Interaction between asymmetric solutes and solvents. Peptide derivatives as stationary phases in gas liquid partition chromatography

Enantiomeric separation of new phytoalexin analogs with cyclofructan chiral stationary phases in normal-phase mode

For the first time, three different derivatized cyclofructan chiral stationary phases were used for the direct high-performance liquid chromatographic enantiomeric separation of 11 new racemic analogs of a natural indole phytoalexin. This class of compounds is known to have significant antiproliferative activity and other potentially useful pharmacological properties. The effect of various experimental factors was investigated to optimize the separations in the normal-phase mode. It was found that the nature of polar modifier and additive in the mobile phase have significant impact on the enantioseparations. Better chiral recognition of analyzed compounds was achieved on (R)-naphthylethyl carbamate cyclofructan 6 than on isopropyl carbamate cyclofructan 6 and dimethylphenyl carbamate cyclofructan 7. The thermodynamic parameters showed that the chiral separation was enthalpy controlled in all cases.

Temperature-Induced Inversion of the Elution Order of Enantiomers in Gas Chromatography: N-Ethoxycarbonyl Propylamides and N-Trifluoroacetyl Ethyl Esters of α-Amino Acids on Chirasil-Val-C11 and Chirasil-Dex Stationary Phases

Inversion of the elution order of enantiomers caused by enthalpy-entropy compensation at the isoenantioselective temperature (Tiso) was experimentally observed by gas chromatography on the diamide-type chiral stationary phase (CSP), Chirasil-L-Val-C11, with N-ethoxycarbonyl propylamide (ECPA) derivatives of a number of alpha-amino acids. For the first time, a clear visual representation of the increase of the apparent enantioseparation factor alpha app from 1.00 to 1.08 as the temperature is raised from 120 to 170 degrees C is presented. The increase of alpha app is accompanied by a concomitant reduction of the retention factors of the enantiomers. The Tiso values were in the range from 110 to 130 degrees C depending on the nature of the alpha-amino acid. On the contrary, the Tiso values of the N(O)-trifluoroacetyl ethyl ester derivatives (TFA-Et) of the same alpha-amino acids were approximately 80 degrees higher than that of ECPA derivatives. The comprehensive thermodynamic investigation of the enantioseparation of ECPA and TFA-Et derivatives of valine and alanine using the retention increment method showed that the Delta L,D(DeltaH) difference between the diastereomeric selector-selectand associates was almost the same for ECPA and TFA-Et derivatives despite a much stronger bonded selector-selectand association taking place for the ECPA derivatives. On the other hand, the Delta L,D(DeltaS) values were found to be more negative in the case of ECPA derivatives, resulting in the unusually low values of Tiso. A temperature-dependent inversion of the elution order of enantiomers was also observed on the cyclodextrin-type CSP, Chirasil-Dex, with TFA-Et derivatives of several alpha-amino acids. The Tiso values were in the range from 20 to 170 degrees C depending on the nature of the alpha-amino acid. The results obtained demonstrate the necessity to conduct temperature-dependent studies in order to optimize the enantiomeric separation of single racemates isothermally or of mixtures of racemates in temperature-programmed runs using enantioselective GC. It is also shown that consideration of the elution order of enantiomers and the value of the apparent enantioseparation factor alpha app alone, without performing temperature-dependent measurements, can easily lead to wrong conclusions regarding the enantiorecognition mechanism.

Study of the mechanism of enantioseparation Part VI: Thermodynamic study of HPLC separation of some enantiomers of phenylcarbamic acid derivatives on a (S,S) Whelk-O 1 column

The enantiomers of thirteen 2-, 3-, and 4-alkoxyphenylcarbamic acid 2-methoxy-1-[(4-methylpiperazino)methyl]ethyl ester were separated on a (S,S) Whelk-O 1 CSP column isothermally in the range of 0-50 degrees C at 10 degrees C increments, using methanol/ water (90/10, v/v) containing 17.5 mmol L(-1) acetic acid and 14.36 mmol L(-1) triethylamine as a mobile phase. The dependence of the natural logarithms of retention and selectivity factors (In k, In alpha, respectively) on the inverse of temperature, 1/T, was used to determine thermodynamic data of enantiomers of alkoxysubstituted phenylcarbamic acid 2-methoxy-1-[(4-methylpiperazino) methyl]ethyl esters. Enthalpyentropy compensation plots showed that all of the compounds in this study separate via the same enthalpy-driven chiral recognition mechanism.

Probability rule for chiral recognition

Molecular Chirality is of central interest in biological studies because enantiomeric compounds, while indistinguishable by most inanimate systems, show profoundly different properties in biochemical environments. Enantioselective separation methods, based on the differential recognition of two optical isomers by a chiral selector, have been amply documented. Also, great effort has been directed towards a theoretical understanding of the fundamental mechanisms underlying the chiral recognition process. Here we report a comprehensive data examination of enantio separation measurements for over 72000 chiral selector-select and pairs from the chiral selection compendium CHIRBASE. The distribution of alpha = k'(D)/k'(L) values was found to follow a power law, equivalent to an exponential decay for chiral differential free energies. This observation is experimentally relevant in terms of the number of different individual or combinatorial selectors that need to be screened in order to observe alpha values higher than a preset minimum. A string model for enantiorecognition (SMED) formalism is proposed to account for this observation on the basis of an extended Ogston three-point interaction model. Partially overlapping molecular interaction domains are analyzed in terms of a string complementarity model for ligand-receptor complementarity. The results suggest that chiral selection statistics may be interpreted in terms of more general concepts related to biomolecular recognition.

Enantiomer separation by gas and high-performance liquid chromatography with tripeptide derivatives as chiral stationary phases

Excellent enantiomer separation of a variety of racemic compounds, including alcohols, amines, amino alcohols, carboxylic acids, hydroxy acids and amino acids, was achieved by GC and HPLC with tripeptide derivatives, containing L-valyl-L-valyl-L-valine isopropyl ester as a chiral selector, bonded to amino silicone oil (CSP-3) and N-(2-aminoethyl)-3-aminopropyl silica gel (CSP-4) via a triazine ring, respectively. These results show that the hydrogen bonding association between solutes and chiral stationary phases (CSPs) can play an important role in chiral recognition in both GC and HPLC. The joint use of two CSPs is promising for the direct separation of racemic compounds.

Direct enantiomeric resolution of mephenytoin and its N-demethylated metabolite in plasma and blood using chiral capillary gas chromatography

A gas chromatographic method was developed for the determination of the R- and S- enantiomers of the anticonvulsant, mephenytoin, and its N-demethylated metabolite, 5-phenyl-5- ethylhydantoin ( PEH ), in plasma and blood. Direct enantiomeric separation of mephenytoin and its internal standard was obtained using a chiral capillary column ( Chirasil -Val) followed by nitrogen specific detection. However, resolution of the enantiomers of PEH and its internal standard required propylation at the 3-position of the hydantoin ring prior to analysis. Similar linear and reproducible standard curves were obtained from both plasma and blood over the concentration range 50 ng/ml to 5 micrograms/ml, and above 100 ng/ml the reproducibility was less than 8% (coefficient of variation). Pronounced stereoselective differences in the plasma concentration--time curves for both mephenytoin and PEH were observed in a normal subject who received a single oral dose of 300 mg racemic mephenytoin. The peak plasma level of S-mephenytoin was only one-fifth that of the R-enantiomer and its elimination half-life was less than 3 h compared to over 70 h for R-mephenytoin. Similarly, S- PEH levels were barely detectable whereas concentrations of R-metabolite steadily increased over 4-6 days before slowly declining.

Recent advances in methods of direct optical resolution

Very great advances have been made in the field of direct optical resolution of organic compounds by chromatographic techniques. Chiral capillary gas chromatography now permits a determination of the enantiomeric composition of a few nanograms of a compound present in a mixture of many others. Coupled with high resolution mass spectrometry the technique will additionally permit structural elucidation; of great interest in pheromone research and related areas. Analytical separations of enantiomers are now also carried out by high-performance liquid chromatography (HPLC) methods based on a variety of principles. Basically, two main types are used, differing as to whether the mobile phase has to be a chiral medium or not. Two-dimensional HPLC, whereby compounds separated on a non-chiral column are progressively and automatically transferred to a chiral column for optical resolution, has been used successsfully for chiral amino acid separations. Many different chiral sorbents for preparative LC and HPLC resolutions have been prepared; some of these are now used in columns capable of producing pure enantiomers from a given racemate at a rate of the order of one gram/hour in continuous, automatic HPLC procedures. Apart from all important applications of these results of optical resolution technology, an increased knowledge of the underlying chiral recognition phenomena responsible for enantioselection has also been achieved.