Separation factor, chiral recognition

In another study, the authors reported a comparative study of the enantiomeric resolution of miconazole and the other two chiral drugs by high performance liquid chromatography on various cellulose chiral columns in the normal phase mode [79], The chiral resolution of the three drugs on the columns containing different cellulose derivatives namely Chiralcel OD, OJ, OB, OK, OC, and OE in normal phase mode was described. The mobile phase used was hexane-isopropanol-diethylamine (425 74 1). The flow rates of the mobile phase used were 0.5, 1, and 1.5 mL/min. The values of the separation factor (a) of the resolved enantiomers of econazole, miconazole, and sulconazole on chiral phases were ranged from 1.07 to 2.5 while the values of resolution factors (Rs) varied from 0.17 to 3.9. The chiral recognition mechanisms between the analytes and the chiral selectors are discussed. 

The chiral recognition ability of a CSP is quantitatively evaluated from the results of chromatographic separation of enantiomers. Figure 3.4 shows a chromatogram of the resolution of benzoin (19) on cellulose tris(3,5-dimethylphenylcarbamate). The (+)-isomer elutes first followed by the (—)-isomer complete baseline separation is achieved. The results of the separation can be expressed by three parameters—capacity factors (k1), separation factor (a), and resolution factor (Rs)—defined as follows ... 

Several CD derivatives (charged and uncharged) are available which should allow the separation of most chiral molecules with at least one of them. However, due to the complexity of chiral recognition mechanisms, the determination of the best selector based on the analyte structure is challenging. Eurthermore, separations using CDs are influenced by numerous factors, so that no general rule can be applied for the successful resolution of enantiomers. ... 

The chiral recognition of enantiomers can be of three types (i) desionoselective, (ii) ionoselective, or (iii) duoselective, in which only the non-dissociated, the dissociated or both forms (charged and uncharged), respectively, of the enantiomers selectively interact with the chiral selector. In the case of ionoselective and duoselective interactions, a reversal of the migration order of the enantiomers is theoretically possible by the appropriate selection of CD concentration and the pH of the BGE. The addition of organic modifier to the BGE can also change selectivity by modifying the solubility of the chiral selector and/or of the solute, the complex equilibrium, the conductivity of the BGE and the electroendos-motic flow (EOE) level. Several other factors, such as the temperature, the type and the concentration of the BGE, and the level of the EOE can influence the separation. 

Chiral separation can be observed when there is a suitable difference between free energies (AG) of diastereomeric associates formed by R- and S-enantiomers of selectand and a chiral selector (SO). The energy differences can be directly attributed to the chiral recognition phenomena, involving complementarity of size, shape, and molecular interaction of SO and SA molecules. These factors are also controlled by experimental conditions such as temperature, mobile phase... 

The enantiomeric separation of some racemic antihistamines and antimalar-ials, namely (+/-)-pheniramine, (+/-)-bromopheniramine, (+/-)-chlorophen-iramine, (+/-)-doxylamine, and (+/-)-chloroquine, were investigated by capillary zone electrophoresis (CZE). The enantiomeric separation of these five compounds was obtained by addition of 7 mM or 1 % (w/v) of sulfated P-cyclo-dextrin to the buffer as a chiral selector. It was found that the type of substituent and degree of substitution on the rim of the cyclodextrin structure played a very important part in enhancing chiral recognition (174). The use of sulfated P-cyclo-dextrin mixtures as chiral additives was evaluated for the chiral resolution of neutral, cyclic, and bicyclic monoterpenes. While there was no resolution of the monoterpene enantiomers with the sulfated P-cyclodextrin, the addition of a-cyclodextrin resulted in mobility differences for the terpenoid enantiomers. Resolution factors of 4-25 were observed. The role of both a-cyclodextrin and sulfated P-cyclodextrin in these separations was discussed (187). The enantiomeric separation of 56 compounds of pharmaceutical interest, including anesthetics, antiarrhythmics, antidepressants, anticonvulsants, antihistamines, antimalarials, relaxants, and broncodilators, was studied. The separations were obtained at pH 3.8 with the anode at the detector end of the capillary. Most of the 40 successfully resolved enantiomers contained a basic functionality and a stereogenic carbon (173). 

S.M. Han, Y.I. Han and D.W. Armstrong, Structural Factors Affecting Chiral Recognition and Separation on P-Cyclodextrin Bonded Phases, J. Chromatogr., 441(1988)376. 

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... 

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