Chiral separation, HPLC, amino

HPLC is one of the most commonly used analytical methods in chiral separations. The direct chiral separation of amino alcohols by HPLC with the chiral stationary phases has been reported in many articles (58-61). 

Ilisz, I., Berkecz, R., and Peter, A., HPLC separation of amino acid enantiomers and small peptides on macrocyclic antibiotic-based chiral stationary phases a review, J. Sep. ScL, 29, 1305, 2006. 

Piccinini, A.-M., Chiral separation of natural and unnatural amino acid derivatives by micro-HPLC on a ristocetin A stationary phase, J. Biochem. Biophys. Methods, 61, 11,2004. 

Lehotay, J. et al.. Chiral separation of enantiomers of amino acid derivatives by HPLC on vancomycin and teicoplanin chiral stationary phases, Pharmazie, 53, 863, 1998. 

CSPs and chiral mobile phase additives have also been used in the separation of amino acid enantiomers. Another technique that should be mentioned is an analysis system employing column-switching. D-and L- amino acids are first isolated as the racemic mixture by reverse-phase HPLC. The isolated fractions are introduced to a second column (a CSP or a mobile phase containing a chiral selector) for separation of enantiomers. Long et al. (2001) applied this technique to the determination of D- and L-Asp in cell culture medium, within cells and in rat blood. 

Since dorzolamide possesses two chiral centers, an indirect chiral separation has been developed [18]. The procedure employs the chemical derivatization of the secondary amino group of the inhibitor, formation of diastereomeric urea derivatives (each with three chiral centers in the molecule), and their separation under non-ehiral HPLC conditions. Using... 

Enantiomers of racemic cis-3H,7H- and frans-3H,7H-3-substituted 7-amino-l,2,3,5,6,7-hexahydropyrido[3,2,l-zy]quinazolin-l-ones were separated by using chiral preparative HPLC method (06USA2006/0004028, 07JP2007131577). 

Macrocyclic antibiotics such as teicoplanin and vancomycin have been used in chiral stationary phases separations of amino acids, drugs, and other species using HPLC and other separations methods. These applications have been reviewed in a number of recent sources, including a 2004 monograph on chiral separations. [97, 101-107]... 

Owing to the different biological activity of D- and L-enantiomers of seleno-amino acids, the chiral separation of optical isomers has been undertaken in sele-nized yeast and in yeast-based commercial supplements. Both, chiral stationary phase (crown ether) and chiral derivatization prior to reversed-phase HPLC were used [16, 77, 78],... 

Finally, libraries aimed to chiral resolution of racemates will be covered here in particular, the use of chiral stationary phases (CSPs) has recently been reported for the identification of materials to be used for chiral separation of racemates by HPLC. The group of Frechet reported the selection of two macroporous poly methacrylate-supported 4-aryl-1,4-dihydropyrimidines (DHPs) as CSPs for the separation of amino acid, anti-inflammatory drugs, and DHP racemates from an 140-member discrete DHP library (214,215) as well as a deconvolutive approach for the identification of the best selector phase from a 36-member pool library of macroporous polymethacrylate-grafted amino acid anilides (216,217). Welch and co-workers (218,219) reported the selection of the best CSP for the separation of a racemic amino acid amide from a 50-member discrete dipeptide iV-3,5-dinitrobenzoyl amide hbrary and the follow-up, focused 71-member library (220). Wang and Li (221) reported the synthesis and the Circular Dichroism- (CD) based screening of a 16-member library of CSPs for the HPLC resolution of a leucine ester. Welch et al. recentiy reviewed the field of combinatorial libraries for the discovery of novel CSPs (222). Dyer et al. (223) reported an automated synthetic and screening procedure based on Differential Scanning Calorimetry (DSC) for the selection of chiral diastereomeric salts to resolve racemic mixtures by crystallization. Clark Still rejxrrted another example which is discussed in detail in Section 9.5.4. 

Chiral separation can also be performed with packed capillaries. /3-CyD-bonded CSPs that are most frequently used in HPLC and CE were successfully applied in CEC. The separation of a variety of chiral compounds, such as some amino acid derivatives benzoin and hexobarbital was achieved by using CSPs bonded with different CyD derivatives [14,15]. Proteins are not ideal for use as buffer additives in CE because of their large detector response however, CEC may be a good way to use this type of chi-... 

TLC using silica gel coated with an L-prolinamide-copper(II) salt mixture (Chiralplates Macherey-Nagel Co.) separates enantiomers using the ligand-exchange principle to give information on the chiral purity of amino acids and peptides. The equivalent HPLC procedure has been used for determining enantiomer ratios. 

HPLC. " Different perfluorinated carboxylic acids have been examined as the ion-pairing agents (pH 2.5. 5) in the analyses of enriched yeast and vegetable extracts Alkylsulfonic acids have been successfully used in speciation analysis carried out on nuts, onion leaves and yeast " " Owing to their different biological activities, optical enantiomers of Se-amino acids were analyzed by several chiral separation techniques - ... 

The design and development of effective chiral separation and recognition of enantiomers is the key point of the chiral technique. Many technologies have been developed for chiral recognition and separation of amino acids and their derivatives including HPLC, CE and electrochemical sensors. However, they are expensive techniques in terms of time and reagent consumption. Accordingly, there is considerable interest in the development of simple, rapid and economical methods that will afford the analysis of enantiomeric species. 

Figure 6 Chiral HPLC separation of amino acid derivatives on imprinted stationary phases packed with Z-L-Glu-OH (a), Boc-L-phe-Gly-OEt (b), Z-L-Ala-L-Ala-Ome (c) Z-L-Ala-Gly-L-Phe-OMe (d). Mobile phase chloroform - acetic acid. Column 250 x 4.6 mm. Flow rate, 1 mL/min. Detection, UV 260 nm. Reproduced from Ref. 22, with permission.

Ding et al. described the chiral separation of enantiomers of several dansyl-amino acids by HPLC in the reversed-phase mode. The natural logarithms of selectivity factors (In a) of all the investigated compounds depended linearly upon the reciprocal of temperature (1/T). For most processes of enantioseparation, enantioselectivity, a, decreased with increasing of temperature, and the processes of chiral recognition were enthalpy-controlled. It is very interesting that enantioselectivity, a, increased with increasing temperature for dansyl-threonine (Dns-Thr) at a... 

Terminal alkynes substituted with chiral substituents have been polymerized by using a rhodium catalyst, [RhCl(NBD)]2 (NBD = norbomadiene) [6]. As shown in Scheme 3, polymerization of a chiral (carbamoyloxy)phenylacetylene 4 forms a cis-substituted polyacetylene 5. Due to the bulkiness of the substituents, these polymers show a helical conformation with no extended conjugation in the polymer chain. These materials are potentially useful as enantioselective permeable membranes to separate racemic amino acids and alcohols in water or in methanol. They can be also used as chiral stationary phase for enantioselective high-performance liquid chromatography (HPLC) analysis. 

Marchelli R, Corradini R, Galavema G et al (2006) Enantioselective separation of amino acids and hydroxy acids by ligand exchange with copptafll) cmnplexes in HPLC (chiral eluent) and fast sensing systems. In Subramanian G (ed) Chiral separation techniques. Wiley-VCH, Weinheim... 

Chiral separations can be carried out using an amino acid-based or a cyclodextrin-type (Section 3.5.5.2) stationary phase. The latter is most common, and the selectivity is based on surface interactions or inclusion. Usually rather low temperatures are used as enantioselectivity is rare above 200 °C. Because of higher plate numbers, capillary GC columns generally give better enantiomeric separations than HPLC columns. 

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