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 a solid phase extraction (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.
Quorum Sensing allows bacteria to sense their population density via diffusible N-acyl homoserine lactone (N-HL) signaling molecules. Upon reaching a high enough cell density, bacteria will collectively exhibit a phenotype. Until recently, methods used for detection of N-HLs have not considered the chirality of these molecules and it was assumed that only the L-enantiomer was produced by bacteria. The production and effects of D-N-HLs have rarely been studied. In this work, the temporal production of D-N-HLs by the plant pathogen Pectobacterium atrosepticum and the human pathogen Pseudomonas aeruginosa are reported. Both bacteria produced D-N-HLs in significant amounts and in some cases their concentrations were higher than other low abundance L-N-HLs. Previously unreported D-enantiomers of N-3-oxoacyl and N-3-hydroxyacyl homoserine lactones were detected in P. atrosepticum. Interestingly, L-N-HLs produced in the lowest concentrations had relatively higher amounts of their corresponding D-enantiomers. Potential sources of D-N-HLs and their significance are considered.
N-Acyl homoserine lactones (N-HLs) are signaling molecules used by Gram-negative bacteria in a phenomenon called quorum sensing (QS). Bacteria will detect N-HLs as a way of monitoring their population which, upon reaching a critical level, will express a specific phenotype. An example is the expression of bioluminescence by Vibrio fischeri. A limited number of studies have considered the chirality of these molecules nor used highly sensitive detection methods. Here, the production of D,L-N-HLs are monitored for V. fischeri, B. cepacia, P. fluorescens and P. putida, using highly sensitive tandem mass spectrometry analysis. Novel N-HLs are reported for both V. fischeri and B. cepacia including a plethora of previously unknown D-N-HLs, including the first D-N-HLs containing oxo and hydroxy functionalities. Anomalously, N-HLs were not detected in any cultures of P. fluorescens and P. putida, which are species that previously were reported to produce N-HLs. However, it is apparent that differences in the reported occurrence and levels of N-HLs can result from: different strains of bacteria, b) different growth media and environmental conditions and c) sometimes false positive results from detection methodologies. Time studies of V. fischeri suggest the possibility that separate synthetic and elimination pathways exist between D- and L-N-HLs. Possible biological processes that could be the source of D-N-HL production are considered.
Derivatives of the nido‐7,8‐C2B9H12(1‐) (dicarbollide ion) and [3,3′‐Co‐(1,2‐C2B9H11)2](1‐) cobalt sandwich (COSAN) ion represent groups of extremely chemically and thermally stable abiotic compounds. They are being investigated in many research areas, that is, medicinal chemistry, material sciences, analytical chemistry, and electrochemistry. The chirality of these compounds remains still grossly overlooked, what is also reflected in limited number of reports on their chiral separations. Continued progress depends on reliable, fast, and cost‐effective methods for such separations. Recently, chiral separations of COSAN derivatives were achieved in liquid chromatography and supercritical fluid chromatography. Only five anionic derivatives of nido‐7,8‐C2B9H12(1‐) were successfully enantioseparated in liquid chromatography. Efforts to separate anionic nido‐7,8‐C2B9H12(1‐) in supercritical chromatography have failed, and only a few dicarbollide ions were separated using liquid chromatography. Generally, all chiral separations in liquid chromatography took about 30 min. Herein, we identify a versatile column capable of separating both COSAN and nido‐7,8‐C2B9H12(1‐) derivatives and achieve faster analyses times employing commercially available superficially porous chiral stationary phases. The semisynthetic hydroxypropyl β‐cyclodextrin‐based column (CDShell‐RSP) is identified as the column of choice from the tested columns by separating 19 of 27 compounds from each structural motifs tested mainly in less than 10 min. The dihydroxyalkyl, oxygen‐bridged hydroxyalkyl, and bisphenylene‐bridged COSAN derivatives were baseline separated in less than 5 min exceeding the results of supercritical fluid chromatography. Methods developed herein will aid synthetic chemists without the possession of a supercritical fluid chromatograph to achieve fast chiral separations of COSAN and derivatives of nido‐7,8‐C2B9H12(1‐) on a common liquid chromatograph without the need of dedicated instrumentation.
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