A Therapeutic Chemical Chaperone Inhibits Cholera Intoxication and Unfolding/Translocation of the Cholera Toxin A1 Subunit

Cholera toxin (CT) travels as an intact AB5 protein toxin from the cell surface to the endoplasmic reticulum (ER) of an intoxicated cell. In the ER, the catalytic A1 subunit dissociates from the rest of the toxin. Translocation of CTA1 from the ER to the cytosol is then facilitated by the quality control mechanism of ER-associated degradation (ERAD). Thermal instability in the isolated CTA1 subunit generates an unfolded toxin conformation that acts as the trigger for ERAD-mediated translocation to the cytosol. In this work, we show by circular dichroism and fluorescence spectroscopy that exposure to 4-phenylbutyric acid (PBA) inhibited the thermal unfolding of CTA1. This, in turn, blocked the ER-to-cytosol export of CTA1 and productive intoxication of either cultured cells or rat ileal loops. In cell culture studies PBA did not affect CT trafficking to the ER, CTA1 dissociation from the holotoxin, or functioning of the ERAD system. PBA is currently used as a therapeutic agent to treat urea cycle disorders. Our data suggest PBA could also be used in a new application to prevent or possibly treat cholera

beta gamma-fused turn structures in sugar amino acid (SAA) containing cyclic tetrapeptides with alpha 3 delta architecture

The current manuscript describes conformational analysis of 15-membered cyclic tetrapeptides (CTPs), with alpha 3 delta architecture, containing sugar amino acids (SAA) having variation in the stereocenter at C5 carbon. Conformational analyses of both the series, in protected and deprotected forms, were carried out in DMSO-d(6) using various NMR techniques, supported by restrained MD calculations. It was intriguing to notice that the alpha 3 delta macrocycles got stabilized by both 10-membered beta-turn as well as a seven-membered gamma-turn, fused within the same macrocycle. The presence of fused sub-structures within a 15-membered macrocycle is rare to see. Also, the stereocenter variation at C5 did not affect the fused turn structures and exhibited similar conformations in both the series. The design becomes highly advantageous as fused reverse turn structures are occurring in the cyclic structure with minimalistic size macrocycle and this can be applied to develop suitable pharmacophores in the drug development process. (C) 2014 Elsevier Ltd. All rights reserved

Stereoselective construction of quaternary chiral centers using Ti(III)-mediated opening of 2,3-epoxy alcohols: studies directed toward the synthesis of penifulvins

A trisubstituted α,β-unsaturated ester moiety was suitably placed in a molecule also bearing an epoxy alcohol moiety at its other end to intramolecularly trap the intermediate radical, which was formed when the molecule was treated with Cp<SUB>2</SUB>Ti(III)Cl to regio- and stereoselectively open its epoxy ring, giving rise to a quaternary chiral center. The method was subsequently used in an attempt to construct the bicyclic core framework of potent insecticides penifulvins

Total Synthesis of Reported Structure of Baulamycin A and Its Congeners

A convergent and flexible strategy for the stereoselective total synthesis of the reported structure of baulamycin A and its congeners has been developed for the first time. Synthetic highlights include a Crimmins aldol reaction to construct the C-1′ and C-14 centers, a Crimmins acetate aldol reaction to generate the hydroxy group at the C-13 position, Horner–Wadsworth–Emmons olefination to form the C₉–C₁₀ bond, and Evans methylation to install the C-8 center. This synthetic study disclosed that the reported structure of baulamycin A needs to be revised, as its spectroscopic data are not identical with those of the synthetic baulamycin A

Robust Turn Structures in α₃β Cyclic Tetrapeptides Induced and Controlled by Carbo-β³ Amino Acid

Designing cyclic tetrapeptides (CTPs), which fold into desired structures, is often a challenging task. While it is difficult to synthesize them, they are also prone to adopt multiple conformations. In this paper we report the synthesis and conformational studies of CTP mimics, having nonconstrained α₃β motif, that exhibit stable β- and γ-turn structures. We also demonstrate the transformation of β-turn to γ-turn structure in similar CTPs by inverting the chirality of β³ carbon in C-linked-carbo-β³-amino acid (Caa) from <i>R</i> to <i>S</i>

Diastereoselective Synthesis of 5‑Heteroaryl-Substituted Prolines Useful for Controlling Peptide-Bond Geometry

A versatile diastereoselective Friedel–Crafts alkylation reaction of heteroaryl systems with a cyclic enecarbamate for the preparation of 5-heteroaryl-substituted proline analogues in good yields has been developed. These heterocyclic tethered cyclic amino acid building blocks constitute important structural motifs in many biologically active molecules. The impact of the substitution on proline <i>cis</i>/<i>trans</i> isomerization was explored by carrying out solution conformational studies by NMR on 5-furanyl-substituted proline-containing peptides. Conformational analysis revealed that the peptide bond is constrained in an exclusively <i>trans</i> conformation

Conformational studies of glycosylated cyclic oligomers of furanoid sugar amino acids

Glycosylation of molecules improve their pharmacological and pharmacokinetic properties. In the current manuscript, we have explored the effect of glycosylation on the structure and function of conformationally well-defined small ring homooligomers derived from a structurally diverse library of sugar amino acids (SAA). Conformational analyses carried out by NMR suggested that these cyclic dimers and trimers have well-defined structures in solution. MD simulations performed based on the restraints obtained from NMR revealed that C2H and CO are positioned outside the plane of the ring and NHs are pointed inside the ring. It was encouraging to note that while the cyclic non-glycosylated homooligomers did not show any antimicrobial activity at all, their glycosylated counterparts showed relatively better activity. The modular design developed here is amenable to further improvement and can serve as a tool to investigate many molecular recognition processes. (C) 2016 Elsevier Ltd. All rights reserved

Influence of Linker Length on Conformational Preferences of Glycosylated Sugar Amino Acid Foldamers

Glycosylation of foldamers derived from furanoid sugar amino acids with mannose and a propyltriazole linker results in an unprecedented 16/10 mixed-turn structure in the glycopeptides in water, with a preference for the higher-order structure irrespective of the stereochemistry of the starting foldamer. This is in stark contrast to the structures displayed by the same oligomers in water when mannosylated with a two-carbon-shorter methyltriazole linker: 16-membered turn structure in the cis-foldamer and 10-membered in its trans congener. This demonstrates the defining influence of the linker length on the structural preference of these novel glycopeptide mimics

Conformation Analysis of GalNAc-Appended Sugar Amino Acid Foldamers as Glycopeptide Mimics

Sugar amino acid (SAA)-based foldamers with well-defined secondary structures were appended with N-acetylgalactosamine (GalNAc) sugars to access sequence-defined, multidentate glycoconjugates with full control over number, spacing and position. Conformation analysis of these glycopeptides by extensive NMR spectroscopic studies revealed that the appended GalNAc units had a profound influence on the native conformational behaviour of the SAA foldamers. Whereas the 2,5-cis glycoconjugate showed a helical structure in water, comprising of two consecutive 16-membered hydrogen bonds, its 2,5-trans congener displayed an unprecedented 16/10-mixed turn structure not seen before in any glycopeptide foldamer

Synthesis, Conformational Studies and Biological Profiles of Tetrahydrofuran Amino-Acid-Containing Cationic Antitubercular Peptides

Linear cationic tetra- and octapeptides containing a tetrahydrofuran amino acid were synthesised, a conformational analysis performed, and their structure-activity relationships explored by evaluating their antibacterial, antitubercular and cytotoxic properties. Only the octapeptides showed activities, which was weak to moderate against Gram-positive and Gram-negative bacteria for only two peptides, but was very significant against Mycobacterium tuberculosis, and peptides were devoid of toxicity towards mammalian Vero cells. N-Terminus-deprotected esters showed the best results with excellent selectivity for M.tuberculosis and these peptides promise to be potential leads