Study of the Bluing Mechanism of Amylose-Iodine Complexes by CD Stopped-Flow Method

The bluing mechanism of amylose-iodine complexes was investigated by simultaneous stopped-flow measurements of circular dichroism (CD) and absorption spectra, using an amylase with an average degree of polymerization (DP) of ca. 20. A two-step process of complexation was observed under the conditions that the bound species of iodine is only I-3 and the concentration of amylose was much larger than that of I-3. The first step was the formation of achromic complex of amylose with intrinsic I-3, accompanied by the appearance of induced positive CD bands around 288 and 353 nm. In the second step, the formation of a dimer I-3·I-3 responsible for the coloring of the complex was observed. On the other hand, under the conditions that the coloring species of iodine was I2·I-3 in the absence of KI, the complexation proceeded by apparent one-step reaction. The apparent initial rate for the formation of I-3·I-3 was estimated to be smaller than that of I2·I-3. Consequently, it was suggested that the formation of I2·I-3 is facilitated by the more cooperative interaction between I2 and I-3, than that between the two I-3 ions.