Adjusting the water-sensitivity of sugar/boronate-based organogels

During the investigation of the water-sensitivity of (arylboronate alkylglucoside)-based organogels, we evaluated a series of twelve potential organogelators. They were synthesised in a single step from the corresponding arylboronic acids and alkylglucosides. Eight of them showed organogelation abilities in three solvents (toluene, cyclohexane, and ethyl myristate). Conformational minimisations of the potential organogelators permitted a clear relationship between the arylboronate orientation and the gelation effectiveness to be established. These gels were characterised by rheometry and SEM which revealed a gel-state originating from the self-assembly of the organogelators into long entangled fibres. SAXS confirmed the mode of packing in a hexagonal phase. Gels in toluene were found to be water-sensitive both after addition of a small amount of water and immersion into water. This study demonstrated that the main parameter impacting the water-sensitivity was the length of the alkyl chain at the anomeric position of the glucoside unit, much more than the functionalisation of an arylboronate moiety.

Irreversible inhibitors of the proline racemase (PRAC) unveil innovative mechanism of action as antibacterial against Clostridioides difficile

Proline racemases (PRAC), catalyzing the L-proline and D-proline interconversion, are essential factors in eucaryotic pathogenes such as Trypanosoma cruzi, Trypanosoma vivax and Clostridioides difficile. If the discovery of irreversible inhibitors of Trypanosoma cruzi PRAC (TcPRAC) led to innovative therapy of the Chagas disease, no inhibitors of CdPRAC have been discovered to date. However, Clostridioides difficile, due to an increased incidence in recent years, is considered as a major cause of health threat. In this work, we have taken into account the similarity between TcPRAC and CdPRAC enzymes to design new inhibitors of CdPRAC. Starting from (E) 4-oxopent-2-enoic acid TcPRAC irreversible inhibitors, we synthesized 4-aryl substituted analogues and evaluated their CdPRAC enzymatic inhibition against eleven strains of Clostridioides difficile. This study resulted in promising candidates and allowed for identification of (E)-4-(3-bromothiophen-2-yl)-4-oxobut-2-enoic acid 20 that was chosen for complementary in vivo studies and did not reveal in vivo toxicity.

Structural revision of the Mcl-1 inhibitor MIM1: synthesis and biological studies on ovarian cancer cells with evaluation of designed analogues

In the area of cancer research, the development of new and potent inhibitors of anti-apoptotic proteins is a very active and promising topic. The small molecule MIM1 has been reported earlier as one of the first selective inhibitors of the anti-apoptotic protein Mcl-1. In the present paper, we first revised the structure of this molecule based on extensive physicochemical analyses. Then we designed and synthesized a focused library of analogues for the corrected structure of MIM1. Next, these molecules were subjected to a panel of in cellulo biological studies, allowing the identification of dual Bcl-x_L/Mcl-1 inhibitors, as well as selective Mcl-1 inhibitors. These results have been complemented by fluorescence polarization assays with the Mcl-1 protein. Preliminary structure-activity relationships were discussed and extensive molecular modelling studies allowed us to propose a rationale for the biological activity of this series of new inhibitors, in particular for the selectivity of inhibition of Mcl-1 versus Bcl-x_L.

Modulation of the Activity and Regioselectivity of a Glycosidase: Development of a Convenient Tool for the Synthesis of Specific Disaccharides

The synthesis of disaccharides, particularly those containing hexofuranoside rings, requires a large number of steps by classical chemical means. The use of glycosidases can be an alternative to limit the number of steps, as they catalyze the formation of controlled glycosidic bonds starting from simple and easy to access building blocks; the main drawbacks are the yields, due to the balance between the hydrolysis and transglycosylation of these enzymes, and the enzyme-dependent regioselectivity. To improve the yield of the synthesis of β-d-galactofuranosyl-(1→X)-d-mannopyranosides catalyzed by an arabinofuranosidase, in this study we developed a strategy to mutate, then screen the catalyst, followed by a tailored molecular modeling methodology to rationalize the effects of the identified mutations. Two mutants with a 2.3 to 3.8-fold increase in transglycosylation yield were obtained, and in addition their accumulated regioisomer kinetic profiles were very different from the wild-type enzyme. Those differences were studied in silico by docking and molecular dynamics, and the methodology revealed a good predictive quality in regards with the regioisomer profiles, which is in good agreement with the experimental transglycosylation kinetics. So, by engineering CtAraf51, new biocatalysts were enabled to obtain the attractive central motif from the Leishmania lipophosphoglycan core with a higher yield and regioselectivity.

Boron Effect on Sugar-Based Organogelators

The reaction of several alkylglucosides with phenyl boronic acid permitted easy access to a series of alkylglucoside phenyl boronate derivatives. This type of compound has structures similar to those of known benzylidene glucoside organogelators except for the presence of a boronate function in place of the acetal one. Low to very low concentrations of these amphiphilic molecules produced gelation of several organic solvents. The rheological properties of the corresponding soft materials characterized them as elastic solids. They were further characterized by SEM to obtain more information on their morphologies and by SAXS to determine the type of self-assembly involved within the gels. The sensitivity of the boronate function towards hydrolysis was also investigated. We demonstrated that a small amount of water (5 % v/v) was sufficient to disrupt the organogels leading to the original alkylglucoside and phenyl boronic acid; an important difference with the stable benzylidene-based organogelators. Such water-sensitive boronated organogelators could be suitable substances for the preparation of smart soft material for topical drug delivery.

6-Deoxy-6-fluoro galactofuranosides: regioselective glycosylation, unexpected reactivity, and anti-leishmanial activity

Selective glycosylation of the C-6 fluorinated galactofuranosyl acceptor 2 was studied with four galactofuranosyl donors. It was highlighted that this electron-withdrawing atom strongly impacted the behavior of the acceptor, thus leading to unprecedented glycosylation pathways. Competition between expected glycosylation of 2, ring expansion of this acceptor and furanosylation, and intermolecular aglycon transfer was observed. Further investigation of the fluorinated synthetic compounds showed that the presence of fluorine atom contributed to increase the inhibition of the growth of Leishmania tarentolae, a non-pathogenic strain of Leishmania.

Improvement of the versatility of an arabinofuranosidase against galactofuranose for the synthesis of galactofuranoconjugates

A new performant biocatalyst was developed for the synthesis ofO-,S- and acyl-galactofuranoconjugates.

Hemisynthesis of selected embelin analogs and investigation of their proapoptotic activity against cancer cells

Embelin is a natural product, inhibitor of XIAP (X-chromosome-linked Inhibitor of APoptosis) with strong proapoptotic properties on cancer cells. In order to clarify the role of two OH groups on benzoquinone core, we have prepared by hemisynthesis close analogs of embelin, where these OH groups have been replaced in a systematic manner by OMe and OAc groups. Proapoptotic activities of six embelin derivatives have been studied as single agent, or in combination with TRAIL, and their abilities to interact with XIAP have been evaluated by Surface Plasmon Biacore. Our results show that these new embelin analogs have good proapoptotic properties against selected cancer cells, often higher than the natural product itself. Further, this activity is not directly mediated by XIAP. Altogether these preliminary results demonstrate that for active embelin analogs, the two OH groups are not absolutely required for anticancer activity, opening new possibilities for the design of proapoptotic derivatives in these series.

Synthesis of enones, pyrazolines and pyrrolines with gem-difluoroalkyl side chains

Starting from easily accessible gem-difluoropropargylic derivatives, a DBU-mediated isomerisation affords enones in fair yields with a gem-difluoroalkyl chain. These derivatives were used to prepare pyrazolines and pyrrolines with the desired gem-difluoroalkyl side chain by cyclocondensations in good yields and with excellent stereoselectivity. A one-pot process was also successfully developed for these sequential reactions. By carrying out various types of Pd-catalyzed coupling reactions for compounds with a p-bromophenyl substituent a route to focused chemical libraries was demonstrated.

Rational design of a low molecular weight, stable, potent, and long-lasting GPR103 aza-β3-pseudopeptide agonist

26RFa, a novel RFamide neuropeptide, is the endogenous ligand of the former orphan receptor GPR103. Intracerebroventricular injection of 26RFa and its C-terminal heptapeptide, 26RFa((20-26)), stimulates food intake in rodents. To develop potent, stable ligands of GPR103 with low molecular weight, we have designed a series of aza-β(3)-containing 26RFa((20-26)) analogues for their propensity to establish intramolecular hydrogen bonds, and we have evaluated their ability to increase [Ca(2+)](i) in GPR103-transfected cells. We have identified a compound, [Cmpi(21),aza-β(3)-Hht(23)]26RFa((21-26)), which was 8-fold more potent than 26RFa((20-26)) in mobilizing [Ca(2+)](i). This pseudopeptide was more stable in serum than 26RFa((20-26)) and exerted a longer lasting orexigenic effect in mice. This study constitutes an important step toward the development of 26RFa analogues that could prove useful for the treatment of feeding disorders.

Incorporation of vinylogous scaffolds in the C-terminal tripeptide of substance P

Glycine-9 and leucine-10 of substance P (SP) are critical for (NK)-1 receptor recognition and agonist activity. Propsi(Z)-CH=CH(CH3)-CONH)Leu (or Met) and Propsi((E)-CH=CH(CH3)-CONH)Leu (or Met) have been introduced in the sequence of SP, in order to restrict the conformational flexibility of the C-terminal tripeptide, Gly-Leu-Met-NH2, of SP. Propsi((Z)-CH=C(CH2CH(CH3)2)-CONH)Met-NH2, with an isobutyl substituent to mimic the Leu side-chain, was also incorporated in place of the C-terminal tripeptide. The substituted-SP analogs were tested for their affinity to human NK-1 receptor specific binding sites (NK-1M and NK-1m) and their potency to stimulate adenylate cyclase and phospholipase C in Chinese Hamster ovary (CHO) cells transfected with the human NK-1 receptor. The most potent SP analogs [Pro9psi((Z)CH=C(CH3)CONH)Leu10]SP and [Pro9psi ((E)CH=C(CH3)CONH)Leu10]SP, are about 100-fold less potent than SP on both binding sites and second messenger pathways. These vinylogous (Z)- or (E)-CH=C(CH3)- or (Z)-CH=C(CH2CH(CH3)2) moieties hamper the correct positioning of the C-terminal tripeptide of SP within both the NK-1M- and NK-1m-specific binding sites. The origin of these lower potencies is related either to an incorrect peptidic backbone conformation and/or an unfavorable receptor interaction of the methyl or isobutyl group.

Anionic copper complex fragmentations from enkephalins under low-energy collision-induced dissociation in an ion trap mass spectrometer

Peptide metallation with Cu2+ was explored in the negative ESI mode using an ion trap mass spectrometer. Under these conditions, the [(M-3H) + CuII]- species formed were investigated under low-energy collision-induced dissociation conditions. MS2 experiments indicate a very different behavior of CuII metallated complexes compared with [M-H]- species. CuII induces an easy loss of CO2 and specific side-chain cleavages (by radical losses) at the C-terminal residue, as observed previously by prompt 'in source' dissociation experiments. The loss of CO2 yields an unstable carbylide that leads to further dissociations involving the migration of a proton or a hydrogen radical (through the reduction of CuII). Multistage MS3 experiments were carried out to rationalize this behavior. Fragmentation pathways are proposed in order to explain the product ions observed. The side-chain radical loss at the C-terminus was demonstrated to be a consecutive process. Finally, evidence is provided that the specific side-chain cleavages can be used for the differentiation of Leu/Ile and Gln/Lys residues when they are located at the C-terminus. The existence of a zwitterionic form in the case of the anionic YGGFK-CuII complex is proposed.

Isotope and Affinity Tags in Photoreactive Substance P Analogues To Identify the Covalent Linkage within the NK-1 Receptor by MALDI-TOF Analysis

Photoreactive analogues of substance P (biotin sulfone-spacer (amino pentanoic or Gly(3))-Arg-Pro-Lys-Pro-(pBzl)Phe-Gln-Phe-Phe-Gly-Leu-Met(O(2))NH(2)) with or without isotope (deuterium) labeling have been synthesized. Deuteriums were present on (d)-biotin or epibiotin sulfone (D(3)), on the Gly(3) spacer linker (D(6)), or on the Gly in position 9 of SP (D(2)). Therefore, peptide analogues could be either unlabeled or tri-, penta-, or hexadeuterated. Results obtained with the use of these peptide analogues show that (d)-biotin sulfone and epibiotin sulfone are not recognized with the same affinity by streptavidin, with (d)-biotin sulfone displaying better affinity for the protein. Photolabeling of the human NK-1 receptor with a 1:1 molar ratio of nondeuterated and deuterated photoreactive substance P (SP) analogues in position 5, followed by combined digestions, purification, and MALDI-TOF mass spectrometry analysis, made the identification of the domain of the receptor covalently linked by the photoreactive SP analogue easier. Indeed, doublets in mass spectra were specific for the covalent complex whereas single peaks could be attributed to contaminating species. This method is particularly suitable when minute amounts of complex have to be analyzed, as in the case of highly hydrophobic G-protein coupled receptors.

Structural and biological effects of a beta2- or beta3-amino acid insertion in a peptide

Molecular mechanics calculations on conformers of Ac-HGly-NHMe, Ac-beta2-HAla-NHMe and Ac-beta3-HAla-NHMe indicate that low-energy conformations of the beta-amino acids backbone, corresponding to gauche rotamers around the Calpha-Cbeta bond, may overlap canonical backbone conformers observed for alpha-amino acids. Therefore, Substance P (SP) was used as a model peptide to analyse the structural and biological consequences of the substitution of Phe7 and Phe8 by (R)-beta2-HPhe and of Gly9 by HGly (R)-beta2-HAla or (S)-beta3-HAla. [(R)-beta2-HAla9]SP has pharmacological potency similar to that of SP while [HGly9]SP and [(S)-beta3-HAla9]SP show a 30- to 50-fold decrease in biological activities. The three analogues modified at position 9 are more resistant to degradation by angiotensin converting enzyme than SP and [Ala9]SP. NMR analysis of these SP analogues suggest that a beta-amino acid insertion in position 9 does not affect the overall backbone conformation. Altogether these data suggest that [HGly9]SP, [(S)-beta3-HAla9]SP and [(R)-beta2-HAla9]SP could adopt backbone conformations similar to that of SP, [Ala9]SP and [Pro9]SP. In contrast, incorporation of beta2-HPhe in position 7 and 8 of SP led to peptides that are almost devoid of biological activity. Thus, a beta-amino acid could replace an alpha-amino acid within the sequence of a bioactive peptide provided that the additional methylene group does not cause steric hindrance and does not confine orientations of the side chain to regions of space different from those permitted in the alpha-amino acid.

Electrospray mass spectrometric study of anionic complexes of enkephalins with Cu(II): regioselective distinction of Leu/Ile at the C-terminus induced by metal reduction

The yield of metallation of methionine-enkephalin and leucine-enkephalin isomers by copper(II) chloride was investigated by electrospray ionization ion trap mass spectrometry (ESI-ITMS) in negative ionization mode. Binary ([(M-3H)+Cu(II)](-)) and ternary ([(M-3H)+Cu(II)Cl](-)) complexes were observed. Soft and hard desolvation conditions (by changing the declustering voltage) were applied to study their influence on the metallation yield and on the observed deprotonated and metallated species. Structures of the binary complexes with defined charge locations are proposed, based on the observed in-source fragmentations. It was demonstrated that the in-source fragmentations under hard desolvation conditions could differentiate the Leu/Ile isomers if located at the C-terminal position but not at the N-terminal position. This behavior was also observed using a triple quadrupole analyzer. This facile distinction, due to a different radical loss from the [(M-3Hbond;CO(2))+Cu(II)](-) species (loss of [C(3)H(7)](.) for YGGFL and [C(2)H(5)](.) for YGGFI), was facilitated by the reduction of the oxidation state of Cu(II). This in-source differentiation of YGGFI and YGGFL was also implemented in LC/ESI-MS analysis by post-column addition of the copper salt with a syringe pump.