Cyclodextrin analyte interaction

Electrokinetic chromatography (EKC) is a more general term that encompasses CE methods using buffer additives, for example cyclodextrins, which interact with analyte species. 

Although the chiral recognition mechanism of these cyclodexttin-based phases is not entirely understood, thermodynamic and column capacity studies indicate that the analytes may interact with the functionalized cyclodextrins by either associating with the outside or mouth of the cyclodextrin, or by forming a more traditional inclusion complex with the cyclodextrin (122). As in the case of the metal-complex chiral stationary phase, configuration assignment is generally not possible in the absence of pure chiral standards. 

The formation of complexes of the fluorescent tracer dye ammonium 1-phenyl-aminonaphthalene-8-sulphonate (10.41) with cyclodextrins has been investigated with favourable results, especially in environmental studies [33]. The ability of this dye to complex with cyclodextrins has been exploited mainly as an analytical tool in the study of cyclodextrin applications, since its fluorescence is easily measured. The interaction of a-, P-and y-cyclodextrins with azo acid dyes containing alkyl chains of different lengths has been reported [36,37]. The formation and isolation of solid complexes between P-cyclodextrin and Cl Acid Red 42, Cl Acid Blue 40 or Erionyl Bordeaux 5BLF (Ciba) have been reported [29]. 

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. 

From Eq. (13.3), it is clear that neutral molecules will have a net velocity. In normal electrophoresis, cations will migrate faster than neutrals, and neutrals will migrate faster than anions. Anions are electrophoretically migrating in a direction opposite to EOF. Separations of neutral molecules, such as organic explosives, can only be achieved by using buffer additives, such as micelles, ionic cyclodextrins, and bile salts. The interaction of neutral analytes with these ionic buffer additives results in a modified mobility that enables separation. 

Cyclodextrins have significantly contributed to the development of enantioseparations in CE, where they represent the most widely used chiral selectors. On the other hand, due to its inherently high separation efficiency and diverse technical advantages, CE has contributed enormously to the better understanding of affinity interactions between CDs and chiral analytes. The following text summarizes the recent developments in this field (3-60). 

One additional advantage of chiral CE over chromatographic techniques is that both the chiral selector and the analyte reside in a free solution in this technique. This makes the investigation of chiral drug-cyclodextrin interactions technically much easier, cheaper, and faster, because unlike chromatographic techniques there is no need to immobilize the receptor (selectors). However, an even more important advantage is that there is no effect of immobilization on the degree of freedom of the selector and the effects of a solid matrix are absent. 

Figure 14.11 shows the separation of R and S isomers of a series of structurally related local anaesthetics. Wide separations were achieved for the compounds in this series where it was proposed that the fit of the hydrophobic portion of the analyte into the cyclodextrin was optimal when one of the substituents at the chiral centre was able to interact with the chiral hydroxyl groups on the rim of the cyclodextrin cavity. Table 14.3 shows the association constants calculated for the interaction of the enantiomeric pairs with the dimethylcyclodextrin. The larger the value of K, the... 

Because the steric effect contributes to the complex formation between guest and host, the chiral resolution on these CSPs is affected by the structures of the analytes. Amino acids, amino alcohols, and derivatives of amines are the best classes for studying the effect of analyte structures on the chiral resolution. The effect of analyte structures on the chiral resolution may be obtained from the work of Hyun et al. [47,48]. The authors studied the chiral resolution of amino alcohols, amides, amino esters, and amino carbonyls. The effects of the substituents on the chiral resolution of some racemic compounds are shown in Table 6. A perusal of this table indicates the dominant effect of steric interactions on chiral resolution. Furthermore, an improved resolution of the racemic compounds, having phenyl moieties as the substituents, may be observed from this Table 6. ft may be the result of the presence of n—n interactions between the CCE and racemates. Generally, the resolution decreases with the addition of bulky groups, which may be caused by the steric effects. In addition, some anions have been used as the mobile phase additives for the improvement of the chiral resolution of amino acids [76]. Recently, Machida et al. [69] reported the use of some mobile phase additives for the improvement of chiral resolution. They observed an improvement in the chiral resolution of some hydrophobic amino compound using cyclodextrins and cations as mobile phase additives. 

Lipodex E [octakis(3-0-butanoyl-2,6-di-0- -pentyl)-y-cyclodextrin] dissolved in a polysiloxane matrix (SE-54) was used as a chiral receptor. The binding strength of the analyte molecules depends on the interaction mech-... 

In LC, cyclodextrins have been used in both the mobile and stationary phases, although the latter application predominates using bonded phases. Both normal and reversed phase separations are possible. With polar mobile phases, the cyclodextrin cavity hosts and retains hydrophobic analytes but when large concentrations of organic solvent are used in the mobile phase, solvent molecules occupy the cavity—inclusion of analytes is suppressed and hydrogen bonding and dipole-dipole interactions with analytes predominate. Applications to electrochromatography have also been described. [96]... 

Additives that specifically interact with an analyte component are also very useful in altering the electrophoretic mobility of that component. For example, the addition of copper(II)-L-histidine (12) or copper(II)-aspartame (54) complexes to the buffer system allows racemic mixtures of derivatized amino acids to resolve into its component enantiomers. Similarly, cyclodextrins have proven to be useful additives for improving selectivity. Cyclodextrins are non-ionic cyclic polysaccharides of glucose with a shape like a hollow truncated torus. The cavity is relatively hydrophobic while the external faces are hydrophilic, with one edge of the torus containing chiral secondary hydroxyl groups (55). These substances form inclusion complexes with guest compounds that fit well into their cavity. The use of cyclodextrins has been successfully applied to the separation of isomeric compounds (56), and to the optical resolution of racemic amino acid derivatives (57). 

Polysaccharide derivatives used as CSPs interact with chiral analytes in much the same manner as cyclodextrins. These molecules have been shown to exhibit high chiral recognition ability in HPLC [155]. The main advantage of CEC over HPLC is the enhanced efficiency. In chiral separations, slow mass transfer kinetics between the CSP and chiral analytes have somewhat diminished the efficiency advantage of the technique. The goal of using polysaccharide derivatives... 

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