Pyridine-based receptors with high affinity for carbohydrates. Influence of the degree of steric hindrance at pyridine nitrogen on the binding mode

Remarkable changes in the binding affinity and selectivity of pyridine-based receptors toward monosaccharides have been observed when the degree of steric hindrance at pyridine nitrogen atom decreases.     

Selective Solid–Liquid and Liquid–Liquid Extraction of Lithium Chloride Using Strapped Calix[4]pyrroles

LiCl is a classic “hard” ion salt that is present in lithium‐rich brines and a key component in end‐of‐life materials (that is, used lithium‐ion batteries). Its isolation and purification from like salts is a recognized challenge with potential strategic and economic implications. Herein, we describe two ditopic calix[4]pyrrole‐based ion‐pair receptors ( 2 and 3 ), that are capable of selectively capturing LiCl. Under solid–liquid extraction conditions, using 2 as the extractant, LiCl could be separated from a NaCl/KCl salt mixture containing as little as 1 % LiCl with circa 100 % selectivity, while receptor 3 achieved similar separations when the LiCl level was as low as 200 ppm. Under liquid–liquid extraction conditions using nitrobenzene as the non‐aqueous phase, the extraction preference displayed by 2 is KCl>NaCl>LiCl. In contrast, 3 exhibits high selectivity towards LiCl over NaCl and KCl, with no appreciable extraction being observed for the latter two salts.     

Molecularly Imprinted Polymer Coated Quantum Dots for Multiplexed Cell Targeting and Imaging

Advanced tools for cell imaging are of great interest for the detection, localization, and quantification of molecular biomarkers of cancer or infection. We describe a novel photopolymerization method to coat quantum dots (QDs) with polymer shells, in particular, molecularly imprinted polymers (MIPs), by using the visible light emitted from QDs excited by UV light. Fluorescent core–shell particles specifically recognizing glucuronic acid (GlcA) or N‐acetylneuraminic acid (NANA) were prepared. Simultaneous multiplexed labeling of human keratinocytes with green QDs conjugated with MIP‐GlcA and red QDs conjugated with MIP‐NANA was demonstrated by fluorescence imaging. The specificity of binding was verified with a non‐imprinted control polymer and by enzymatic cleavage of the terminal GlcA and NANA moieties. The coating strategy is potentially a generic method for the functionalization of QDs to address a much wider range of biocompatibility and biorecognition issues.     

Stapled Peptides with γ‐Methylated Hydrocarbon Chains for the Estrogen Receptor/Coactivator Interaction

“Stapled” peptides are typically designed to replace two non‐interacting residues with a constraining, olefinic staple. To mimic interacting leucine and isoleucine residues, we have created new amino acids that incorporate a methyl group in the γ‐position of the stapling amino acid S5. We have incorporated them into a sequence derived from steroid receptor coactivator 2, which interacts with estrogen receptor α. The best peptide (IC₅₀=89 nm ) replaces isoleucine 689 with an S‐γ‐methyl stapled amino acid, and has significantly higher affinity than unsubstituted peptides (390 and 760 nm ). Through X‐ray crystallography and molecular dynamics studies, we show that the conformation taken up by the S‐γ‐methyl peptide minimizes the syn‐pentane interactions between the α‐ and γ‐methyl groups.     

Solid-phase organic tagging resins for labeling biomolecules by 1,3-dipolar cycloaddition: application to the synthesis of a fluorescent non-peptidic vasopressin receptor ligand

Two novel solid-phase organic tagging (SPOrT) resins were synthesized to facilitate the labeling of peptides and small organic compounds with a fluorescent probe. Both resins were obtained from the commercially available backbone amide linker (BAL) resin. Following the solid-phase synthesis of model compounds, a tripeptide and benzazepine, the fluorescent probe derived from Lissamine Rhodamine B was incorporated through CuI-catalyzed 1,3-dipolar cycloaddition. Final cleavage in acidic media enabled access to both types of molecules in good yield with high purity. The SPOrT resin was successfully applied to the preparation of the first non-peptidic fluorescent compound with a nanomolar affinity for the human vasopressin V2 receptor (V2R) subtype. This molecule will find application in binding assays that use polarization or fluorescence resonance energy-transfer (FRET) techniques. The SPOrT resins are also well suited for other tags and the parallel synthesis of a fluorescently tagged library for protein screening.     

Metal-controlled anion-binding tendencies of the thiourea unit of thiosemicarbazones

The terdentate ligand 3 (LH, 2-formylpyridine 4-thiosemicarbazone) forms with FeII and NiII 2:1 complexes of octahedral geometry of formula [MII(LH)2]2+. X-ray diffraction studies have shown that in both complexes the thiourea moieties of the coordinated thiosemicarbazones are exposed to the outside and are prone to establish hydrogen-bonding bifurcate interactions with oxoanions. However, spectrophotometric studies in CHCl3 solution have shown that only the poorly basic NO3 - ion is able to form authentic hydrogen-bond complexes with thiourea subunits, whereas all the other investigated anions (CH3COO-, NO2 -, F-) induce deprotonation of the N-H fragment. The extreme enhancement of the thiourea acidity is based on the coordinative interaction of the sulphur atom with the metal, which stabilises the thiolate form, and it is much higher than that exerted by any other covalently linked electron-withdrawing substituent, for example, --NO2.     

Synthesis, DNA-binding, cleavage, and cytotoxic activity of new 1,7-dioxa-4,10-diazacyclododecane artificial receptors containing bisguanidinoethyl or diaminoethyl double side arms

Novel 1,7-dioxa-4,10-diazacyclododecane artificial receptors with two pendant aminoethyl (3) or guanidinoethyl (4) side arms have been synthesized. Spectroscopy, including fluorescence and CD spectroscopy, of the interactions of 3, 4, and their copper(II) complexes with calf thymus DNA indicated that the DNA binding affinity of these compounds follows the order Cu(2+)-4>Cu(2+)-3>4>3, and the binding constants of Cu(2+)-3 are Cu(2+)-4 are 7.2x10(4) and 8.7x10(4) M(-1), respectively. Assessment by agarose gel electrophoresis of the plasmid pUC 19 DNA cleavage activity in the presence of the receptors showed that the complexes Cu(2+)-3 and Cu(2+)-4 exhibit powerful supercoiled DNA cleavage efficiency. Kinetic data of DNA cleavage promoted by Cu(2+)-3 and Cu(2+)-4 under physiological conditions fit to a saturation kinetic profile with kmax values of 0.865 and 0.596 h(-1), respectively, which give about 10(8)-fold rate acceleration over uncatalyzed supercoiled DNA. This acceleration is due to efficient cooperative catalysis of the copper(II) center and the functional (diamino or bisguanidinium) groups. In-vitro cytotoxic activities toward murine melanoma B16 cells and human leukemia HL-60 cells were also examined: Cu(2+)-4 shows the highest activity with IC(50) values of 1.62x10(-4) and 1.19x10(-5) M, respectively.     

Synthetic studies of neoclerodane diterpenoids from Salvia splendens and evaluation of opioid receptor affinity

Salvinorin A (1), a neoclerodane diterpene from the hallucinogenic mint Salvia divinorum, is the only known non-nitrogenous and specific κ-opioid agonist. Several structural congeners of 1 isolated from Salvia splendens (2-8) together with a series of semisynthetic derivatives (9-24), some of which possess a pyrazoline structural moiety (9, 19-22), have been tested for affinity at human μ, δ, and κ opioid receptors. None of these compounds showed high affinity binding to these receptors. However, 10 showed modest affinity for κ receptors suggesting that other natural neoclerodanes from different Salvia species may possess opioid affinity.     

New Open‐Chain and Cyclic Tetrapeptides,Consisting of α‐, β2‐, and β3‐Amino‐Acid Residues,as Somatostatin Mimics – A Survey

Cyclo‐β‐tetrapeptides are known to adopt a conformation with an intramolecular transannular hydrogen bond in solution. Analysis of this structure reveals that incorporation of a β²‐amino‐acid residue should lead to mimics of ‘α‐peptidic β‐turns’ (cf. A, B, C ). It is also known that short‐chain mixed β/α‐peptides with appropriate side chains can be used to mimic interactions between α‐peptidic hairpin turns and G protein‐coupled receptors. Based on these facts, we have now prepared a number of cyclic and open‐chain tetrapeptides, 7 – 20 , consisting of α‐, β²‐, and β³‐amino‐acid residues, which bear the side chains of Trp and Lys, and possess backbone configurations such that they should be capable of mimicking somatostatin in its affinity for the human SRIF receptors (hsst_1–5). All peptides were prepared by solid‐phase coupling by the Fmoc strategy. For the cyclic peptides, the three‐dimensional orthogonal methodology (Scheme 3) was employed with best success. The new compounds were characterized by high‐resolution mass spectrometry, NMR and CD spectroscopy, and, in five cases, by a full NMR‐solution‐structure determination (in MeOH or H₂O; Fig. 4). The affinities of the new compounds for the receptors hsst_1–5 were determined by competition with [¹²⁵I]LTT‐SRIF₂₈ or [¹²⁵I] [Tyr¹⁰]‐CST₁₄. In Table 1, the data are listed, together with corresponding values of all β‐ and γ‐peptidic somatostatin/Sandostatin^® mimics measured previously by our groups. Submicromolar affinities have been achieved for most of the human SRIF receptors hsst_1–5. Especially high, specific binding affinities for receptor hsst₄ (which is highly expressed in lung and brain tissue, although still of unknown function!) was observed with some of the β‐peptidic mimics. In view of the fact that numerous peptide‐activated G protein‐coupled receptors (GPCRs) recognize ligands with turn structure (Table 2), the results reported herein are relevant far beyond the realm of somatostatin: many other peptide GPCRs should be ‘reached’ with β‐ and γ‐peptidic mimics as well, and these compounds are proteolytically and metabolically stable, and do not need to be cell‐penetrating for this purpose (Fig. 5).