AHL-dependent quorum sensing inhibition: Synthesis and biological evaluation of α-(N-alkyl-carboxamide)-γ-butyrolactones and α-(N-alkyl-sulfonamide)-γ-butyrolactones

Programmed alternating current optimization of Cu-catalyzed C-H bond transformations

Direct current (DC) electrosynthesis, which has undergone optimization over the past century, plays a pivotal role in a variety of industrial processes. Alternating current (AC) electrosynthesis, characterized by polarity reversal and periodic fluctuations, may be advantageous for multiple chemical reactions, but apparatus, principles, and application scenarios remain underdeveloped. In this work, we introduce a protocol for programmed AC (pAC) electrosynthesis that systematically adjusts currents, frequencies, and duty ratios. The application of representative pAC waveforms facilitates copper-catalyzed carbon-hydrogen bond cleavage in cross-coupling and difunctionalization reactions that exhibit suboptimal performance under DC and chemical oxidation conditions. Moreover, observing catalyst dynamic variation under diverse waveform applications provides mechanistic insight.

Amide bioisosteric replacement in the design and synthesis of quorum sensing modulators

The prevention or control of bacterial infections requires continuous search for novel approaches among which bacterial quorum sensing inhibition is considered as a complementary antibacterial strategy. Quorum sensing, used by many different bacteria, functions through a cell-to-cell communication mechanism relying on chemical signals, referred to as autoinducers, such as N-acyl homoserine lactones (AHLs) which are the most common chemical signals in this system. Designing analogs of these autoinducers is one of the possible ways to interfere with quorum sensing. Since bioisosteres are powerful tools in medicinal chemistry, targeting analogs of AHLs or other signal molecules and mimics of known QS modulators built on amide bond bioisosteres is a relevant strategy in molecular design and synthetic routes. This review highlights the application of amide bond bioisosteric replacement in the design and synthesis of novel quorum sensing inhibitors.

Synthesis of Mixed Phosphonate Esters and Amino Acid-Based Phosphonamidates, and Their Screening as Herbicides

While organophosphorus chemistry is gaining attention in a variety of fields, the synthesis of the phosphorus derivatives of amino acids remains a challenging task. Previously reported methods require the deprotonation of the nucleophile, complex reagents or hydrolysis of the phosphonate ester. In this paper, we demonstrate how to avoid these issues by employing phosphonylaminium salts for the synthesis of novel mixed n-alkylphosphonate diesters or amino acid-derived n-alkylphosphonamidates. We successfully applied this methodology for the synthesis of novel N-acyl homoserine lactone analogues with varying alkyl chains and ester groups in the phosphorus moiety. Finally, we developed a rapid, quantitative and high-throughput bioassay to screen a selection of these compounds for their herbicidal activity. Together, these results will aid future research in phosphorus chemistry, agrochemistry and the synthesis of bioactive targets.

Synthetic non-toxic anti-biofilm agents as a strategy in combating bacterial resistance

The tremendous increase in the bacterial resistance to the available antibiotics is a serious problem for the treatment of various infections. Biofilm formation in bacteria significantly contributes to the bacterial survival in host cells, and is considered as an crucial factor, responsible for bacterial resistance. The response of the bacterial cells in the biofilm to antibiotics is completely different from that of the free floating planktonic cells of the same strain. The anti-biofilm agents that could inhibit the biofilm production without affecting the bacterial growth, apply less selective pressure over the bacterial strains than the traditional antibiotics; thus the development of bacterial resistance would be of low incidence. Many attempts have been performed to discover novel agents capable of interfering with the bacterial biofilm life cycle, and several compounds have shown promising activities in suppressing the biofilm production or in dispersing mature existing biofilms. This review describes the different chemical classes that have anti-biofilm effects against different Gram-positive and Gram-negative bacteria without affecting the bacterial growth.

Modified N-acyl-L-homoserine lactone compounds abrogate Las-dependent quorum-sensing response in human pathogen Pseudomonas aeruginosa

Quorum sensing (QS) is a mode of cell-cell communication that bacteria use to sense population density and orchestrate collective behaviors. The common opportunistic human pathogen Pseudomonas aeruginosa employs QS to regulate a large set of genes involved in virulence and host-pathogen interactions. The Las circuit positioned on the top of the QS hierarchy in P. aeruginosa makes use of N-acyl-L-homoserine lactones (AHLs) as signal molecules, like N-3-oxo-dodecanoyl-L-homoserine lactone (3O-C12-HSL). Disabling QS circuits by certain small-molecule compounds, known as quorum-sensing inhibitors (QSIs), has been proposed as a strategy to attenuate bacterial pathogenicity. In this study, four new AHL analogs were designed by incorporating a tert-butoxycarbonyl Boc group in amide and β-keto (3-oxo) moiety. Compounds were evaluated on a molecular and phenotypic basis as a QSI using the screening strategy linked to the assignment of the Las QS system in P. aeruginosa. Using a LasR-based bioreporter, we found that the compounds decreased LasR-controlled light activity and competed efficiently with natural 3O-C12-HSL. The compounds reduced the production of the cognate 3O-C12-HSL and certain virulence traits, like total protease activity, elastase activity, pyocyanin production, and extracellular DNA release. Furthermore, a quantitative proteomic approach was used to study the effect of the compounds on QS-regulated extracellular proteins. Among the four compounds tested, one of them showed the most significant difference in the appearance of the 3O-C12-HSL-responsive reference proteins related to QS communication and virulence, i.e., a distinct activity as a QSI. Moreover, by combining experimental data with computational chemistry, we addressed the effect of LasR protein flexibility on docking precision and assessed the advantage of using a multi-conformational docking procedure for binding mode prediction of LasR modulators. Thus, the four new AHL compounds were tested for their interaction with the AHL-binding site in LasR to identify the key interferences with the activity of LasR. Our study provides further insight into molecular features that are required for small-molecule modulation of LasR-dependent QS communication in P. aeruginosa. This should facilitate rational design of the next generation of antivirulence tools to study and manipulate QS-controlled fitness in bacteria and, thereby, handle bacterial infections in a new way.

An Overview of Biofilm Formation-Combating Strategies and Mechanisms of Action of Antibiofilm Agents

Biofilm formation on surfaces via microbial colonization causes infections and has become a major health issue globally. The biofilm lifestyle provides resistance to environmental stresses and antimicrobial therapies. Biofilms can cause several chronic conditions, and effective treatment has become a challenge due to increased antimicrobial resistance. Antibiotics available for treating biofilm-associated infections are generally not very effective and require high doses that may cause toxicity in the host. Therefore, it is essential to study and develop efficient anti-biofilm strategies that can significantly reduce the rate of biofilm-associated healthcare problems. In this context, some effective combating strategies with potential anti-biofilm agents, including plant extracts, peptides, enzymes, lantibiotics, chelating agents, biosurfactants, polysaccharides, organic, inorganic, and metal nanoparticles, etc., have been reviewed to overcome biofilm-associated healthcare problems. From their extensive literature survey, it can be concluded that these molecules with considerable structural alterations might be applied to the treatment of biofilm-associated infections, by evaluating their significant delivery to the target site of the host. To design effective anti-biofilm molecules, it must be assured that the minimum inhibitory concentrations of these anti-biofilm compounds can eradicate biofilm-associated infections without causing toxic effects at a significant rate.

(2R)- and (2S)- 2-hydroxy- hexanoyl and octanoyl-l-homoserine lactones: New highly potent Quorum Sensing modulators with opposite activities

The synthesis and the QS modulation activity of diastereoisomerically pure 2-hydroxy-N-acyl-l-homoserine lactones (2-OH-AHLs) are unveiled. (2R)- and (2S)- 2-hydroxy-N-hexanoyl-l-homoserine lactone and 2-hydroxy-N-octanoyl-l-homoserine lactone have been identified as very potent QS agonists and antagonists on the Vibrio fischeri-quorum sensing system with opposite activities depending on the configuration of the carbon atom with the hydroxyl group. Flexible molecular docking showed that the (2R)-OH configuration in the antagonist isomer induces new hydrogen bonds with Tyr70 and Asp79, two importantly conserved residues in the LuxR protein family, while the (2S)-OH agonist configuration exhibits a binding mode comparable to the natural ligand 3-oxo-hexanoyl-l-homoserine lactone (OHHL). For the analogs with long alkyl chain 3a and 3b and aromatic analogs, all are antagonists with no effect of the configuration at C-2.

Biological Evaluation and Docking Studies of New Carbamate, Thiocarbamate, and Hydrazide Analogues of Acyl Homoserine Lactones as Vibrio fischeri-Quorum Sensing Modulators

A series of carbamate, thiocarbamate, and hydrazide analogues of acylhomoserine lactones (AHLs) were synthesized and their ability to modulate Vibrio fischeri-quorum sensing was evaluated. The compounds in the series exhibit variable side chain length and the possible presence of a diversely substituted phenyl substituent. Biological evaluation on the Vibrio fischeri quorum sensing system revealed that the ethyl substituted carbamate (1) display a weak agonistic activity whereas compounds with longer chain length or benzyl substituents display significant antagonistic activity. The most active compounds in the series were the 4-nitrobenzyl carbamate and thiocarbamate 7 and 11 which exhibited an IC₅₀ value of about 20 µM. These activities are in the range of other reported of AHL-structurally related quorum sensing (QS) inhibitors. Docking experiments conducted on the LuxR model showed that, compared to the natural ligand OHHL, the additional heteroatom of the carbamate group induces a new hydrogen bond with Tyr70 leading to a different global hydrogen-bond network. Tyr70 is an important residue in the binding site and is strictly conserved in the LuxR family. For the 4-nitrobenzyl carbamate and thiocarbamate analogues, the docking results highlight an additional hydrogen bond between the nitro group and Lys178. For hydrazide analogues, which are deprived of any activity, docking shows that the orientation of the carbonyl group is opposite as compared with the natural ligand, leading to the absence of a H-bond between the C=O with Tyr62. This suggests that, either this later interaction, or the influence of the C=O orientation on the overall ligand conformation, are essential for the biological activity.

Furanone quorum-sensing inhibitors with potential as novel therapeutics against Pseudomonas aeruginosa

Micro-organisms use quorum sensing (QS), a cell density-dependent process, to communicate. This QS mode of interchange leads to the production of a variety of virulence factors, co-ordination of complex bacterial behaviours, such as swarming motility, degradation of host tissue and biofilm formation. QS is implicated in numerous human infections and consequently researchers have sought ways of effectively inhibiting the process in pathogenic bacteria. Two decades ago, furanones were the first class of chemical compounds identified as Pseudomonas aeruginosa QS inhibitors (QSIs). P. aeruginosa is a ubiquitous organism, capable of causing a wide range of infections in humans, including eye and ear infections, wound infections and potentially fatal bacteraemia and thus novel treatments against this organism are greatly needed. This review provides a brief background on QS and the use of furanones as QSIs. Based on the effectiveness of action, both in vivo and in vitro, we will explore the use of furanones as potential antimicrobial therapeutics and conclude with open questions.

Influence of the d/l configuration of N-acyl-homoserine lactones (AHLs) and analogues on their Lux-R dependent quorum sensing activity

Whereas l-3-oxo-hexanoyl homoserine lactone (OHHL) is the active enantiomer of the of LuxR-regulated quorum sensing (QS) autoinducer, its d isomer is implicitly considered as inactive. The present work aims to clarify this l-specificity and investigate whether it extends to some analogues in the acyl homoserine lactone (AHL) family. For this purpose, OHHL and a series of AHL analogs were synthesized in racemic and enantiomerically pure d and l forms and their ability to induce or attenuate bioluminescence in the LuxR-dependent QS system was evaluated. In this study, l-isomers are confirmed as either the only, or as the most active, enantiomers. However, in several cases, especially for the natural ligand of LuxR (OHHL) and the very similar AHL agonist analogue 2, the d-isomer cannot be considered as totally inactive on QS. Molecular modelling suggests that when the lactone moiety of the d-isomer is able to twist, enabling the lactone carbonyl group and the amide function to interact with the key residues in the binding site, then the d-isomer can exhibit some activity.

Low-Temperature, Transition-Metal-Free Cross-Dehydrogenative Coupling Protocol for the Synthesis of 3,3-Disubstituted Oxindoles

A new low-temperature procedure for the synthesis of 3,3-disubstituted 2-oxindoles via cross-dehydrogenative coupling (CDC) is reported. The use of a strong, nonreversible base in these reactions has been found to effect a dramatic drop in reaction temperature (to room temperature) relative to the current state-of-the-art (>100 °C) procedure. When employing iodine as an "oxidant", new evidence suggests that this transformation may occur via a transiently stable iodinated intermediate rather than by direct single-electron oxidation.

The Effect of Carboxamide/Sulfonamide Replacement in Arylpiperazinylalkyl Derivatives on Activity to Serotonin and Dopamine Receptors

A series of carboxamide and sulfonamide alkyl (p-xylyl and benzyl) 1-(2-methoxyphenyl)piperazine (o-OMe-PhP) and 1-(2,3-dichlorophenyl)piperazine (2,3-DCPP) analogs were prepared and tested for their affinity to bind to serotonin 5-HT_1A /5-HT₆ /5-HT₇ and dopamine D₂ receptors. This chemical modification let us explore the impact of the replacement of the carboxamide by the sulfonamide group on the affinity changes. In both the o-OMe-PhP and 2,3-DCPP series, the relative activities of the carboxamides versus sulfonamides toward the 5-HT_1A /5-HT₆ /5-HT₇ and D₂ receptors show similar trends. Varied or similar activities for particular receptors were found for the carboxamides/sulfonamides with p-xylyl spacer, while of the two classes of carboxamides and sulfonamides examined, benzyl derivatives of the sulfonamides displayed the highest serotoninergic affinity, in particular to the 5-HT₇ receptors (K_i 8-85 nM). The K_i values revealed that, irrespective of the carboxamide/sulfonamide zone, both p-xylyl and benzyl derivatives had the highest affinity for the dopamine D₂ receptor (i.e., 16 out of 24 compounds investigated have an affinity below 100 nM). A molecular modeling study of carboxamide 9a and sulfonamide 9b showed that their binding effects to each of 5-HT_1A R and D₂ R created binding modes interaction with different conserved receptors residues. Structural similarities of carboxamide 9a in complexes with a 5-HT_1A R (9aI) and D₂ R (9aII) are over 83%, while the respective similarities of sulfonamide 9b structures (9bI/9bII) are only about 40%.

Burkholderia cenocepacia Infections in Cystic Fibrosis Patients: Drug Resistance and Therapeutic Approaches

Burkholderia cenocepacia is an opportunistic pathogen particularly dangerous for cystic fibrosis (CF) patients. It can cause a severe decline in CF lung function possibly developing into a life-threatening systemic infection known as cepacia syndrome. Antibiotic resistance and presence of numerous virulence determinants in the genome make B. cenocepacia extremely difficult to treat. Better understanding of its resistance profiles and mechanisms is crucial to improve management of these infections. Here, we present the clinical distribution of B. cenocepacia described in the last 6 years and methods for identification and classification of epidemic strains. We also detail new antibiotics, clinical trials, and alternative approaches reported in the literature in the last 5 years to tackle B. cenocepacia resistance issue. All together these findings point out the urgent need of new and alternative therapies to improve CF patients’ life expectancy.

Ultrasonic-assisted synthesis of 1,4-disubstituted 1,2,3-triazoles via various terminal acetylenes and azide and their quorum sensing inhibition

An efficient synthesis of 1,4-disubstituted 1,2,3-triazole derivatives was studied. 1,4-Disubstituted 1,2,3-triazoles containing isoxazole and thymidine structures were synthesized in 84-96% yields starting from various terminal isoxazole ether alkynes and β-thymidine azide derivatives via a 1,3-dispolar cycloaddition using copper acetate, sodium ascorbate as the catalyst under ultrasonic assisted condition. All the target compounds were characterized by HRMS, FT-IR, ¹H NMR and ¹³C NMR spectroscopy. Furthermore, the quorum sensing inhibitory activities of synthesized compounds were evaluated with Chromobacterium violaceum (C. Violaceum CV026) based on their inhibition of violacein production, with compound C₁₀-HSL as a positive control. The compounds 8a, 8c and 8f exhibited considerable levels of inhibitory activity against violacein production, and IC₅₀ values were 217±19, 223±20 and 42.8±4.5μM, respectively, which highlighted the potential of these compounds as lead structures for further research towards the development of novel QS inhibitors.

Quorum Sensing Inhibition and Structure-Activity Relationships of β-Keto Esters

Traditional therapeutics to treat bacterial infections have given rise to multi-drug resistant pathogens, which pose a major threat to human and animal health. In several pathogens, quorum sensing (QS)-a cell-cell communication system in bacteria-controls the expression of genes responsible for pathogenesis, thus representing a novel target in the fight against bacterial infections. Based on the structure of the autoinducers responsible for QS activity and other QS inhibitors, we hypothesize that β-keto esters with aryl functionality could possess anti-QS activity. A panel of nineteen β-keto ester analogs was tested for the inhibition of bioluminescence (a QS-controlled phenotype) in the marine pathogen Vibrio harveyi. Initial screening demonstrated the need of a phenyl ring at the C-3 position for antagonistic activity. Further additions to the phenyl ring with 4-substituted halo groups or a 3- or 4-substituted methoxy group resulted in the most active compounds with IC50 values ranging from 23 µM to 53 µM. The compounds additionally inhibit green fluorescent protein production by E. coli JB525. Evidence is presented that aryl β-keto esters may act as antagonists of bacterial quorum sensing by competing with N-acyl homoserine lactones for receptor binding. Expansion of the β-keto ester panel will enable us to obtain more insight into the structure-activity relationships needed to allow for the development of novel anti-virulence agents.

Novel reporter for identification of interference with acyl homoserine lactone and autoinducer-2 quorum sensing

Two reporter strains were established to identify novel biomolecules interfering with bacterial communication (quorum sensing [QS]). The basic design of these Escherichia coli-based systems comprises a gene encoding a lethal protein fused to promoters induced in the presence of QS signal molecules. Consequently, these E. coli strains are unable to grow in the presence of the respective QS signal molecules unless a nontoxic QS-interfering compound is present. The first reporter strain designed to detect autoinducer-2 (AI-2)-interfering activities (AI2-QQ.1) contained the E. coli ccdB lethal gene under the control of the E. coli lsrA promoter. The second reporter strain (AI1-QQ.1) contained the Vibrio fischeri luxI promoter fused to the ccdB gene to detect interference with acyl-homoserine lactones. Bacteria isolated from the surfaces of several marine eukarya were screened for quorum- quenching (QQ) activities using the established reporter systems AI1-QQ.1 and AI2-QQ.1. Out of 34 isolates, two interfered with acylated homoserine lactone (AHL) signaling, five interfered with AI-2 QS signaling, and 10 were demonstrated to interfere with both signal molecules. Open reading frames (ORFs) conferring QQ activity were identified for three selected isolates (Photobacterium sp., Pseudoalteromonas sp., and Vibrio parahaemolyticus). Evaluation of the respective heterologously expressed and purified QQ proteins confirmed their ability to interfere with the AHL and AI-2 signaling processes.

N,N'-alkylated Imidazolium-derivatives act as quorum-sensing inhibitors targeting the Pectobacterium atrosepticum-induced symptoms on potato tubers

Bacteria belonging to the Pectobacterium genus are the causative agents of the blackleg and soft-rot diseases that affect potato plants and tubers worldwide. In Pectobacterium, the expression of the virulence genes is controlled by quorum-sensing (QS) and N-acylhomoserine lactones (AHLs). In this work, we screened a chemical library of QS-inhibitors (QSIs) and AHL-analogs to find novel QSIs targeting the virulence of Pectobacterium. Four N,N′-bisalkylated imidazolium salts were identified as QSIs; they were active at the μM range. In potato tuber assays, two of them were able to decrease the severity of the symptoms provoked by P. atrosepticum. This work extends the range of the QSIs acting on the Pectobacterium-induced soft-rot disease.

Design, synthesis and biological evaluation of N-sulfonyl homoserine lactone derivatives as inhibitors of quorum sensing in Chromobacterium violaceum

A novel series of N-sulfonyl homoserine lactone derivatives 5a–l has been designed, synthesized and evaluated for quorum sensing inhibitory activities towards violacein production. Of the compounds synthesized, compound 5h was found to possess an excellent level of enantiopurity (99.2% e.e.). The results indicated that compounds bearing an ortho substituent on their phenyl ring exhibited excellent levels of inhibitory activity against violacein production. Compounds 5h and 5k in particular, with IC₅₀ values of 1.64 and 1.66 µM, respectively, were identified as promising lead compounds for further structural modification.

Synthesis and biological evaluation of triazole-containing N-acyl homoserine lactones as quorum sensing modulators

Many bacterial species are capable of assessing their local population densities through a cell-cell signaling mechanism termed quorum sensing (QS). This intercellular communication process is mediated by small molecule or peptide ligands and their cognate protein receptors. Numerous pathogens use QS to initiate virulence once they achieve a threshold cell number on a host. Consequently, approaches to intercept QS have attracted considerable attention as potential anti-infective therapies. Our interest in the development of small molecule tools to modulate QS pathways motivated us to evaluate triazole-containing analogs of natural N-acyl L-homoserine lactone (AHL) signals as non-native QS agonists and antagonists in Gram-negative bacteria. We synthesized 72 triazole derivatives of five broad structure types in high yields and purities using efficient Cu(I)-catalyzed azide-alkyne couplings. These compounds were evaluated for their ability to activate or inhibit two QS receptors from two prevalent pathogens - LasR from Pseudomonas aeruginosa and AbaR from Acinetobacter baumannii- using bacterial reporter strains. Several triazole derivatives were identified that were capable of strongly modulating the activity of LasR and AbaR. These compounds represent a new and synthetically accessible class of AHL analogs, and could find utility as chemical tools to study QS and its role in bacterial virulence.

Synthesis and evaluation of the quorum sensing inhibitory effect of substituted triazolyldihydrofuranones

Acylhomoserine lactone (AHL) analogues in which the amide function is replaced by a triazole group were synthesized and evaluated for their effect on quorum sensing (QS) and biofilm formation in Burkholderia cenocepacia and Pseudomonas aeruginosa. In addition, the influence of the length of the acyl-mimicking chain was investigated. The compounds showed selectivity between two different AHL QS systems. 3-(1H-1,2,3-Triazol-1-yl)dihydrofuran-2(3H)-ones, in which the 4-substituent best resembled the acyl chain of the native AHL molecule exhibited significant QS agonistic and antagonistic activities. Replacing this aliphatic substituent by a phenyl-containing moiety resulted in active inhibitors of QS. The most active compounds showed biofilm inhibitory as well as biofilm eradicating activities in both test organisms.

Design, synthesis and biological evaluation of non-natural modulators of quorum sensing in Pseudomonas aeruginosa

Many species of bacteria employ a mechanism of intercellular communication known as quorum sensing which is mediated by small diffusible signalling molecules termed autoinducers. The most common class of autoinducer used by Gram-negative bacteria are N-acylated-L-homoserine lactones (AHLs). Pseudomonas aeruginosa is a clinically important bacterium which is known to use AHL-mediated quorum sensing systems to regulate a variety of processes associated with virulence. Thus the selective disruption of AHL-based quorum sensing represents a strategy to attenuate the pathogenicity of this bacterium. Herein we describe the design, synthesis and biological evaluation of a collection of structurally novel AHL mimics. A number of new compounds capable of modulating the LasR-dependent quorum sensing system of P. aeruginosa were identified, which could have value as molecular tools to study and manipulate this signalling pathway. Worthy of particular note, this research has delivered novel potent quorum sensing antagonists, which strongly inhibit the production of virulence factors in a wild type strain of this pathogenic bacterium.