Enantiomeric separation of carboxylic acids having chiral centers remote from the carboxyl group by labelling with a chiral fluorescent derivatization reagent

Polymer degrading marine Microbulbifer bacteria: an un(der)utilized source of chemical and biocatalytic novelty

Microbulbifer is a genus of halophilic bacteria that are commonly detected in the commensal marine microbiomes. These bacteria have been recognized for their ability to degrade polysaccharides and other polymeric materials. Increasingly, Microbulbifer genomes indicate these bacteria to be an untapped reservoir for novel natural product discovery and biosynthetic novelty. In this review, we summarize the distribution of Microbulbifer bacteria, activities of the various polymer degrading enzymes that these bacteria produce, and an up-to-date summary of the natural products that have been isolated from Microbulbifer strains. We argue that these bacteria have been hiding in plain sight, and contemporary efforts into their genome and metabolome mining are going to lead to a proliferation of Microbulbifer-derived natural products in the future. We also describe, where possible, the ecological interactions of these bacteria in marine microbiomes.

Branched medium-chain fatty acid profiling and enantiomer separation of anteiso-forms of teicoplanin fatty acyl side chain RS3 using UHPLC-MS/MS with polysaccharide columns

This work reports on targeted UHPLC-tandem mass spectrometry methods for the chiral separation of anteiso-methyl branched fatty acids (aiFAs). The methods involve precolumn derivatization with 1-naphthylamine and chiral separation on Chiralpak IG-U. anteiso-Methyl branched fatty acids with up to eight carbons can be separated. A method was used for the assignment of the absolute configuration of an aiFA present as fatty acyl residue of the teicoplanin mixture, namely teicoplanin RS3. Furthermore, the excellent methylene selectivity and improved selectivity for constitutional isomers of the polysaccharide columns was exploited for the elucidation and structural confirmation of previously unknown fatty acyl residues in teicoplanin. This shows the versatility and practical applicability of polysaccharide columns as orthogonal stationary phases to reversed-phase for structural elucidation of natural compounds. The developed methods are useful tools for related subdisciplines such as targeted metabolomics and lipidomics.

Nocarimidazoles C and D, antimicrobial alkanoylimidazoles from a coral-derived actinomycete Kocuria sp.: application of 1 J C,H coupling constants for the unequivocal determination of substituted imidazoles and stereochemical diversity of anteisoalkyl chains in microbial metabolites

Chemical investigation of secondary metabolites from a marine-derived actinomycete strain of the genus Kocuria, isolated from a stony coral Mycedium sp., led to the identification of two new alkanoylimidazoles, nocarimidazoles C (1) and D (2) as well as three known congeners, nocarimidazoles A (3) and B (4) and bulbimidazole A (5). Structure analysis of 1 and 2 by NMR and MS revealed that both are 4-alkanoyl-5-aminoimidazoles with a 6-methyloctanoyl or decanoyl chain, respectively. Two possible positions of the amino group on the imidazole rings (C-2 and C-5) posed a challenge in the structure study, which was settled by the measurement of ¹J_C,H coupling constants for comparison with those of synthetically prepared model imidazoles. The absolute configurations of the anteisoalkanoyl group present in 1, 4, and 5 were determined by low-temperature HPLC analysis of the degradation products labeled with a chiral anthracene reagent, which revealed that 1 is a mixture of the R- and S-enantiomers with a ratio of 73:27, 4 is the pure (S)-enantiomer, and 5 is the (S)-enantiomer with 98% ee. The present study illustrates the diversity in the stereochemistry of anteiso branching in bacterial metabolites. Compounds 1−4 were moderately antimicrobial against Gram-positive bacteria and fungi, with MIC ranges of 6.25–25 μg/mL.

Bulbimidazoles A-C, Antimicrobial and Cytotoxic Alkanoyl Imidazoles from a Marine Gammaproteobacterium Microbulbifer Species

Three new alkanoyl imidazoles, designated bulbimidazoles A-C (1-3), were found from the culture extract of the gammaproteobacterium Microbulbifer sp. DC3-6 isolated from a stony coral of the genus Tubastraea. The absolute configuration of the anteiso-methyl substitution in 1 was established to be a mixture of (R)- and (S)-configurations in a ratio of 9:91 by applying the Ohrui-Akasaka method. Compounds 1-3 displayed unique broad-spectrum antimicrobial activity against Gram-positive and -negative bacteria and fungi with MICs ranging from 0.78 to 12.5 μg/mL. They also exhibited cytotoxicity toward P388 murine leukemia cells with IC₅₀ in the micromolar range.

Enatiomeric Separation of Branched Fatty Acids after Conversion with trans-2-(2,3-Anthracenedicarboximido)cyclohexanol, a Highly Sensitive Chiral Fluorescent Conversion Reagent

(1R,2R)-2-(2,3-Anthracenedicarboximido)cyclohexanol was synthesized as a highly sensitive chiral fluorescent conversion reagent. The diastereomeric derivatives of chiral branched fatty acids that had methyl ethyl chirality from the 2 to 12 position were separated into 2 peaks by reversed-phase HPLC and detected at the 10(-15) mole level by fluorometry.

Combined urea-thin layer chromatography and silver nitrate-thin layer chromatography for micro separation and determination of hard-to-detect branched chain fatty acids in natural lipids

A simple, fast and efficient procedure was developed for micro separation and enrichment of branched chain fatty acids (BCFA) from natural products using successive thin layer chromatography (TLC) technique coupling novel urea-TLC with AgNO3-TLC, which rely on the formation of urea adduction and AgNO3 bonding in methanol. These natural lipids contain a significant amount of straight chain fatty acids (FA). Fresh and fast urea-TLC and AgNO3-TLC plate making techniques were developed with more even coating and less coating material contamination before being utilized for separation. Goat milk fat was used as a model. Various experimental parameters that affect urea-TLC and AgNO3-TLC separation of BCFA were investigated and optimized, including coating of urea, concentration of original oil sample, mobile phase and sample application format. High efficiency of removal of straight chain FA was achieved with a low amount of sample in an easy and fast way. A total BCFA mix with much higher purity than previous studies was successfully achieved. The developed method has also been applied for the concentration and analysis of BCFA in cow milk fat and Anchovy oil.

A polyaromatic molecular tube that binds long hydrocarbons with high selectivity

Long hydrocarbon chains are essential components of biomolecules used for structure and function in living organisms. The selective recognition and effective binding of hydrocarbons within synthetic host compounds are problematic owing to their conformational flexibility and the lack of specific binding sites. Here we report a molecular tube with polyaromatic frameworks prepared by the Zincke cross-coupling reaction. The tube has a well-defined cylindrical cavity with a diameter and length of ~1 nm encircled by multiple anthracene panels and thereby binds long hydrocarbons containing branched methyl groups and/or unsaturated carbon-carbon double bonds (for example, heptamethylnonane, nervonic acid ester and squalene) with high selectivity in aqueous solutions.

Total synthesis and complete structural assignment of gambieric acid A, a large polycyclic ether marine natural product

More than thirty years after the discovery of polycyclic ether marine natural products, they continue to receive intense attention from the chemical, biological, and pharmacological communities because of their potent biological activities and highly complex molecular architectures. Gambieric acids are intriguing polycyclic ethers that exhibit potent antifungal activity with minimal toxicity against mammals. Despite the recent advances in the synthesis of this class of natural products, gambieric acids remain unconquered due to their daunting structural complexity, which poses a formidable synthetic challenge to organic chemists. This paper reviews our long-term studies on the total synthesis, complete configurational reassignment, and structure-activity relationships of gambieric acid A over the last decade.

Development of Highly PotentD-Glucosamine-Based Chiral Fluorescent Labeling Reagents and a Microwave-Assisted β-Selective Glycosidation of a Methyl Glycoside Reagent

We synthesized new chiral fluorescence labeling reagents having a 2,3-anthracenedicarboximide group from D-glucosamine, and it was possible to introduce target alcohols at the anomeric positions of the reagents with beta-selectivity by glycosidations. Especially, it was possible to use methyl glycoside reagent as a glycosyl donor with a Lewis acid and microwave irradiation, and it gave selectively beta-glycoside while the reaction without microwave irradiation gave alpha- and beta-mixed glycosides. Those reagents showed very high chiral discrimination ability, and they made it possible to separate the eight stereoisomers of 4,8,12,16-tetramethylheptadecanol by HPLC after derivatizations.

Direct Determination of the Stereoisomeric Composition of Callosobruchusic Acid, the Copulation Release Pheromone of the Azuki Bean Weevil, Callosobruchus chinensis L., by the 2D-Ohrui-Akasaka Method

The stereoisomeric composition of the copulation release pheromone of the azuki bean weevil, Callosobruchus chinensis L., was determined to be R:S=3.3-3.4:1 by the 2D-Ohrui-Akasaka method.

Chiral discrimination of primary amines by HPLC after labeling with a chiral derivatization reagent,trans-2-(2,3-anthracenedicarboximido)-cyclohexanecarbonyl chloride

Enantiomeric discrimination of chiral primary amines was performed by both reversed-phase HPLC and normal-phase HPLC after labeling with a chiral fluorescent derivatization reagent, (1R,2R)- and (1S,2S)-trans-2-(2,3-anthracenedicarboximido)cyclohexanecarbonyl chloride. Use of HPLC permits separation of diastereomeric derivatives of amines up to C30 which have a primary amino group at the middle of the alkyl chain. The derivatives of primary amines having an anteiso alkyl chain, which has a chiral branched-methyl at the n-3 position of the alkyl chain, were also separated by HPLC, and it was also possible to separate niphatesine D by reversed-phase HPLC after derivatization.

Chiral discrimination of secondary alcohols by both1H-NMR and HPLC after labeling with a chiral derivatization reagent, 2-(2,3-anthracenedicarboximide)cyclohexane carboxylic acid

Enantiomeric discrimination of chiral secondary alcohols was performed by both reversed-phase HPLC and 1H-NMR after labeling with a chiral fluorescent derivatization reagent, 2-(2,3-anthracenedicarboximide)cyclohexanecarboxylic acid. It was possible to discriminate by HPLC the chirality of alcohols up to C30 having a chiral hydroxyl group at the middle of the straight alkyl chain, and, by 1H-NMR, alcohols up to C16. For alcohols having one straight alkyl and one isoalkyl group with the same carbon numbers at both sides of a hydroxyl group, it was possible to discriminate the chiralities of alcohols up to C19 by both 1H-NMR and HPLC. The 1H-NMR methods also made it possible to determine absolute configurations empirically.

Highly Potent Chiral Labeling Reagents for the Discrimination of Chiral Alcohols*

Highly sensitive chiral labeling reagents, (1R,2R)- and (1S,2S)-2-(2,3-anthracenedicarboximido)cyclohexanecarboxylic acids [(1R,2R)- and (1S,2S)-A] and (1R,2R)- and (1S,2S)-2-(2,3-naphthalenedicarboximido)cyclohexanecarboxylic acids [(1R,2R)- and (1S,2S)-A'], were prepared. Reagent A has enabled us to discriminate the enantiomers of anteiso fatty alcohols up to C9 methyl branching by 1H NMR, and both reagents A and A' have allowed up to C16 methyl branching at the 10(-15) molar level by fluorescence detected reversed-phase HPLC.

Resolution of chiral drugs by liquid chromatography based upon diastereomer formation with chiral derivatization reagents

Chiral derivatization reagents for resolution of biologically important compounds, such as chiral drugs by high-performance liquid chromatography (HPLC), based upon pre-column derivatization and diastereomer formation, are reviewed. The derivatization reagents for various functional groups, i.e., amine, carboxyl, carbonyl, hydroxyl and thiol, are evaluated in terms of reactivity, stability, wavelength, handling, versatility, sensitivity, and selectivity. The applicability of the reagents to the analyses of drugs and bioactive compounds are included in the text.

Pharmaceutical and biomedical applications of enantioseparations using liquid chromatographic techniques

The chiral separation methods using liquid chromatographic techniques can be divided into two categories: one is a direct method, which is based on a diastereomer formation on stationary phase or in mobile phase. The other is an indirect method, which is based on a diasteromer formation by reaction with a homochiral reagent. The enantiomer separation on a chiral stationary phases followed by derivatization with an achiral reagent is also dealt with this review article as the indirect method. The pharmaceutical and biomedical applications of enantioseparations using the direct and indirect methods have been considered in this review.

Absolute configuration of a ceramide with a novel branched-chain fatty acid isolated from the epiphytic dinoflagellate, Coolia monotis

The absolute configuration of the chiral center at the C15 position of a novel branched-chain fatty acid derived from a new ceramide isolated from the epiphytic dinoflagellate Coolia monotis was determined to be of R from by reversed-phase HPLC after cleavage to 12-methylpentadecanoic acid and subsequent conversion with the chiral fluorescent reagent, (1R,2R)-2-(2,3-anthracenedicarboximido)cyclohexanol.