Synthesis and Functions of Branched Cyclodextrins

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The conditions for the synthesis of branched cyclodextrins (CDs) by the reverse action of Klebsiella pullulanase, the procedures for isolation of the branched CDs from reaction mixtures, and their properties were investigated.The rates for the formation of branched CDs with pullulanase were in the order of <gamma> -> <alpha> -> <beta> -CD, while those with Pseudomonas isoamylase were in <gamma> -> <beta> -> <alpha> -CD. Maltose ( <G_2> ) is the better substrate than maltotriose ( <G_3> ) for <alpha> -CD as to the counter substrate but <G_2> and <G_3> were equally effective substrates for <beta> - and <gamma> -CDs. The optimal conditions for the formation of <G_2> - <gamma> -CD were as follows : <gamma> -CD 0.4 M, <G_2> 1.8 M, pH 6.0 and temperature 50゜C. Under these conditions, 37% of <gamma> -CD was converted into <G_2> - <gamma> -CD by incubation for 36 hrs with 21.6 U/ml pullulanase.The two minor products, di <G_2> - <gamma> -CD and branched maltoteraose (B <G_4> ) were also found besides <G_2> - <gamma> -CD by the above reaction and these products were identified by <^(13)C> -NMR spectra and chemical and enzymic analyses. The water solubility of <gamma> -CD was enhanced about 6-fold by binding a side-chain and the hemolytic activity for human red blood cells was reduced to about a half. These results suggest that the modification of CDs to branched CDs is useful for their applications.