Cyclic Inulooligosaccharides

Inulin can be used to produce cyclic inulooligosaccharides in which the fructose chain closes back on itself, eliminating the presence of a reducing end (Figure 5.4). These compounds are thought to be potentially useful for food, drug, cosmetic, surfactant, catalyst, and purification and separation applications. The cyclic compounds contain six, seven, or eight fructose subunits (i.e., cycloinulo-hexaose (1), cycloinuloheptaose, cycloinulooctaose), with the distribution favoring the six-subunit form (i.e., 1.23, 0.44, and 0.09 g, respectively) (Oba et al., 1992). The primary attribute of these compounds is the presence of a hollow cavity in the center, which is surrounded by a relatively hydrophobic surface. This allows certain hydrophobic molecules to enter and form a stable inclusion complex. Cyclic inulooligosaccharides, for example, can be used to form water-soluble complexes with hydrophobic drugs, fragrances, or oil-based flavors (Okamura et al., 1997). They can also potentially be used in deodorizing sprays where they complex with odorants, thereby removing them.

Cycloinulohexaose is synthesized using Bacillus circulans (Kuwamura et al., 1989) or a similar microorganism containing a fructanotransferase (e.g., B. polymyxa, B. subtilis) in a shake-culture medium of inulin, yeast extract, and salts at 30°C for 30 h and then heated to 100°C for deactivation (Oba et al., 1992) or by using the cycloinulooligosaccharide fructanotransferase directly (Nanjo, 2004). Cycloinulohexaose has a characteristic 18-crown-6 skeleton (Figure 5.4) and forms 1:1 complexes with metal ions such as Ba2+ (Uchiyama et al., 1993). The permethylated derivative of a cycloinulohexaose has been synthesized and its metal binding association constants in acetone determined (Li+ < Na+ < Cs+ < K+ < Ba2+) (Takai et al., 1994) (See Section 5.9.19). Interestingly, the metal ions are not found in the central cavity but rather in a pocket formed by the upper rim 3-OMe oxygens and the crown ether oxygens.

Other uses of cyclic inulooligosaccharides include the removal of harsh tastes from alcohol and spirits (Katsuragi and Nishimura, 1992) and the trapping of metal ions such as Ba2+, K+, Rb+, Cs+, Ag+, and Pb2+ (Uchama, 1993).

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