Cyclodextrin concentration chromatography

The utility of the highly soluble 6-cyclodextrin derivatives (soluble polymer and dimethyl-6-cyclodextrin) in RPTLC is illustrated in the separation of barbiturates. The lipophilicity of a barbiturate or any guest decreases when included in a cyclodextrin-cavity. Therefore its mobility is modified in reversed phase thin layer chromatography. With this simple and rapid method, the stability of a complex can be estimated empirically (Table II). The "b" value of the following equation is characteristic for the complex stability (in water ethanol =4 1 solution, R determined at 5 different cyclodextrin concentrations for 21 barbiturates) ... 

The use of cyclodextrins as the mobile phase components which impart stereoselectivity to reversed phase high performance liquid chromatography (RP-HPLC) systems are surveyed. The exemplary separations of structural and geometrical isomers are presented as well as the resolution of some enantiomeric compounds. A simplified scheme of the separation process occurring in RP-HPLC system modified by cyclodextrin is discussed and equations which relate the capacity factors of solutes to cyclodextrin concentration are given. The results are considered in the light of two phenomena influencing separation processes adsorption of inclusion complexes on stationary phase and complexation of solutes in the bulk mobile phase solution. 

Strength of interaction and the greater the rate of decrease in k with increasing cyclodextrin concentration. Cyclodextrin mobile phase chromatography provides an effective way to determine inclusion complex formation constants, and their magnitudes are useful in evaluating solute-CD interactions. Also, the Kf values obtained chromatographically can be confirmed spectroscopically if identical experimental conditions are employed. 

In addition to high-performance liquid chromatography (HPLC), the chiral resolution using CMPAs was also carried out by supercritical fluid chromatography (SFC) [91] and capillary electrochromatography (CEC) [92-98]. Salvador et al. [91] used dimethylated /1-cyclodextrin as the mobile phase additive on porous graphite carbon as the solid phase for the chiral resolution of tofizopam, warfarin, a benzoxazine derivative, lorazepam, flurbiprofen, temazepam, chlorthalidone, and methyl phehydantoin by SFC. The authors also studied the effect of the concentration of dimethylated /1-cyclodextrin, the concentration of the mobile phase, the nature of polar modifiers, outlet pressure, and the column temperature on the chiral resolution. 

Adsorption chromatography on cellulose was examined for azo dyes that form complexes with CDs using aqueous solutions of a-CD as eluents. The cyclodextrin substantially increases the RF values [61], Commercial a-, [>-. and y-CD polymers were used for the same purpose, however changes of RF were observed only for some azo dyes [62], 2-Hydroxypropyl-P-cyclodextrin used in 10-20% concentration causes complete elution of many compounds [63],... 

An advantage of cyclodextrins over the common stationary phases is the high selectivity toward the isomeric substances. It has been demonstrated that many positional and geometric isomers can be separated by packed-column GSC in a very short time (i.e. analysis time does not exceed 2 minute) in separation of a mixture of o-, m- and p-isomers. From the analytical viewpoint, the low efficiency of the columns used is a disadvantage. Also, there ar other drawbacks of the gas-solid chromatography using CO s nonlinearity of the separation isotherm over a wider concentration range and poor reproducibility in the preparation of the CD columns utilized. 

The FITC labeling method was also applied to chiral separations of amino acids on a microchip to determine the enantiomeric ratios of amino acids found on a meteorite [27], Since biotic amino acids are normally single enantiomers, chiral separations of amino acids are not truly clinical in nature, but illustrate the potential for chiral separations of small molecules of clinical interest. Ma-thies and co-workers used this technique to search for evidence of life in extraterrestrial environments. Enantiomeric forms of Val, Ala, Glu, and Asp could be discriminated by addition of a-, (3-, or y-cyclodextrin (CD) to the run buffer. Improved resolution with faster separations was found with respect to conventional CE. This method has been modified, by addition of SDS to the buffer, to perform cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) [28]. Increasing the SDS concentration decreased the magnitude of elec-troosmotic flow (EOF), increasing the effective migration distance, and therefore the resolution on the microchips. 

Micelles and cyclodextrins are the most common reagents used for this technique. Micellar electrokinetic capillary chromatography (MECC or MEKC) is generally used for the separation of small molecules [6], Sodium dodecyl sulfate at concentrations from 20 to 150 mM in conjunction with 20 mM borate buffer (pH 9.3) or phosphate buffer (pH 7.0) represent the most common operating conditions. The mechanism of separation is related to reversed-phase liquid chromatography, at least for neutral solutes. Organic solvents such as 5-20% methanol or acetonitrile are useful to modify selectivity when there is too much retention in the system. Alternative surfactants such as bile salts (sodium cholate), cationic surfactants (cetyltrimethy-lammonium bromide), nonionic surfactants (poly-oxyethylene-23-lauryl ether), and alkyl glucosides can be used as well. 

Chiral mobile phase additives provide a more versatile and cost-effective approach for enantiomer separations in thin-layer chromatography. Typically, chemically bonded layers with cyclodextrin and its derivatives, bovine serum albumin, or macrocyclic glycopeptides are used as chiral additives in the reversed-phase mode [59,60,172-178]. For [5- and y-cyclodextrins and their derivatives, a 0.1 to 0.5 M aqueous methanol or acetonitrile solution of the chiral selector is used as the mobile phase. Bovine serum albumin is generally used at concentrations of 1-8 % (w/v) in an aqueous acetate buffer of pH 5 to 7 or in a 0.5 M acetic acid solution, in either case with from 3-40 % (v/v) propan-2-ol (or another aliphatic alcohol), added to control retention. Enantioselectivity usually increases with an increase in concentration of the chiral selector, and may be non existent at low concentrations of the chiral selector. 

A three-phase equilibrium model for partitioning of solute, E, between a bulk aqueous phase and dissolved cyclodextrin, and between the bulk aqueous phase and the stationary phase, L3, allows one to derive equations relating capacity factor, k, to the molar concentration of CD. The equation given below is similar to that derived for micellar chromatography which also assumes a three-phase model (12. 131,... 

Tesaf ova et al. [87] used a modified version of SIMUL, which they called SIMULMIC to simulate the separation of neutral analytes in a system with a neutral cyclodextrin and anionic micelles. A number of systems were examined in which various combinations of the inlet and outlet vials and the capillary itself were filled with cyclodextrin. Simulation results were used to examine the micellar/cyclodextrin boundary at various times and concentrations although no simulation results for a chiral separation were reported. To the best of our knowledge, this is the only dynamic simulation of an electrokinetic chromatography (EKC) separation to date. 

Micelles and cyclodextrins are the most common reagents used for this technique. Micellar capillary electro-kinetic chromatography (MECC) or MEKC is generally used for the separation of small molecules.Sodium dodecyl sulfate at concentrations from 20 to 150 mM in conjunction with 20 roM borate buffer (pH 9.3) or phosphate buffer (pH 7.0) represent the most common operating... 

COP-hfacac 50 (5.8 mg, 0.0065 mmol, 5 mol%) was added to a solution of ( )-hex-2-enyl 2,2,2-trichloroacetimidate (48, R = n-Pr) (40 mg, 0.13 mmol) in THF (0.05 mL), and the reaction vial was sealed, protected from light, and maintained at 50 °C. After 6 h, the orange solution was concentrated under reduced pressure. Purification of the residue by silica gel chromatography eluting with 10% ethyl acetate/90% hexanes to give the allylic trichloroacetamide 49 as a colorless solid (37 mg, 0.12 mmol, 93%). Chiral GC (Chiraldex y cyclodextrin trifluoroacetyl, 20 m x 0.25 m initial temperature 50 °C (1 min), final temperature 150 °C, 5 °C/min) showed that compound 48 had been formed in 96% ee. 

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