Influence of amino acid residue on chromatographic behaviour of µ-opioid receptor agonist tetrapeptide analogue on crown ether based chiral stationary phase.

The influence of amino acid residue on µ-opioid receptor agonist tetrapeptide Tyr-Arg-Phe-Lys-NH2 analogue chromatographic behaviour on crown ether based chiral stationary phases has been investigated. S- and R-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 chiral selectors in commercially available CROWNPAK CR-I (+) and (-) columns are both each other enantiomers, thus, under the same LC conditions, retention of Tyr-Arg-Phe-Lys-NH2dxxx enantiomers (fixed in d-tyrosine position) on S-chiral selector do not significantly differ from the retention times of their lxxx antipodes, obtained on R-chiral selector (and vice versa), allowing us to study the apparent separation of a specific tetrapeptide enantiomeric pair, without obtaining the actual racemate. Ten tetrapeptides (llll-isomers) have been synthesized with the aim to cover a wider range of different amino acid classes. Histidine, glutamic acid, cysteine, leucine and tryptophan were introduced at the N-terminus or Phe position of Tyr-Arg-Phe-Lys-NH2 tetrapeptide structure. The effects of the amino acid residue with emphasis on retention, enantioseparation, as well as the influence of position of the amino acid residue in tetrapeptide sequence, are discussed.

A study of tetrapeptide enantiomeric separation on crown ether based chiral stationary phases.

The chiral separations of small peptides is an important challenge in the biological and medical sciences, because different stereoisomers of chiral drugs can often possess different pharmacological, pharmacokinetic, and/or toxicological activities. Commercially available crown ether chiral stationary phases based on S-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 (CROWNPAK CR-I (+)) and (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (ChiroSil RCA (+)) have been successfully used for separating enantiomers of various racemic compounds containing primary amino groups. In this investigation, enantioresolution of more complex model analyte - tetrapeptide Tyr-Arg-Phe-Lys-NH2, has been reported on crown ether chiral stationary phases. Organic and acidic modifier content in aqueous mobile phase was tested. All Tyr-Arg-Phe-Lys-NH2 stereoisomers showed U-shaped retention plots, based on ACN content in mobile phase. Increased retention of tetrapeptide stereoisomers was observed at low (<35%) and at high (>70%) acetonitrile content in the mobile phase, indicating that different separation mechanisms are most likely involved. As a result, baseline separation of all eight tetrapeptide enantiomer pairs was achieved under isocratic elution mode on both chiral columns.

Separation of piracetam derivatives on polysaccharide-based chiral stationary phases.

High-performance liquid chromatography was used for the enantiomeric separation of two chiral piracetam derivatives. The suitability of six commercially available polysaccharide-based chiral stationary phases (CSPs) under normal phase mode for direct enantioseparation has been investigated. The influence of the CSPs as well the nature and content of an alcoholic modifier in the mobile phase on separation and elution order was studied. It was established that CSP Lux Amylose-2 shows high chiral recognition ability towards 4-phenylsubstituted piracetam derivatives.

Application of Diol column under adsorption and mixed partition-adsorption normal-phase liquid chromatography mode for the separation of purines and pyrimidines.

The diol-bonded phase (column LiChrospher 100 Diol) has been studied for the separation of some purines and pyrimidines under normal-phase liquid chromatography (NPLC) conditions. Retention time, column efficiency, and selectivity of column with diol-phase were compared to those of unmodified silica (column LiChrospher SI-60). It was established that under adsorption NPLC mode application of diol-phase can reduce the separation time and a little improve the column efficiency. Significant improvement of the column efficiency for polar solutes is observed if mixed partition-adsorption NPLC mode is used. The investigation has shown that application of diol-phase instead of bare silica is useful not only under adsorption, but also under mixed partition-adsorption mode if the system with specific selectivity is necessary.

HPLC separation of some purine and pyrimidine derivatives on Chromolith Performance Si monolithic column.

The chromatographic behavior of some purines and pyrimidines on a monolithic Chromolith Performance Si column under normal-phase high-performance liquid chromatography mode has been studied. Column pressure, column efficiency and selectivity of Chromolith Performance Si column were compared to those of conventional spherical 5 microm silica packed columns Econosphere Silica and Zorbax Rx-SIL. The investigation has shown that application of Chromolith Performance Si column for analysis of polar solutes can reduce the separation time without sacrificing column efficiency and selectivity. Improvement of the monolithic silica column efficiency for polar solutes is observed when ternary mobile phases (mixtures of hexane-isopropanol with ethylene glycol, water or acetonitrile) are applied.

Determination of uracil in 5-fluorouracil substance by high-performance liquid chromatography.

Separation of 5-fluorouracil and uracil in chromatographic systems consisting of a silica column as a stationary phase and ethyl acetate or ethyl acetate partially saturated with water as a mobile phase has been studied. The results indicated that when saturation of ethyl acetate with water was chosen to reach more than 60%, such mobile phases [e.g., 2.4% (v/v) water in ethyl acetate] are useful in determining uracil in 5-fluorouracil substances.

Analysis of purines and pyrimidines by mixed partition-adsorption normal-phase high-performance liquid chromatography.

This paper summarizes the results in the development of mixed partition-adsorption (MPA) normal-phase high-performance liquid chromatography published in the last 10 years. The MPA normal-phase systems are an alternative approach not only to the adsorption normal-phase mode but also to the most widely used reversed-phase mode in the separation area of purine and pyrimidine derivatives. It is shown that the MPA systems are applicable in analytical practice.

Application of mixed partition-adsorption systems in high-performance liquid chromatography of purines and pyrimidines.

Separation of the test mixtures of some purine and pyrimidine derivatives on silicas (types A and B) in adsorption normal-phase (A-NP) and mixed partition-adsorption normal-phase (MPA-NP) mode has been studied. When the A-NP mode is used the peak shapes are unsatisfactory (especially on type A silica). At the same time MPA-NP systems show a good peak symmetry on all silica types. The findings have demonstrated that the MPA-NP mode offers a specific selectivity. This allows MPA-NP systems to supersede reversed-phase systems in some application areas.

Separation of inosine, hypoxanthine, and guanosine by high-performance liquid chromatography on silica.

The separation of inosine (Ino), hypoxanthine (Hyp), and guanosine (Guo) on silica has been studied. In adsorption normal-phase systems the peak shapes are unsatisfactory; a low selectivity has been observed for the Ino-Hyp and Guo-Hyp pairs. Chromatograms can be significantly improved if systems with a mixed partition-adsorption retention mechanism are applied.

Separation of purines and pyrimidines by normal-phase high-performance liquid chromatography using dimethyl sulfoxide in binary and ternary eluents.

Chromatographic systems with a silica sorbent and mobile phases containing dimethyl sulfoxide have been studied. It has been established that the substitution of isopropanol by dimethyl sulfoxide in binary eluents results in a specific selectivity of the chromatographic system and shows an improvement of the peak shape for the solutes under study. When mobile phases consisting of hexane, isopropanol and dimethyl sulfoxide (solvents with a limited mutual solubility) are used, changes in retention characteristics and peak symmetry are caused by a transition from adsorption to partition sorption mechanism. The stationary liquid phase is generated dynamically in the pores of silica, even in the mobile phases not saturated with a polar component. If the phase ratio of the column reaches 0.1, partition dominates over adsorption and such mixed partition-adsorption (MPA) systems show very good peak symmetry for the solutes under study. The investigation has shown that dimethyl sulfoxide-containing MPA systems are applicable in analytical practice.