Cyclodextrin separation

Fig. 3-114. Separation of a-, / -, and y-cyclodextrin. -Separator column CarboPac PA-1 eluent (A) 0.1 mol/L NaOH, (B) 0.1 mol/L NaOH + 0.5 mol/L NaOAc gradient linear, 100% A to 100% B in 20 min flow rate 1 vaU min detection and injection volume see Fig. 3-105 solute concentrations 500 ppm each of a- (1), ft- (2), and y-cyclo-dextrin (3).

Figure 3.240 Separation of a-, fi-, and /-cyclodextrin. Separator column CarboPac PA1 eluent (A) 0.1 mol/L NaOH, (B) 0.1 mol/L NaOH-l-0.5 mol/L NaOAc gradient linear,...

Triton X-100 and y-cyclodextrin, and subsequent charge separation via reductive quenching Chem. Phys. Lett. 223 511-16... 

This experiment introduces the use of a chiral column (a 3-cyclodextrin-bonded Cjg column) to separate the beta-blocker drugs Inderal LA (S-propranolol and... 

Conradi, S. Vogt, C. Rohde, E. Separation of Enatiomeric Barbiturates by Capillary Electrophoresis Using a Cyclodextrin-Containing Run Buffer, /. Chem. Educ. 1997, 74, 1122-1125. 

Fig. 3. The chiral separation obtained for oxa2epam on a sulfated cyclodextrin hplc column (4.6 mm ID x 25 cm) using a 10% acetonitrile/buffer (25 mM...

Gyclodextrins. As indicated previously, the native cyclodextrins, which are thermally stable, have been used extensively in Hquid chromatographic chiral separations, but their utihty in gc appHcations was hampered because their highly crystallinity and insolubiUty in most organic solvents made them difficult to formulate into a gc stationary phase. However, some functionali2ed cyclodextrins form viscous oils suitable for gc stationary-phase coatings and have been used either neat or diluted in a polysiloxane polymer as chiral stationary phases for gc (119). Some of the derivati2ed cyclodextrins which have been adapted to gc phases are 3-0-acetyl-2,6-di-0-pentyl, 3-0-butyryl-2,6-di-0-pentyl,... 

Inclusion compounds of the Cg aromatic compounds with tris((9-phenylenedioxy)cyclotriphosphazene have been used to separate the individual isomers (43—47). The Schardinger dextrins, such as alpha-cyclodextrin, beta-dextrin, and gamma-dextrin are used for clathration alpha-dextrin is particularly useful for recovering PX from a Cg aromatic mixture (48,49). PyromeUitic dianhydride (50) and beryllium oxybenzoate (51) also form complexes, and procedures for separations were developed. 

Analytically, the inclusion phenomenon has been used in chromatography both for the separation of ions and molecules, in Hquid and gas phase (1,79,170,171). Peralkylated cyclodextrins enjoy high popularity as the active component of hplc and gc stationary phases efficient in the optical separation of chiral compounds (57,172). Chromatographic isotope separations have also been shown to occur with the help of Werner clathrates and crown complexes (79,173). 

Immobilization. The abiUty of cyclodextrins to form inclusion complexes selectively with a wide variety of guest molecules or ions is well known (1,2) (see INCLUSION COMPOUNDS). Cyclodextrins immobilized on appropriate supports are used in high performance Hquid chromatography (hplc) to separate optical isomers. Immobilization of cyclodextrin on a soHd support offers several advantages over use as a mobile-phase modifier. For example, as a mobile-phase additive, P-cyclodextrin has a relatively low solubiUty. The cost of y- or a-cyclodextrin is high. Furthermore, when employed in thin-layer chromatography (tic) and hplc, cyclodextrin mobile phases usually produce relatively poor efficiencies. 

Cyclodextrin stationary phases utilize cyclodextrins bound to a soHd support in such a way that the cyclodextrin is free to interact with solutes in solution. These bonded phases consist of cyclodextrin molecules linked to siUca gel by specific nonhydrolytic silane linkages (5,6). This stable cyclodextrin bonded phase is sold commercially under the trade name Cyclobond (Advanced Separation Technologies, Whippany, New Jersey). The vast majority of all reported hplc separations on CD-bonded phases utilize this media which was also the first chiral stationary phase (csp) developed for use in the reversed-phase mode. 

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

Fig. 2. Chromatogram showing (a) the Ic separation of A, (+) (T)-(l-ferrocenyl-ethyl)thioethanol B, (+) 1-ferrocenyl-l-methoxyethane and C, (+) 1-mthenocenylethanol, on a 25-cm P-cyclodextrin column (see Table 2), and (b) the potential use of a P-cyclodextrin column to determine optical purity...

Table 2. Retention Data for Racemic Compounds Separated on a p-Cyclodextrin Stationary Phase ...

Low cholesterol egg products are formed by extraction of cholesterol from the egg. Attempts have been made to extract cholesterol by using hexane or by supercritical CO2 extraction methods (24,25). A whole egg product in which 80% of the cholesterol is removed by a process using beta-cyclodextrin, a starch derivative, added to egg yolks has been introduced. The cyclodextrin binds up to 80% of the cholesterol, the mixture is centrifuged, and the Hquid separated. The cholesterol-reduced yolk is then blended with egg white, pasteurized, and packed in asceptic containers to give a Hquid whole egg product having a shelf Hfe of 60 days under refrigeration (see Eood packaging). 

Catechin and epicatechin are two flavanols of the catechin family. They are enantiomers. The capillary zone electrophoresis (CE) methods with UV-detection were developed for quantitative determination of this flavanols in green tea extracts. For this purpose following conditions were varied mnning buffers, pH and concentration of chiral additive (P-cyclodextrin was chosen as a chiral selector). Borate buffers improve selectivity of separation because borate can make complexes with ortho-dihydroxy groups on the flavanoid nucleus. 

Finally, temperature can have a profound effect on column performance. Tanaka et al. (48) studied the effects of temperature on the separation of hydrolyzed /3-cyclodextrin. In their studies, resolution increased with temperature on a Toyopearl HW-40S column. 

Figure 3.7 shows some early examples of this type of analysis (39), illustrating the GC determination of the stereoisomeric composition of lactones in (a) a fruit drink (where the ratio is racemic, and the lactone is added artificially) and (b) a yoghurt, where the non-racemic ratio indicates no adulteration. Technically, this separation was enabled on a short 10 m slightly polar primary column coupled to a chiral selective cyclodextrin secondary column. Both columns were independently temperature controlled and the transfer cut performed by using a Deans switch, with a backflush of the primary column following the heart-cut. 

Figure 3.7 [continued) (b) Chromatograms of (iii) the dichloromethane extract of strawberry fruit yoghurt analysed with an apolar primary column, with the heart-cut regions indicated, and (iv) a non-racemic mixture of y-deca-(Cio) and 7-dodeca-Cj2 lactones isolated by heart-cut transfer, and separated by using a chiral selective modified cyclodextrin column. Reproduced from A. Mosandl, et al J. High Resol. Chromatogr. 1989, 12, 532 (39f. 

It is in the study of this phenomenon where two-dimensional GC offers by far the most superior method of analysis. The use of chiral selector stationary phases, in particular modified cyclodextrin types, allows apolar primary and atropisomer selective secondary separation. Reported two-dimensional methods have been successful... 

Figure 3.8 Second-dimension chiral cyclodextrin capillary column separation of a non-racemic pair of nonachlorobomane compounds extracted from dolphin blubber, shown with expanded attenuation in the inset. The primary separation (not shown) was performed on an apolar primary capillary column. Reproduced from H.-J. de Geus et al. J. High Resol. Chromatogr. 1998, 21, 39 (59).

Recently, multidimensional GC has been employed in enantioselective analysis by placing a chiral stationary phase such as a cyclodextrin in the second column. Typically, switching valves are used to heart-cut the appropriate portion of the separation from a non-chiral column into a chiral column. Heil et al. used a dual column system consisting of a non-chiral pre-column (30 m X 0.25 mm X 0.38 p.m, PS-268) and a chiral (30 m X 0.32 mm X 0.64 p.m, heptakis(2,3-di-(9-methyl-6-(9-tert-butyldimethylsilyl)-(3-cyclodextrin) (TBDM-CD) analytical column to separate derivatized urinary organic acids that are indicative of metabolic diseases such as short bowel syndrome, phenylketonuria, tyrosinaemia, and others. They used a FID following the pre-column and an ion trap mass-selective detector following the... 

The type of CSPs used have to fulfil the same requirements (resistance, loadabil-ity) as do classical chiral HPLC separations at preparative level [99], although different particle size silica supports are sometimes needed [10]. Again, to date the polysaccharide-derived CSPs have been the most studied in SMB systems, and a large number of racemic compounds have been successfully resolved in this way [95-98, 100-108]. Nevertheless, some applications can also be found with CSPs derived from polyacrylamides [11], Pirkle-type chiral selectors [10] and cyclodextrin derivatives [109]. A system to evaporate the collected fractions and to recover and recycle solvent is sometimes coupled to the SMB. In this context the application of the technique to gas can be advantageous in some cases because this part of the process can be omitted [109]. 

Recently, two examples of the separation of enantiomers using CCC have been published (Fig. 1-2). The complete enantiomeric separation of commercial d,l-kynurenine (2) with bovine serum albumin (BSA) as a chiral selector in an aqueous-aqueous polymer phase system was achieved within 3.5 h [128]. Moreover, the chiral resolution of 100 mg of an estrogen receptor partial agonist (7-DMO, 3) was performed using a sulfated (3-cyclodextrin [129, 130], while previous attempts with unsubstituted cyclodextrin were not successful [124]. The same authors described the partial resolution of a glucose-6-phosphatase inhibitor (4) with a Whelk-0 derivative as chiral selector (5) [129]. 

One of the latest resolutions of the anesthetic enflurane (8) has been performed by preparative GC on a y-cyclodextrin CSP, the process later being scaled-up via SMB [109] (Fig. 1-4). This is the first GC-SMB separation described. 

Recently, the separation of some milligram quantities of terbutaline by classical gel electrophoresis has been reported [194]. A sulfated cyclodextrin impregnated on the agarose gel was used as a chiral selector and the complete resolution was achieved in 5 h. Analogously, small amounts of enantiomers can be isolated using thin-layer... 

T. J. Ward, D. W. Armstrong, Cyclodextrin-stationary phases in Chromatographic chiral separations, M. Zief, L. J. Crane (Eds.), Chromatographic Science Series, Vol. 40, Marcel Dekker, New York (1988) Chapter 5. 

A. M. Stalcup, Cyclodextrin bonded chiral stationary phases in enantiomer separations in A practical approach to chiral separations by liquid chromatogra.phy, G. Subramanian, VCH, Weinheim (1994) Chapter 5. 

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