Alpha-cyclodextrin structure

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. 

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. 

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. 

Cyclodextrins, products of the degradation of starch by an amylase of Bacillus macerans(1), have been studied in terms of chemical modifications, mainly for the purpose of developing efficient enzyme mimics(2). Not only their unique cyclic structures, but also their ability to form Inclusion complexes with suitable organic molecules, led us to Investigate the total synthesis of this class of molecules(3) We describe here an approach to a total synthesis of alpha(l), gamma(2), and "iso-alpha" cyclodextrin (3). 

Odagaki, Y., Hirotsu, K., Higuchi, T., Harada, A., Takahashi, S., X-ray structure of the alpha-cyclodextrin ferrocene (2-1) inclusion compound. J. Chem. Soc.-Perkin Trans. 1 1990, 1230-1231. 

Kihara F, et al. (2002). Effects of structure of polyamidoamine dendrimer on gene transfer efficiency of the dendrimer conjugate with alpha-cyclodextrin. Bioconiue. Chem. 13 1211-1219. 

Cyclodextrin glycosyllransferases [EC 2.4.1.19] share family 13 with alpha-amylases, although the latter are formally classed as hydrolases [EC 3.2.1.1], The CGTases utilize starch, maltodextrins, and a-maltosyl fluoride to synthesize cyclic compounds of six, seven, or eight a-(l -a 4)-linked glu-copyranose units as well as related noncyclic dextrins. The Bacillus circulans strain 251 enzyme complexed with acarbose shows an active-site structure... 

Cyclodextrins are another category of substances that are used for complex formation. Cyclodextrins are ring-shaped oligosaccharides consisting of six, seven or eight glucose units referred to as alpha-, beta-, and gammacyclodextrin, respectively (see Fig. 18.2 for the chemical structure of betacyclodextrin) [15]. 

Chetcuti, R A., Moser, P., and Rihs, G. 1991. MetaUacarborane complexes as guest for cyclodextrins—Molecular-structure of the inclusion complex Cs[Closo-3,3,3-(Co)3-3,l,2-Rec2b9hll.Alpha-Cd].8h2o. Organometallics 10 2895-2897. 

Ohira, A. Sakata, M. Taniguchi, L Hirayama, C. Kunitake, M. 2003. Comparison of nanotube structures constructed from alpha-, beta-, and gamma-cyclodextrins by potential-controlled adsorption. J. Am. Chem. Soc. 125 5057-5065. 

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