MEASUREMENT OF BINDING CONSTANT OF CYCLODEXTRIN BY ULTRAFILTRATION

概要

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We proposed a new measuring method of the binding constant KB, by proving that the inclusion degree α in a host-guest reaction is equal to the rise ratio of rejection η in ultrafiltration. The KB for nitrophenol (NP) and cyclodextrin (CD) system was measured. The data were compared with those of other investigators. The KB measured by the proposed method decreased with increase in operating pressure, but a constant value of KB was obtained below a certain pressure. The pressure was nearly 1 MPa for the NP-CD system. The constant value was the equilibrium binding constant KB. With an increase in pH, the KB became large because guest molecules were charged and the anion and hydrogen bonds between host and guest molecules became strong. With a decrease in the concentration ratio ε of guest molecules to host molecules, the inclusion degree α became large, but the binding constant KB remained almost constant. After checking the validity of the new method by NP-CD system, the KB value for optical isomers of amino acids, threonine and phenylalanine, were measured to investigate whether the optically selective inclusion of guest molecules by CD was possible or not. CD, which is an optically active D-form molecule, tended to include optical L-acid rather than D-acid under the present conditions. The optimum conditions were referred to resolute optical isomers.
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