Cyclodextrins chemical mechanism

I Cyclodextrins are excellent enzyme models Catalysis and induced fit. Due to their cavities, which are able to accommodate guest (substrate) molecules, and due to the many hydroxyl groups lining this cavity, cyclodextrins can act catalytically in a variety of chemical reactions and they therefore serve as good model enzymes. Thus, benzoic acid esters are hydrolyzed in I aqueous solution by factors up to 100 times faster if cyclodextrins are added. The reaction in- j volves an acylated cyclodextrin as intermediate which is hydrolyzed in a second step of the j reaction, a mechanism reminiscent of the enzyme chymotrypsin. The catalytic efficiency can. be further enhanced if the cyclodextrins are suitably modified chemically so that a whole range of artificial enzymes have been synthesized [551-555, 556, 563, 564]. 

Cyclodextrin Complexes. The same physicochemical characteristics that allow successful chemical delivery also complicate the development of acceptable pharmaceutical formulations. Increased lipophilicity allows partition into deep brain compartments, but also confers poor aqueous solubility. The oxidative lability, which is needed for the lock-in mechanism, and the hydrolytic instability, which releases the modifier functions or the... 

The radical cation of (163) can be formed by electron transfer photochemistry and in methanol addition products are formed. A study of the photochemical reactivity of some novel 3-phenylnorbomadienes has been reported. Bichromo-phoric norbomadiene derivatives have also been synthesized and studied photo-chemically. A study of the photoisomerization of some norbomadienes has been carried out within the constrained environment of P-cyclodextrin. The bichromophoric system (164) undergoes intramolecular electron transfer by a through-bond mechanism on irradiation. The transfer is from the benzidine... 

F. Tazerouti, A. Y. Badjah-Hadj-Ahmed and T. Hanai, Analysis of the mechanism of retention on a modified P-cyclodextrin/silica chiral stationary phase using a computational chemical method,/. Liq. Chromatogr. Relat. Technal, 2007, 30, 3043-3057. 

A different kind of selectivity has been observed with cyclodextrin bisimidazoles. These species were synthesized as mimics of the enzyme ribonuclease, and indeed they do perform the cleavage of certain bound phosphate esters with a mechanism closely related to that of the natural enzyme [26]. Of course in the natural enzyme there is considerable selectivity, including regioselectivity. For instance, the natural enzyme can cleave a cyclic phosphate ester between C-2 and C-3 of a ribose unit in such a way as to leave the phosphate group entirely on C-3. Ordinary chemical hydrolysis of such a molecule would lead to a mixture of C-2 and C-3 phosphates. Our cyclodextrin bisimidazole is able to imitate such selectivity. 

Proton nuclear magnetic resonace ( H NMR) spectra were recorded in DMSO-d or trifluoroacetic acid (TFA-di) solution on a Varian EM-360L 60-MHz NMR spectrometer, chemical shifts are repotted in parts per million from internal tetramethylsilane (TMS). Ultraviolet (UV) spectra were recorded on a Gilford Response UV-visible spectrometer using deionized water as solvent Infrared (IR) spe were recorded on a Beckman AccuLab 8 spectrometer and were calibrated with the 3027.9, 1601.8, and 1028.3 cm bands of polystyrene. Potassium bromide (KBr) disks of herbicide-cyclodextrin complexes and of propoitional mechanical mixtures of heibicide and cyclodextrin were prepared for the analysis. 

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