Amino acid enantiomers separation factors

The use of chiral membrane solvent to achieve transport stereoselectivity is not very widespread. Probably, the only report describing chiral hquid-phase-enhanced stereoselective transport is amino acid enantiomers separation by means of chiral alcohols, nopol and (2S)-(—)-methylbutan-l-ol [38] (see Fig. 3.9). It was shown that optical separation of six pairs of enantiomers of amino acids is possible in this way. However, the chiral discrimination (expressed as a flux ratio for both enantiomers and denoted as a) was moderate and the best result was 1for serine. As a main conclusion, it was stated that the factor involved in chiral discrimination was an asymmetry of the amino acid molecule. 

Structurally to vancomycin (Fig. 1), shows promising efficacy toward native amino acid enantiomer separation (Table 2) [8], The eremomycin enantioselectivity factors of amino acids are comparable if not higher than those of teicoplanin (Table 2). 

The capacity ratios and the separation factors of amino acid enantiomers derivatized with (+)-l-(9-fluororenyl)ethyl chloroformate measured in... 

In the case of DNB amino acids, the relevance of the mechanism was established by the following results (i) Methyl esterification of the amino acid carboxylic group cancels all chiral recognition making the docking approach impossible (ii) A relationship between log a, the enanfioselecfivity factor, and log 2. the retention factor of the most retained enantiomer, was found depending on the amino acid side chain size. The retention factors, k, of the first eluting DNB amino acids were similar [41] (iii) The enanfioselecfivity factors and elution order, respectively, obtained on the quinine and quinidine CSPs are similar and opposite for the same enantiomeric pairs (iv) Native amino acid enantiomers are not separated (no n-n interaction) [41]. 

The importance of chemical syntheses of a-amino acids on industrial scale is limited by the fact that the standard procedure always yields the racemic mixture (except for the achiral glycine H2N-CH2-COOH and the corresponding amino acid from symmetrical ketones R-CO-R). A subsequent separation of the enantiomers then is a major cost factor. Various methods for the asymmetric synthesis of a-amino acids on laboratory scale have been developed, and among these are asymmetric Strecker syntheses as well. ... 

The improvements in resolution achieved in each deconvolution step are shown in Figure 3-3. While the initial library could only afford a modest separation of DNB-glutamic acid, the library with proline in position 4 also separated DNP derivatives of alanine and aspartic acid, and further improvement in both resolution and the number of separable racemates was observed for peptides with hydrophobic amino acid residues in position 3. However, the most dramatic improvement and best selectivity were found for c(Arg-Lys-Tyr-Pro-Tyr-(3-Ala) (Scheme 3-2a) with the tyrosine residue at position 5 with a resolution factor as high as 28 observed for the separation of DNP-glutamic acid enantiomers. 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

In simple experiments, particulate silica-supported CSPs having various cin-chonan carbamate selectors immobilized to the surface were employed in an enantioselective liquid-solid batch extraction process for the enantioselective enrichment of the weak binding enantiomer of amino acid derivatives in the liquid phase (methanol-0.1M ammonium acetate buffer pH 6) and the stronger binding enantiomer in the solid phase [64]. For example, when a CSP with the 6>-9-(tcrt-butylcarbamoyl)-6 -neopentoxy-cinchonidine selector was employed at an about 10-fold molar excess as related to the DNB-Leu selectand which was dissolved as a racemate in the liquid phase specified earlier, an enantiomeric excess of 89% could be measured in the supernatant after a single extraction step (i.e., a single equilibration step). This corresponds to an enantioselectivity factor of 17.7 (a-value in HPLC amounted to 31.7). Such a batch extraction method could serve as enrichment technique in hybrid processes such as in combination with, for example, crystallization. In the presented study, it was however used for screening of the enantiomer separation power of a series of CSPs. 

Later, a commercially available TAG CSP was tested in the enantioseparation of 10 secondary a-amino acids, by using RP mobile mode systems [154]. The chromatographic results, compared with those obtained on a native teicoplanin CSP, were given as the retention, separation, and resolution factors, together with the enanti-oselective free energy difference corresponding to the separation of the investigated enantiomers. 

The resorcarene 23a, on the other hand, was obtained by O-alkylation of the corresponding octol. Its incorporation in a dimethylpolysiloxane backbone led to a stationary phase by which proteinogenic amino acids could be separated into their enantiomers by GC of their 7V(0,S)-trifluoroacetylmethylesters with separation factors aLD = 1.025-1.102.50 The question remains in this case (and in similar cases), whether the chiral amide functions have to be attached to the resorcarene skeleton, or if a direct attachment to the polymer backbone via suitable spacers would lead to similar results. The chiral resorcarene octaamides 23b prepared by... 

Jandera et al. [35] measured by frontal analysis the competitive isotherms of the enantiomers of mandeHc acid, phenyl-glycine and tryptophan on the glyco-peptide Teicoplanin, in water/methanol or ethanol solutions. The less retained L enantiomers of the two amino acids follow Langmuir isotherm behavior while the D isomers foUow bi-Langmuir behavior. The enantiomeric separation factors increase with increasing alcohol concentration while the solubilities of these com-poimds decrease. Similar results were reported by Loukih et al. [36] for the separation of the enantiomers of tryptophan on a teicoplanin- based CSR The authors insisted on the importance of the nature of the ions in a supporting salt. Optimization of the experimental conditions for maximum production rate must take this effect into account. 

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