles

Macrocyclic peptides are an important class of bioactive substances. When inserting an aromatic foldamer segment in a macrocyclic peptide, the strong folding propensity of the former may influence the conformation and alter the properties of the latter. Such an insertion is relevant because some foldamer–peptide hybrids have recently been shown to be tolerated by the ribosome, prior to forming macrocycles, and can thus be produced using an in vitro translation system. We have investigated the interplay of peptide and foldamer conformations in such hybrid macrocycles. We show that foldamer helical folding always prevails and stands as a viable means to stretch, i.e. unfold, peptides in a solvent dependent manner. Conversely, the peptide systematically has a reciprocal influence and gives rise to strong foldamer helix handedness bias as well as foldamer helix stabilisation. The hybrid macrocycles also show resistance towards proteolytic degradation

Combining local conformational preferences and solvophobic effects in helical aromatic oligoamide foldamers

Removing features that locally control conformations in aromatic oligoamides hampers helical folding in DMSO but not in water where the loss is compensated by hydrophobic effects. Aromatic oligoamide foldamers were designed using a newly-developed monomer so that helical folding was promoted by both local conformation preferences and solvophobic effects. Solid phase synthesis provided quick access to the desired sequences.Sharp solvent-driven conformational transitions that depended on sequence length were evidenced by both NMR and UV absorption spectroscopie

Molecular torsion springs: alteration of helix curvature in frustrated tertiary folds

The first abiotic foldamer tertiary structures have been recently reported in the form of aromatic helix–turn–helix motifs based on oligo-quinolinecarboxamides held together by intramolecular hydrogen bonds. Tertiary folds were predicted by computational modelling of the hydrogen-bonding interfaces between helices and later verified by X-ray crystallography. However, the prognosis of how the conformational preference inherent to each helix influences the tertiary structure warranted further investigation. Several new helix–turn–helix sequences were synthesised in which some hydrogen bonds have been removed. Contrary to expectations, this change did not strongly destabilise the tertiary folds. On closer inspection, a new crystal structure revealed that helices adopt their natural curvature when some hydrogen bonds are missing and undergo some spring torsion upon forming the said hydrogen bonds, thus potentially giving rise to a conformational frustration. This phenomenon sheds light on the aggregation behaviour of the helices when they are not linked by a turn unit.

Catalytic Alkyne Semihydrogenation with Polyhydride Ni/Ga Clusters

International audienceThe bimetallic, decanuclear Ni3Ga7-cluster of the formula [Ni-3(GaTMP)(3)(and mu;(2)-GaTMP)(3)(and mu;(3)-GaTMP)] (1, TMP=2,2,6,6-tetramethylpiperidinyl) reacts reversibly with dihydrogen under the formation of a series of (poly-)hydride clusters 2. Low-temperature 2D NMR experiments at -80 and DEG;C show that 2 consist of a mixture of a di- (2(Di)), tetra- (2(Tetra)) and hexahydride species (2(Hexa)). The structures of 2(Di) and 2(Tetra) are assessed by a combination of 2D NMR spectroscopy and DFT calculations. The cooperation of both metals is essential for the high hydrogen uptake of the cluster. Polyhydrides 2 are catalytically active in the semihydrogenation of 4-octyne to 4-octene with good selectivity. The example is the first of its kind and conceptually relates properties of molecular, atom-precise transition metal/main group metal clusters to the respective solid-state phase in catalysis