Cyclodextrin anion

Scheme 12.1 Acetylation of a cyclodextrin anion by a bound substrate via a tetrahedral intermediate.

The other point that was discovered was that some reaction rates were accelerated by operating in a mixed solvent rather than in pure water. The one that was examined most carefully was the acetyl transfer from bound ra-f-butylphenyl acetate to /3-cyclodextrin with buffers that in water give a pH of 9.5. It was observed that the reaction was almost 50-fold accelerated in a 60% DMS0-H20 solvent compared with the reaction rate in pure water. Part of this acceleration came from an increase in the apparent basicity of the medium, since relative pK s are solvent dependent part of it was also a solvent effect on the reaction rate of the cyclodextrin anion with the substrate. Thus, in 60% DMSO-H20 the /3-cyclodextrin reaction with this substrate was 13,000-fold faster than was the rate of hydrolysis of the substrate in an aqueous buffer of the same composition. Of this approximately 50-fold acceleration over cyclodextrin in water, about 10-fold was caused by changes in the pK s in the system and about 5-fold was caused by a change in the reaction rate of the cyclodextrin. 

Anionic Amphiphilic Cyclodextrins Anionic amphiphilic CDs possess a sulfate group in their structure to render an anionic nature to the molecule. An efficient regiospecific synthetic route to obtain acyl-sulfated P-CDs was introduced in which the upper rim is functionalized with sulfates and the lower rim with fatty acid esters [69], These derivatives were able to form aggregates in aqueous medium. 

Modified cyclodextrins anionic Carbon based derivatives ... 

Anionic Cyclodextrins Anionic CDs are often carbox-yalkylated, sulfoalkylated, sulfated, and phosphorylated. The most commonly used derivatives are S-p-CD, SBE-(3-CD, and sulfoethyl ether-p-CD. " Some examples from the literature are given below. Wu et al." describe a CE method with electrospray ionization-tandem MS (ESI-MS/MS) for the quantification of 1,2,3,4-tetrahydroisoqui-noline derivatives (TlQs). They performed the enantioseparation in a 50-p,m i.d., 75-cm-long fused-silica capillary filled with a 20-mM acetic acid/ammonium acetate buffer with pH 5.5 containing 1.0-mM S- -CD. The TIQ derivatives, i.e., salsolinol, 1-benzyl-TIQ, and A/-methyl salsoli-nol, could be baseline separated with resolutions ranging from 3 (for salsolinol) to 4.5 (for 1-benzyl-TIQ). 

Silipo and Hansch 77) have developed correlation equations for the formation of a-cyclodextrin-substituted phenyl acetate complexes (Eq. 13), a-cyclodextrin-RCOO complexes (Eq. 14), and P-cyclodextrin-substituted phenylcyanoacetic acid anion complexes (Eq. 15). 

A few examples have been reported in which no steric parameter is involved in the correlation analysis of cyclodextrin catalysis. Straub and Bender 108) showed that the maximal catalytic rate constant, k2, for the (5-cyclodextrin-catalyzed decarboxylation of substituted phenylcyanoacetic acid anions (J) is correlated simply by the Hammett a parameter. 

An artificial metalloenzyme (26) was designed by Breslow et al. 24). It was the first example of a complete artificial enzyme, having a substrate binding cyclodextrin cavity and a Ni2+ ion-chelated nucleophilic group for catalysis. Metalloenzyme (26) behaves a real catalyst, exhibiting turnover, and enhances the rate of hydrolysis of p-nitrophenyl acetate more than 103 fold. The catalytic group of 26 is a -Ni2+ complex which itself is active toward the substrate 1, but not toward such a substrate having no metal ion affinity at a low catalyst concentration. It is appearent that the metal ion in 26 activates the oximate anion by chelation, but not the substrate directly as believed in carboxypeptidase. 

Teranishi, K. (2003). Cyclodextrin-bound 6-(4-methoxyphenyl)imidazo[l,2-a]-pyrazin-3(7H)-one as chemiluminescent probe for superoxide anions. ITE Letters on Batteries, New Technologies and Medicine 4 201-205. 

The method for creating acceptor sink condition discussed so far is based on the use of a surfactant solution. In such solutions, anionic micelles act to accelerate the transport of lipophilic molecules. We also explored the use of other sink-forming reagents, including serum proteins and uncharged cyclodextrins. Table 7.20 compares the sink effect of 100 mM (5-cyclodextrin added to the pH 7.4 buffer in the acceptor wells to that of the anionic surfactant. Cyclodextrin creates a weaker sink for the cationic bases, compared to the anionic surfactant. The electrostatic binding force between charged lipophilic bases and the anionic surfactant micelles... 

It has been claimed that complexes of P-cyclodextrin with anionic surfactants, notably higher fatty alcohol ethoxylates, improve scouring efficiency on cotton and wool in laboratory-scale processing [34]. Residual surfactants carried over from preparation can have undesirable effects in subsequent processing. When cyclodextrins complex with surfactants, their surface activity is reduced. Hence cyclodextrins are potentially useful for the removal of residual amounts of surfactants from substrates [35]. The use of a- and P-cyclodextrins has been studied in this context with one cationic, one anionic and four... 

When p-nitrophenolate is incrementally added to a-cyclodextrin in water, the ultraviolet spectrum of this anion changes such that successive spectra give rise to two isobestic points (Figure 5.2). Such behaviour is in accord with the formation of a 1 1 species. The spectral changes may be used for the direct calculation of K, which in this case was found to be approximately 104 dm3 mol-1 (Cramer, Saenger Spatz, 1967). 

Inclusion complexation, 77 552-553 Inclusion compounds, 74 159-190, 170-182 amylose, 74 168 anionic guest, 74 170 cailixarene, 74 165-166 categories of, 74 160 crown macroring, 74 160-161 cucurbituril, 74 168-169 cyclodextrin, 74 166-167... 

Table 2 Thermodynamic parameters for the /1-cyclodextrin-catalysed decarboxylation of the 4-chlorophenylcyanoacetate anion."...

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. 

A combined -cyclodextrin quatemary ammonium salt catalyst promotes the addition of the trichloromethyl anion to aromatic aldehydes and enhances the yield of the a-hydroxy acid [7],... 

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