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 amylose 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 I3- and the concentration of amylose was much larger than that of I3-. The first step was the formation of achromic complex of amylose with intrinsic I3-, accompanied by the appearance of induced positive CD bands around 288 and 353 nm. In the second step, the formation of a dimer I3-.I3- responsible for the coloring of the complex was observed. On the other hand, under the conditions that the coloring species of iodine was I2.I3- in the absence of KI, the complexation proceeded by apparent one-step reaction. The apparent initial rate for the formation of I3-.I3- was estimated to be smaller than that of I2.I3-. Consequently, it was suggested that the formation of I2.I3- is facilitated by the more cooperative interaction between I2 and I3-, than that between the two I3- ions.