Cyclodextrins formation

ALCOHOLS,HIGHERALIPHATIC - SURVEY AND NATURALALCOHOLSMANUFACTURE] (Voll) -use in cyclodextrin formation [BIOPOLYMERS - ANALYTICAL TECHNIQUES] (V ol 4)... 

A few recent reports have indicated that multiple cyclodextrin formation in aqueous solution is more common than once believed (1-8). Gelb et al. determined 1 2 (substratercyclodextrin CO ) bTnJing constant ratios but failed to obtain individual constant values (2). Connor et al. used potentiometric, spectrophotometric, solubility and competitive indicator methods to evaluate the binding constants of one substrate molecule bound to two CO molecules (5-7). Some of these methods gave substantial relative errors for the binding constants while other techniques were applicable to a limited number of compounds. 

Bonomo, RP and V Cucinotta (1991). Conformational features and coordination properties of functionalized cyclodextrins. Formation, stability, and structnre of proton andcopper(ll) complexes of histamine-bearing beta-cyclodextrin in aqueons solntion. Inorganic Chemistry, 30(13), 2708-2713. 

Possible formation of hydrogen bonds between substrate and a-cyclodextrin (formation of hydrogen bonds has been shown to be an extremely fast process and therefore cannot be rate determining for the inclusion reaction). 

Several procedures are used to control the ratios of cyclodextrins produced. One is addition of a substance to the reaction mixture that can gready affect the formation of one specific cyclodextrin over another. For example, in the presence of 1-decanol and 1-nonanol, a-cyclodextrin is produced almost exclusively whereas hexane or toluene promote the production of P-cyclodextrin. Conversely both cyclodextrins are produced simultaneously in the presence of 1-heptanol (2,4). 

As a final example we consider noncovalent molecular complex formation with the macrocyclic ligand a-cyclodextrin, a natural product consisting of six a-D-glucose units linked 1-4 to form a torus whose cavity is capable of including molecules the size of an aromatic ring. Table 4-3 gives some rate constants for this reaction, where L represents the cyclodextrin and S is the substrate ... 

Table 4-3. Binding Constants and Rate Constants for Complex Formation between a-Cyclodextrin and Azo Dyes ...

Figure 5-9. Free energy reaction coordinate diagram for System 2 of Table 4-3, the formation of a cyclodextrin inclusion complex.

Binding Forces Contributing to the Formation of Cyclodextrin Inclusion... 

Several intermolecular interactions have been proposed and discussed as being responsible for the formation of cyclodextrin inclusion complexes in an aqueous solution 6-10). They are... 

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). 

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

The theory and development of a solvent-extraction scheme for polynuclear aromatic hydrocarbons (PAHs) is described. The use of y-cyclodextrin (CDx) as an aqueous phase modifier makes this scheme unique since it allows for the extraction of PAHs from ether to the aqueous phase. Generally, the extraction of PAHS into water is not feasible due to the low solubility of these compounds in aqueous media. Water-soluble cyclodextrins, which act as hosts in the formation of inclusion complexes, promote this type of extraction by partitioning PAHs into the aqueous phase through the formation of complexes. The stereoselective nature of CDx inclusion-complex formation enhances the separation of different sized PAH molecules present in a mixture. For example, perylene is extracted into the aqueous phase from an organic phase anthracene-perylene mixture in the presence of CDx modifier. Extraction results for a variety of PAHs are presented, and the potential of this method for separation of more complex mixtures is discussed. 

Cyclodextrins can solubilize hydrophobic molecules in aqueous media through complex formation (5-8). A nonpolar species prefers the protective environment of the CDx cavity to the hulk aqueous solvent. In addition, cyclodextrins create a degree of structural rigidity and molecular organization for the included species. As a result of these characteristics, these macrocycles are used in studies of fluorescence and phosphorescence enhancement (9-11), stereoselective catalysis (.12,13), and reverse-phase chromatographic separations of structurally similar molecules (14,15). These same complexing abilities make cyclodextrins useful in solvent extraction. 

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