Macrocyclic Antibiotics Research Articles

A classic antibiotic reimagined: Rationally designed bacitracin variants exhibit potent activity against vancomycin-resistant pathogens

Bacitracin is a macrocyclic peptide antibiotic that is widely used as a topical treatment for infections caused by gram-positive bacteria. Mechanistically, bacitracin targets bacteria by specifically binding to the phospholipid undecaprenyl pyrophosphate (C55PP), which plays a key role in the bacterial lipid II cycle. Recent crystallographic studies have shown that when bound to C55PP, bacitracin adopts a highly ordered amphipathic conformation. In doing so, all hydrophobic side chains align on one face of the bacitracin-C55PP complex, presumably interacting with the bacterial cell membrane. These insights led us to undertake structure-activity investigations into the individual contribution of the nonpolar amino acids found in bacitracin. To achieve this we designed, synthesized, and evaluated a series of bacitracin analogues, a number of which were found to exhibit significantly enhanced antibacterial activity against clinically relevant, drug-resistant pathogens. As for the natural product, these next-generation bacitracins were found to form stable complexes with C55PP. The structure-activity insights thus obtained serve to inform the design of C55PP-targeting antibiotics, a key and underexploited antibacterial strategy.

Semi-synthesis and structure-activity relationship study yield antibacterial vicenistatin derivatives with low cytotoxicity.

Vicenistatin (1) is a 20-membered polyketide macrocyclic antibiotic with potent antimicrobial and cytotoxic activities. In this study, to further explore the potential of 1 as candidates of antibacterial drug development, 4'-N-demethyl vicenistatin (2), a secondary metabolite obtained from the ∆vicG mutant strain of Monodonata labio-associated Streptomyces parvus SCSIO Mla-L010, was utilized as a starting material for modifications of 4'-amino group of vicenistatin. Six new vicenistatin derivatives (3-8) were semi-synthesized through a concise route of amino modification with various aliphatic and aromatic aldehydes. Our study reveals that the bioactivity of vicenistatin is closely related to amino modification in sugar moiety, which results from the length of alkyl side chain as well as the presence of electron withdrawing/denoting group on the benzene ring. Importantly, compounds 4 with a butyl group and 8 with a 3,5-dihydroxyl-benzyl group at 4'-amino group, respectively, exhibited good antimicrobial activities, with MIC values spanning 0.5-4 μg ml-1 to Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus, methicillin-resistant Staphylococcus epidermidis, Micrococcus luteus and Bacillus subtilis, with low cytotoxicity. This research promotes the further exploration of structure-activity relationships of vicenistatin and provides new vicenistatin derivatives for development of future anti-infectious agents with reduced cytotoxicity.

A new family of macrocyclic antibiotics based-on Pillar[5]arene concluding multi quinoline moieties

Due to the many negative effects of increasing resistance to traditional antibiotics on living organisms, the development of new antibiotic classes has become imperative and macrocyclic compounds come to the fore for the solution of this problem. In this paper, the Pillar [5]arenes including ten and five quinoline units were synthesized and characterized by some procedures such as magnetic susceptibility, FT-IR, elemental analyses, 1H NMR, 13C NMR, melting point. Moreover, zinc(II) and copper (II) complexes were prepared to compare the antimicrobial effects with macrocyclic ligands. In the complex compounds, the metal:ligand ratios were determined with Job method as 1:2 and 1:1 for deca and penta units, respectively. The magnetic properties of the copper (II) complexes are 1.83 BM and 1.87 BM while zinc complexes have a diamagnetic character. The target macrocyclic ligands (Compound 7 and 8) and their metal complexes (Cu(II) and Zn(II)) were screened for the antimicrobial properties against the bacterial strains of Escherichia coli ATCC 25922, Staphylococcus aureus (MRSA) ATCC 43300, Klebsiella pneumoniae. ATCC 70603, Salmonella enteritidis ATTC 13076, Pseudomonas aeruginosa ATCC 27853, Bacillus cereus ATTC 11778, Sarcina lutea ATCC 9341, and yeast Candida albicans NRRL Y-417 by using broth microdilution method. Compound 7 has the optimum MIC values for S. aureus (MRSA), B. cereus, C. albicans and P. aeruginosa microorganisms that they were determined as 23.43, 46.87, 93.75 and 187 μg/mL, respectively. Moreover, Compound 8-Cu(II) complex was defined as an excellent antimicrobial agent against MRSA, B. cereus and C. albicans strains that the MIC values were determined as 3.9 μg/mL for both bacteria. The metal complexes of the target macrocyclic ligands have a more antimicrobial effect than the reactants and the target macrorings. Moreover, they have more successful results for Gram positive derivatives rather than Gram negative microorganisms.

In vitro activity of fidaxomicin and combinations of fidaxomicin with other antibiotics against Clostridium perfringens strains isolated from dogs and cats

BackgroundPrevious studies have demonstrated that fidaxomicin, a macrocyclic lactone antibiotic used to treat recurrent Clostridioides difficile-associated diarrhea, also displays potent in vitro bactericidal activity against Clostridium perfringens strains isolated from humans. However, to date, there is no data on the susceptibility to fidaxomicin of C. perfringens strains of animal origin. On the other hand, although combination therapy has become popular in human and veterinary medicine, limited data are available on the effects of antibiotic combinations on C. perfringens. We studied the in vitro response of 21 C. perfringens strains obtained from dogs and cats to fidaxomicin and combinations of fidaxomicin with six other antibiotics.ResultsWhen tested by an agar dilution method, fidaxomicin minimum inhibitory concentrations (MICs) ranged between 0.004 and 0.032 µg/ml. Moreover, the results of Etest-based combination assays revealed that the incorporation of fidaxomicin into the test medium at a concentration equivalent to half the MIC significantly increased the susceptibility of isolates to metronidazole and erythromycin in 71.4% and 61.9% of the strains, respectively, and the susceptibility to clindamycin, imipenem, levofloxacin, and vancomycin in 42.9–52.4% of the strains. In contrast, ¼ × MIC concentrations of fidaxomicin did not have any effect on levofloxacin and vancomycin MICs and only enhanced the effects of clindamycin, erythromycin, imipenem, and metronidazole in ≤ 23.8% of the tested strains.ConclusionsThe results of this study demonstrate that fidaxomicin is highly effective against C. perfringens strains of canine and feline origin. Although fidaxomicin is currently considered a critically important antimicrobial that has not yet been licensed for veterinary use, we consider that the results reported in this paper provide useful baseline data to track the possible emergence of fidaxomicin resistant strains of C. perfringens in the veterinary setting.

Enantioseparation of a-substituted proline analogs with macrocyclic glycopeptide-based chiral stationary phases immobilized on superficially porous particles of silica applying liquid chromatography with ultraviolet and mass spectrometric detection

In this study, the liquid chromatography-based direct enantioseparation of the stereoisomers of α-substituted proline analogs has been investigated utilizing chiral stationary phases with UV and/or mass spectrometric (MS) detection. Macrocyclic antibiotics, such as vancomycin, teicoplanin, modified teicoplanin, and teicoplanin aglycone, all covalently immobilized to 2.7 μm superficially porous silica particles have been applied as stationary phases. Mobile phases utilizing mixtures of methanol and acetonitrile with different additives (polar-ionic mode) were optimized during method development. Best separations were achieved with mobile phases of 100% MeOH containing either 20mM acetic acid or 20mM triethylammonium acetate. Special attention was given to the applicability of MS-compatible mobile phases. Acetic acid was found to be advantageous as a mobile phase additive for MS detection. Enantioselective chromatographic behaviors are interpreted based on the explored correlations between the analytes' structural features and those of the applied chiral stationary phases. For the thermodynamic characterization, separations were studied in the temperature range of 5-50°C. Generally, retention and selectivity decreased with increasing temperature, and in most cases, enthalpy-driven enantiorecognition was observed, but entropic contributions also were present. Unexpectedly, unusual shapes for the van Deemter curves were registered in the kinetic evaluations. General trends could be observed in the enantiomeric elution orders: S < R on VancoShell and NicoShell, and opposite R < S on TeicoShell and TagShell columns.

Teicoplanin aglycone media and carboxypeptidase Y: Tools for finding low-abundance D-amino acidsandepimericpeptides.

D-amino acids and epimeric peptides/proteins can play crucial biological roles and adversely affect protein folding and oligopeptide aggregation in age-related pathologies in humans. This has ignited interest in free D-amino acids as well as those incorporated in peptides/proteins and their effects in humans. However, such stereoisomeric analytes are often elusive and in low abundance with few existing methodologies capable of scouting for and identifying them. In this work, we examine the feasibility of using teicoplanin aglycone, a macrocyclic antibiotic, which has been reported to strongly retain D-amino acids and peptides with a D-amino acid on the C-terminus, for use as asolidphaseextraction(SPE)medium. The HPLC retention factors of L-/D-amino acids and C-terminus modified D-amino acid-containing peptides and their L-amino acid exclusive counterparts on teicoplanin aglycone are presented. Retention curve differences between amino acids and peptides highlight regions of solvent composition that can be utilized for their separation. This approach is particularly useful when coupled with enzymatic hydrolysis via carboxypeptidase Y to eliminate all L-amino acid exclusive peptides. The remaining peptides with carboxy-terminal D-amino acids are then more easily concentrated and identified.

Structurally Guided Synthesis and Development of Novel Macrocyclic Antibiotics

Structurally Guided Synthesis and Development of Novel Macrocyclic Antibiotics

Real-World Budget Impact of Fidaxomicin versus Vancomycin or Metronidazole for In-Hospital Treatment of Clostridioides difficile Infection

Fidaxomicin, a macrocyclic antibiotic, selectively kills Clostridioides difficile and reduces C. difficile infection (CDI) recurrence compared with vancomycin, but some studies and guidelines report fidaxomicin as being less cost-effective. The aim of this study was to compare the cost-effectiveness and budget impact of fidaxomicin versus vancomycin or metronidazole for treating CDI in a real-world UK setting. Data were retrospectively collected from medical records of 86 patients with CDI treated with vancomycin or metronidazole at a single UK hospital between April 2011 and March 2012, and prospectively from 62 patients with CDI treated with fidaxomicin between August 2012 and July 2013. CDI cases were matched by age, financial year, and healthcare resource use to control cases. CDI recurrence rates were lower with fidaxomicin (6.5%) than vancomycin/metronidazole (19.8%). An estimated 12 additional recurrent CDIs were prevented with fidaxomicin treatment. Patients with CDI had significantly higher healthcare costs than those without CDI, with a mean excess spend of GBP 10,748 and GBP 17,451 per patient in the fidaxomicin (p = 0.015) and vancomycin/metronidazole cohorts (p < 0.001), respectively. A second CDI was associated with mean excess costs of GBP 8373 and GBP 20,249 per patient in the fidaxomicin and vancomycin/metronidazole cohorts, respectively. Despite higher fidaxomicin drug costs, overall cost savings were estimated at GBP 140,292 (GBP 2125 per CDI). In this real-world study, first-line CDI treatment with fidaxomicin reduced healthcare costs versus vancomycin/metronidazole, consistent with previous studies.

Separation of cinchona alkaloid stereoisomers and analogues by ion mobility and chemical theoretical calculation

In this study, a simple and accurate method for the separation and quantitative analysis of cinchona alkaloid stereoisomers and analogues was developed using trapped ion mobility spectrometry (TIMS)-mass spectrometry (MS) and theoretical calculations. Stereoisomer studied were N-(9-Deoxy-epi-cinchonin-9-yl) picolinamide (N-CNP)/N-(9-Deoxy-epi-cinchonidin-9-yl) picolinamide (N-CDP), N-Benzylcinchoninium (BCN)/N-Benzylcinchonidinium (BCN), and Cinchonine (CCN)/Cinchonidine (CCD). TIMS results revealed slight mobility differences in the protonated stereoisomers, [CCD + H]+/[CCN + H]+ complexes showed the most effective separation, with a resolution (RP–P) of 0.431. When the macrocyclic antibiotics rifamycin (Rif) and natamycin (Nat) were introduced as separation reagents, the binary diastereomer complexes [Nat/Rif + CCD/CCN + H]+, [Rif + N-CDP/N-CNP + H]+, and [Rif + BCD/BCN + H]+ were formed by non-covalent interaction with the stereoisomers. The TIMS analysis revealed that the stereoisomers were separated by the formed binary complexes: CCD/CCD was separated by [CCD/CCN + Nat + H]+ (RP–P = 1.771), N-CDP/N-CNP was separated by [N-CDP/N-CNP + Rif + H]+ (RP–P = 0.914), and the BCD/BCN was separated by [BCD/BCN + Rif]+ (RP–P = 0.713). The conformations of the binary complexes were optimized using chemical theoretical calculations which revealed the microscopic interaction between the stereoisomers was different, and that the theoretical collision cross section (CCS) was consistent with the observed experimental results. An additional quantitative analysis of the stereoisomers yielded a limit of detection (LOD) ≦ 0.408 μg L−1 and satisfactory linearities (R2 > 0.99). The proposed method is simple, highly selective, highly sensitive, and without the need for chromatographic separation, and is therefore a promising option for the separation and analysis of cinchoninestereoisomers or analogues, as well as in desirable figures-of-merit for forensics.

The Application of Different Technologies for Removal of Rifampicin From Aquatic Environments: A Recent Review

Antibiotics are a group of drugs widely used as human and veterinary drugs and in aquaculture and agriculture. Recently, parent compounds and their metabolites are constantly excreted and released into environmental matrices, due to the fact that antibiotics cannot be completely metabolized after consumption by humans and animals and cannot be completely removed by conventional wastewater treatment plants. The accumulation and persistence of antibiotics in environmental matrices can lead to harmful effects on ecosystems, even at concentration levels as low as ng/L to μg/L. Rifampicin (RIF), which belongs to the macrocyclic antibiotic class, is the most important antibiotic widely used in the tuberculosis treatment. Lately, the RIF was detected in aquatic environments and needs to be removal effectively. This review considers the current state of knowledge regarding the sources, fate, effects and removal processes of the antibiotic RIF. In this review, the different treatment techniques such as adsorption, advanced oxidation processes (AOPs) and other technologies (membrane process and moving bed biofilm reactor) for RIF removal were evaluated and compared. A comparison between these techniques was made focusing on performance and efficiency. As a result, it was found that adsorption and AOPs were the most studied method and almost all of the studied RIF removal methods were also to be successful.

Silica Immobilised Chloro- and Amido-Derivatives of Eremomycine as Chiral Stationary Phases for the Enantioseparation of Amino Acids by Reversed-Phase Liquid Chromatography.

Macrocyclic glycopeptide antibiotics immobilized on silica are one of the effective classes of stationary phases for chiral recognition and HPLC separation of a wide range of optically active compounds. Enantioselectivity primarily depends on the chemical structure of the chiral ligand, immobilization chemistry, and separation conditions. In the present work, three new chiral stationary phases (CSPs) based on macrocyclic antibiotic eremomycin were prepared and investigated for enantioseparation of amino acids. Two eremomycin derivatives, including simple non-substituted amide and bulky adamantyl amide, provided important information on the role of the carboxylic group in the eremomycin structure in the chiral recognition mechanism concerning amino acid optical isomers. One more CSP having a chlorine atom in the same position elucidates the role of the first aromatic ring in the eremomycin structure as a crucial point for chiral recognition. CSP with immobilized chloreremomycin was the most successful among the phases prepared in this work. It was additionally investigated under various separation conditions, including the type and content of the organic solvent in the eluent, the effects of different additives, and the concentration and pH of the buffer. Importantly, an efficient enantioselective separation of amino acids was achieved with pure water as the eluent.

Integrated Metabolomic and Network Analysis to Explore the Potential Mechanism of Three Chemical Elicitors in Rapamycin Overproduction.

Rapamycin is a polyketide macrocyclic antibiotic with exceptional pharmacological potential. To explore the potential mechanism of rapamycin overproduction, the intracellular metabolic differences of three chemical elicitor treatments were first investigated by combining them with dynamic metabolomics and network analysis. The metabolic response characteristics of each chemical elicitor treatment were identified by a weighted gene co-expression network analysis (WGCNA) model. According to the analysis of the identified metabolic modules, the changes in the cell membrane permeability might play a key role in rapamycin overproduction for dimethyl sulfoxide (DMSO) treatment. The enhancement of the starter unit of 4,5-dihydroxycyclohex-1-ene carboxylic acid (DHCHC) and the nicotinamide adenine dinucleotide phosphate (NADPH) availability were the main functions in the LaCl3 treatment. However, for sodium butyrate (SB), the improvement of the methylmalonyl-CoA and NADPH availability was a potential reason for the rapamycin overproduction. Further, the responsive metabolic pathways after chemical elicitor treatments were selected to predict the potential key limiting steps in rapamycin accumulation using a genome-scale metabolic network model (GSMM). Based on the prediction results, the targets within the reinforcement of the DHCHC and NADPH supply were selected to verify their effects on rapamycin production. The highest rapamycin yield improved 1.62 fold in the HT-aroA/zwf2 strain compared to the control.

Terahertz time domain spectroscopic response towards different macrocyclic antibiotics using a one-step transfer graphene metamaterial absorber

To determine concentration of different antibiotics, a absorption device based on one-step transfer of graphene was designed and fabricated. The transmission spectrum of the resonant cavity is simulated and measured by using a terahertz time domain spectroscopy system, and the characteristic resonance frequency shift is observed. It is found that different concentrations of samples will lead to the blue shift of the resonant position of graphene-metamaterial absorber, and the degree of blue shift is directly proportional to the concentration of antibiotics. It provides evidence that the terahertz spectroscopy based on a metamaterial absorber may be used to develop a method for the determination of macrocyclic antibiotics.

Repurposing of potential antiviral drugs against RNA-dependent RNA polymerase of SARS-CoV-2 by computational approach

The high incidences of COVID-19 cases are believed to be associated with high transmissibility rates, which emphasizes the need for the discovery of evidence-based antiviral therapies for curing the disease. The rationale of repurposing existing classes of antiviral small molecule therapeutics against SARS-CoV-2 infection has been expected to accelerate the tedious and expensive drug development process. While Remdesivir has been recently approved to be the first treatment option for specific groups of COVID-19 patients, combinatory therapy with potential antiviral drugs may be necessary to enhance the efficacy in different populations. Hence, a comprehensive list of investigational antimicrobial drug compounds such as Favipiravir, Fidaxomicin, Galidesivir, GC376, Ribavirin, Rifabutin, and Umifenovir were computationally evaluated in this study. We performed in silico docking and molecular dynamics simulation on the selected small molecules against RNA-dependent RNA polymerase, which is one of the key target proteins of SARS-CoV-2, using AutoDock and GROMACS. Interestingly, our results revealed that the macrocyclic antibiotic, Fidaxomicin, possesses the highest binding affinity with the lowest energy value of −8.97 kcal/mol binding to the same active sites of RdRp. GC376, Rifabutin, Umifenovir and Remdesivir were identified as the next best compounds. Therefore, the above-mentioned compounds could be considered good leads for further preclinical and clinical experimentations as potentially efficient antiviral inhibitors for combination therapies against SARS-CoV-2.

Macrocyclic glycopeptides- and derivatized cyclofructan-based chiral stationary phases for the enantioseparation of fluorinated ß-phenylalanine analogs

The enantioseparation of five fluorinated β-phenylalanine analogs together with the nonfluorinated α- and β-phenylalanines has been investigated utilizing chiral stationary phases. The employed chiral selectors include macrocyclic antibiotics, such as vancomycin, teicoplanin, and teicoplanin aglycone, isopropyl carbamate functionalized cyclofructan-6, and Cinchona alkaloid-based tert.-butyl carbamate quinine, all covalently bonded to 2.7 µm superficially porous silica particles. The applied conditions included reversed-phase and polar-ionic modes where the vancomycin-, and the cyclofructan-6-based core-shell particles proved to offer suitable efficiency. Under reversed-phase conditions typical hydrophobic chromatographic behavior was observed, especially in the H2O/MeOH system. The improved selectivity with increasing MeOH content observed in polar ionic mode suggests that H-bonding may not play a major role in the chiral recognition. The stoichiometric displacement model was probed to gather information on the ionic interactions. The ion-exchange process was found to affect retention, but it has no essential contribution to chiral recognition. Without paying special attention to the optimization of the system volume of the UHPLC instrument plate heights varying in the range of 10–50 µm were obtained. In all cases, retention and selectivity decreased with increasing temperature, and enthalpy-driven enantiorecognition was observed. Elution sequences were determined in all cases.

Factor Analysis of Disinfection Byproduct Formation in Drinking Water Distribution Systems Through the Bayesian Network

Disinfection byproducts (DBPs) in drinking water distribution systems are affected by multi-factors, such as basic water quality parameters, microbial community structures, and residual organic pollutants that cannot be removed by the water treatment process. The relationship between the above-mentioned factors that forms a complicated network structure, which causes the dominating factor that affects DBPs formation unclear. This study investigated the water quality in regional tap water in January-February 2021. Trihalomethanes were determined using P&T-GC-MS, and antibiotics and nitrosamines were determined using UPLC-MS/MS. Microbial communities were determined using Illumina 16S rRNA gene sequencing. A Bayesian network was constructed to evaluate the intercorrelation between the factors. Three species of trihalomethanes, six species of nitrosamines, 23 types of antibiotics, and 236 OTUs were detected in the tap water. The mass concentrations of trihalomethanes, nitrosamines, and antibiotics were 18.33-32.09 μg·L-1, 13.08-53.50 ng·L-1, and 47.92-210.33 ng·L-1, respectively. The dominant microbial orders were Rhizobiales and Caulobacterales. Based on the Bayesian-network inference, tetracycline, sulfonamides, and macrocyclic antibiotics were precursors of trihalomethanes, whereas tetracyclines were the nitrosamine precursor. The abundances of Caulobacterales and Corynebacteriales were both affected by antibiotics and associated with DBPs formation. The extracellular polymeric substances of these bacteria were highly suspected to be important DBPs precursors. The results of the proposed project revealed the internal relationship between multi-water-quality parameters and DBPs formation, which could provide a theoretical support to guarantee the safety of drinking water.

Crossregulation of rapamycin and elaiophylin biosynthesis by RapH in Streptomyces rapamycinicus.

Rapamycin is an important macrocyclic antibiotic produced by Streptomyces rapamycinicus. In the rapamycin biosynthetic gene cluster (BGC), there are up to five regulatory genes, which have been shown to play important roles in the regulation of rapamycin biosynthesis. Here, we demonstrated that the rapamycin BGC-situated LAL family regulator RapHco-ordinately regulated the biosynthesis of both rapamycin and elaiophylin. We showed that rapH overexpression not only resulted in enhanced rapamycin production but also led to increased synthesis of another type I polyketide antibiotic, elaiophylin. Consistent with this, rapH deletion resulted in decreased production of both antibiotics. Through real-time RT-PCR combined with β-glucuronidase reporter assays, four target genes controlled by RapH, including rapL (encoding a lysine cyclodeaminase)/rapH in the rapamycin BGC and ela3 (encoding a LuxR family regulator)/ela9 (encoding a hypothetical protein) in the elaiophylin BGC, were identified. A relatively conserved signature sequence recognized by RapH, which comprises two 4-nt inverted repeats separated by 8-nt, 5'-GTT/AC-N8-GTAC-3', was defined. Taken together, our findings demonstrated that RapH was involved in co-ordinated regulation of two disparate BGCs specifying two unrelated antibiotics, rapamycin and elaiophylin. These results further expand our knowledge of the regulation of antibiotic biosynthesis in S. rapamycinicus. KEY POINTS: • The cluster-situated regulator RapH controlled the synthesis of two antibiotics. • Four promoter regions recognized by RapH were identified. • A 16-nt signature DNA sequence essential for RapH regulation was defined.

Recent progress in the development of chiral stationary phases for high-performance liquid chromatography.

Separations and analyses of chiral compounds are important in many fields, including pharmaceutical production, preparation of chemical intermediates, and biochemistry. High-performance liquid chromatography using a chiral stationary phase is regarded as one of the most valuable methods for enantiomeric separation and analysis because it is highly efficient, is broadly applicable, and has powerful separation capability. The focus for development of this method is the identification of novel chiral stationary phases with superior recognition performance and good stability. The present article reviews recent progress in the development of new chiral stationary phases for high-performance liquid chromatography between January 2018 and June 2021. These newly reported chiral stationary phases are divided into three categories: small organic molecule-based (cyclodextrin and its derivatives, macrocyclic antibiotics, cinchona alkaloids, and other low molecular weight chiral molecules), macromolecule-based (cellulose and amylose derivatives, chitin and chitosan derivatives, and synthetic helical polymers) and chiral porous material-based (chiral metal-organic frameworks, chiral covalent organic frameworks, and chiral inorganic mesoporous silicas). Each type of chiral stationary phase is discussed in detail.

Adsorption of Binary Solvents on Chiral Stationary Phases with Grafted Macrocyclic Antibiotics

A study is performed of the adsorption of water–methanol and water–acetonitrile mixtures on chiral stationary phases (CSPs) Chirobiotic R, Chirobiotic T, and Nautilus-E with grafted macrocyclic antibiotics ristocetin A, teicoplanin, and eremomycin, respectively. The patterns of adsorption on the indicated CSPs are qualitatively the same, and differ only by quantitative indicators. Adsorption isotherms of excess water from binary solvents have adsorption azeotrope points and show the preferred absorption of water in the range of pure organic component to an azeotrope point in the range of 60–75 mol % for H2О–МеОН and 80–90 mol % for H2O–MeCN systems. It is shown that the thickness of the adsorption phase in the first case is less than one nominal molecular layer (0.10–0.13 nm). For H2O–MeCN, it is 3–4 molecular layers (0.88–1.05 nm). Activity coefficients are calculated for the components of solutions in surface layers. The coefficients indicate the systems deviate considerably from the properties of an ideal adsorption solution. Reasons for this behavior are discussed.

Enantioselective metabolomics by liquid chromatography-mass spectrometry

Metabolomics strives to capture the entirety of the metabolites in a biological system by comprehensive analysis, often by liquid chromatography hyphenated to mass spectrometry. A particular challenge thereby is the differentiation of structural isomers. Common achiral targeted and untargeted assays do not distinguish between enantiomers. This may lead to information loss. An increasing number of publications demonstrate that the enantiomeric ratio of certain metabolites can be meaningful biomarkers of certain diseases emphasizing the importance of introducing enantioselective analytical procedures in metabolomics. In this work, the state-of-the-art in the field of LC-MS based metabolomics is summarized with focus on developments in the recent decade. Methodologies, tagging strategies, workflows and general concepts are outlined. Selected biological applications in which enantioselective metabolomics has documented its usefulness are briefly discussed. In general, targeted enantioselective metabolomics assays are often based on a direct approach using chiral stationary phases (CSP) with polysaccharide derivatives, macrocyclic antibiotics, chiral crown ethers, chiral ion exchangers, donor-acceptor phases as chiral selectors. Rarely, these targeted assays focus on more than 20 analytes and usually are restricted to a certain metabolite class. In a variety of cases, pre-column derivatization of metabolites has been performed, especially for amino acids, to improve separation and detection sensitivity. Triple quadrupole instruments are the detection methods of first choice in targeted assays. Here, issues like matrix effect, absence of blank matrix impair accuracy of results. In selected applications, multiple heart cutting 2D-LC (RP followed by chiral separation) has been pursued to overcome this problem and alleviate bias due to interferences. Non-targeted assays, on the other hand, are based on indirect approach involving tagging with a chiral derivatizing agent (CDA). Besides classical CDAs numerous innovative reagents and workflows have been proposed and are discussed. Thereby, a critical issue for the accuracy is often neglected, viz. the validation of the enantiomeric impurity in the CDA. The majority of applications focus on amino acids, hydroxy acids, oxidized fatty acids and oxylipins. Some potential clinical applications are highlighted.