Enantioselective resolution of biologically active dipeptide analogs by high-performance liquid chromatography applying Cinchona alkaloid-based ion-exchanger chiral stationary phases.

Sixteen pairs of enantiomeric dipeptides were separated on four chiral ion-exchanger-type stationary phases based on Cinchona alkaloids. Anion-exchangers (QN-AX, QD-AX) and zwitterionic phases [ZWIX(+)™ and ZWIX(-)™] were studied in a comparative manner. The effects of the nature and concentrations of the mobile phase solvent components and organic salt additives on analyte retention and enantioseparation were systematically studied in order to get a deeper insight into the enantiorecognition mechanism. Moreover, experiments were performed in the temperature range 10-50 °C to calculate thermodynamic parameters like changes in standard enthalpy, Δ(ΔH°), entropy, Δ(ΔS°), and free energy, Δ(ΔG°) on the basis of van't Hoff plots derived from the ln α vs. 1/T curves. Elution sequences of the dipeptides were determined in all cases and, with a few exceptions, they were found to be opposite on the pseudoenantiomeric stationary phases as of QN-AX/QD-AX and of ZWIX(+) and ZWIX(-). The stereoselective retention mechanism is based on electrostatically driven intermolecular interactions supported by additional interaction increments mainly determined by the absolute configuration of the chiral C8 and C9 atoms of the quinine and quinidine moieties.

Liquid chromatographic enantioseparation of limonene-based carbocyclic ß-amino acids on zwitterionic Cinchona alkaloid-based chiral stationary phases.

The eight stereoisomers of limonene-based carbocyclic ß-amino acids containing three chiral centers have been directly separated on chiral stationary phases containing Cinchona alkaloid-based zwitterionic selectors. The effects of bulk solvent composition of the mobile phase, the nature of base additives, counterion concentration, and the structure of selector on the enantiorecognition were studied. Experiments were performed at constant mobile phase composition in the temperature range 5-40°C to study the effect of temperature. Thermodynamic parameters were calculated on the basis of the plots of ln α versus 1/T curves. The enthalpically or entropically driven enantioseparations were found to depend strongly on the structures of analyte and selector. The eight stereoisomers of limonene-based carbocyclic ß-amino acids could be differentiated as well-separated peaks in a traditional 1D chromatographic system in two runs by applying the two complementary ZWIX(+)™ and ZWIX(-)™ columns.

Time-course of kynurenic acid concentration in mouse serum following the administration of a novel kynurenic acid analog.

The changes in concentration of kynurenic acid (KYNA) in different biological samples are of great interest in the pathomechanism and medication of several disorders, and especially those affecting the nervous system. Besides the recent pharmaceutical advances targeting the kynurenine pathway, there is a constant need for further drug development through the synthesis of novel analogs. Reliable analytical methods should be set up to monitor the metabolism and effects of these analogs in both preclinical experiments and human studies. Following a sample preparation procedure based on protein precipitation, new high-performance liquid chromatographic methods with fluorescence and mass spectrometric detection were developed for the determination of KYNA and a novel KYNA analog (N-(2-N,N-dimethylaminoethyl)-4-oxo-1H-quinoline-2-carboxamide hydrochloride; KYNA amide) in mouse serum samples. The analytical parameters obtained in the validation procedure suggest that the developed method with mass spectrometric detection is simple, fast, accurate and suitable for the measurement of KYNA and its analogs. The results reveal the good in vivo stability of the novel KYNA amide.

HPLC separation of amino acid enantiomers and small peptides on macrocyclic antibiotic-based chiral stationary phases: a review.

The search for new and effective chiral selectors capable of separating a wide variety of enantiomeric compounds is an ongoing process. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of HPLC, TLC and electrophoresis. More chiral analytes have been resolved through the use of glycopeptides than with all the other macrocyclic antibiotics combined (ansamycins, thiostrepton, aminoglycosides, etc.). The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. Teicoplanin, vancomycin, their analogs and ristocetin A seem to be the most useful glycopeptide HPLC bonded phases for the enantioseparation of proteins and unusal native and derivatized amino acids. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these antibiotics and their application in the enantioseparations of amino acids. The mechanism of separation, the sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.