Cyclodextrins alpha

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. 

Panja S, Chakravorti S (2002) Photophysics of 4-(N, N-dimethylamino)cinnamaldehyde/ alpha-cyclodextrin inclusion complex. Spectrochim Acta A Mol Biomol Spectrosc 58(1) 113-122... 

A. Kitajima, T. Teranishi, and M. Miyake, Detection of nitric oxide on carbon electrode modified with ionic polymers and alpha-cyclodextrin. Electrochemistry 69, 16-20 (2001). 

Convergence properties of free energy calculations Alpha-cyclodextrin complexes as a case study. J. Am. Chem. Soc. 116 6293 (1994). 

Zhu, X., Lin, B., Jakob, A., Wuerthner, S., Koppenhoefer, B. Separation of drugs by capillary electrophoresis, part 10. Permethyl-alpha-cyclodextrin as chiral solvating agent. Electrophoresis 1999, 20,1878-1889. 

Since the time of Schardinger, one of the most important reasons for studying the cyclodextrins was for the information they might yield on the structure of starch and of the well known blue iodine-starch complex. In fact, the similarity between the iodine-starch reaction and the iodine-alpha cyclodextrin reaction was first noted by Schardinger in 1911, in his final paper on the cyclodextrins. 

G1c -6 + alpha cyclodextrin G1c -7 + beta cyclodextrin G1c -8 + gamma cyclodextrin... 

The first direct evidence for molecular inclusion came from X-ray crystallography. Hybl and coworkers determined the structure of the alpha cyclodextrin-potassium acetate complex by using three-dimensional X-ray diffraction data. They found that, in the solid state, the acetate anions are included by the alpha cyclodextrin. In the process, they also found that every D-glucosyl residue of the alpha cyclodextrin is in the C conformation. 

Yamamoto and coworkers used two-dimensional, nuclear Over-hauser effect experiments (NOESY) to determine the proximity of particular protons situated on an included p-nitrophenolate ion to particular protons of a host alpha cyclodextrin molecule. The experiments showed cross-peaks connecting the H-3 resonance of alpha cyclodextrin to both meta and ortho proton resonances of the p-nitrophenolate ion, whereas H-5 of the alpha cyclodextrin gave a cross-peak only with the resonance of the meta proton thereof. As a consequence, it was unequivocally confirmed that the p-nitrophenolate ion is, in solution, preferentially included with its nitro group oriented to the narrow end of the alpha cyclodextrin... 

Fig. 4.—Enthalpy-Entropy Compensation for Alpha Cyclodextrin Inclusion-Complexes. [The guest identification numbers are as given in Table II.]...

The results just discussed suggest that it is only alpha cyclodextrin that shows a conformational change on complex-formation, due to the rotation of one of its D-glucosyl units. Whether this conformational change contributes to the stability of the complex, as proposed by Saenger, is, however, debatable. Bergeron and Meeley " showed that methylation of the... 

Unusual behavior was reported" for the complexation of the ions IO4 and CIO4 with beta cyclodextrin. Rohrbach and coworkers found" that these ions show values of the forward rate-constant that approach the diffusion-controlled limit that is, 10 to 10 dm mol". s , whereas all of the other ions studied show values significantly less than this limit. In order to explain this observation, Rohrbach suggested" that, because of their larger radii, the CIO and IO4 ions may be too large to fit into the alpha cyclodextrin cavity, but, instead, may form a complex in which the ions straddle one of the entrances to the cavity. 

Until 1984, all of the stopped-flow and temperature-jump kinetic studies of alpha cyclodextrin inclusion-complex formation were explainable in terms of a single-step, binding mechanism. According to this mechanism, the observed rate constant, kobs, (for stopped-flow) and the reciprocal relaxation time, 1/t, (for temperature-jump) should show a linear dependence on the edpha cyclodextrin concentration. Sano and coworkers, however, in the case of the iodide-alpha cyclodextrin interaction, and Hersey and Robinson,in the case of various azo dye-alpha cyclodextrin interactions (see Fig. 7), found that certain guest species exhibit a limiting value of kobs and 1/t at high concentrations of alpha cyclodextrin. This behavior can most simply be explained in terms of a mechanism of the type,... 

Fig. 7.—Variation of kobs Characterizing the Mordant Yellow 7-Alpha Cyclodextrin Systemwith Variation of Total Concentration of Alpha Cyclodextrin. [Mordant Yellow 7] = 3.0 X 10- mol. dm-5 T = 15.0 (x), 25.0 (o), 35.0 (OrC. Adapted from Fig. 2 of Ref. 142. ...

Hersey and Robinson also foundthat many guest species that show kinetic behavior apparently explicable in terms of a single-step binding, give a discrepancy between the values of the equilibrium constant determined kinetically and those determined from equilibrium studies. It was found that the equilibrium constant, deterrmned spectrophotometrically, was usually greater than the ratio of the forward and backward rate-constants, determined kinetically. They therefore suggested that this discrepancy could be adequately explained if the two-step mechanism just described was used to interpret the results. A similar proposal has also been made by Hall and coworkers, who observed a large discrepancy between AV° values for the inclusion of 1-butanol and 1-pentanol by alpha cyclodextrin, calculated from equilibrium-density measurements and kinetic, ultrasonic-absorption data. 

A study similar to that of Hersey and Robinson has been reported by Seiyama and coworkers.From a stopped-flow, kinetic study of the interaction of various azo dyes and some azo dye-metal complexes with alpha cyclodextrin, they observed two kinetic processes. The dependence of the observed rate-constants for these two processes on the alpha cyclodextrin concentration was found to be explainable in terms of a mechanism identical to that proposed earlier by Hersey and Robinson. In the case of the guests used by Seiyama and coworkers, however, values for the rate constants of the binding step could be determined from the concentration dependence of kobs for the faster process thus,... 

Further support for the presence of a conformational change has come from the work of Orstan and Wojcik, who also studied azo dye-alpha cyclodextrin complexes by using the temperature-jump technique. The dyes that they investigated showed three different types of behavior. Certain dyes showed a limiting value of 1/t at high alpha cyclodextrin concentrations, others showed a linear dependence of 1/t on alpha cyclo-... 

Up to now, evidence for the presence of a conformational change during the inclusion process has been presented only for alpha cyclodextrin, and then only in the case of certain guest molecules. Whether the two-step mechanism is generally applicable is not known, because, owing to the nature of kinetic studies, the results are frequently open to more than one interpretation. Nevertheless, sufficient support for the two-step mechanism has appeared to necessitate its consideration in any future kinetic studies. 

Some modifications to the cyclodextrin structure have also been found to improve their complexing ability. Casu and coworkers prepared 2,3,6-tri-O-methyl and 2,6-di-O-methyl derivatives of alpha and beta cyclodextrin. They observed that tri-O-methyl-alpha cyclodextrin shows an almost ten-fold increased stability of the complex with the guest, Methyl Orange, compared with the unmodified alpha cyclodextrin. A possible reason for this increase in stability is that the methyl groups are responsible for an extension of the hydrophobic cavity of the cyclodextrin. Other workers,however, observed a much smaller enhancement of stability of complexes on methylation of the cyclodextrin, and a decrease in stability has even been reportedfor the one host-two guests complex of tropaeolin with beta cyclodextrin. Thus, the effect of methylation on the stability of a complex varies with the guest species involved, and cannot be readily predicted. 

Reactions capable of catalysis by the cyclodextrins also show some degree of chiral selectivity. For example, van Hooidonk and Breebaart-Hansen found that the (-)-R-isomer of isopropyl methylphos-phonofluoridate is hydrolyzed by alpha cyclodextrin much more rapidly than the (+)-5-isomer. Their measured dissociation constants, however. 

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