Kinetic Studies of Amylose-iodine-iodide Reaction by Stopped-flow Method*

1. The kinetics of the complex formation between iodine-iodide and amyloses of various number-average degrees of polymerization (DP_n) at large excess of amylose was studied by stopped-flow method, over the wavelength range of 280–800 mμ, at pH 4.9 and 24.7°C. 2. With amyloses of $DP¯n=15$ and 28, the reaction was found to be completed within about 1 msec, the dead-time of the apparatus. However, with amyloses of DPn above about 40, at least two stages with considerably different velocities were observed. 3. A typical example with amylose of $DP¯n=8,700$ and at low iodine concentration showed that the spectral change which was developed within the dead-time had an absorption peak around 570 mμ, while that developed in the first stage which was observable by the stopped-flow method (stage C) had a peak at longer wavelength around 660 mμ. Most of the free triiodide ions have been taken up within the dead-time. 4. The time course of stage C followed apparent first order kinetics, and the first order rate constant, k, of this stage was roughly proportional to the fifth power of added iodine concentration, (I₂)₀, at lower (I₂)₀range, and tended to saturate at higher (I₂)₀. k was almost independent of amylose concentration over the range studied (0.0025−0.1%). At fixed concentration of added iodine, k increased with about the fifth power of DP_n up to $DP¯n←→140$⁠, above which it was independent of $DP¯n$ of amylose. 5. These results are essentially consistent with a simplified mechanism, in which a complex with shorter poly-iodine chain is formed rapidly and reversibly within the dead-time (about 1 msec), and this complex subsequently transforms into a complex with longer poly-iodine chain without appreciable participation of free triiodide ion in solution. The latter change is considered to correspond to stage C. 6. Evidence has been obtained which indicated that stages following stage C may involve the rearrangement of poly-iodine chain once formed in stage C into a ther-modynamically more stable state, which could have shorter poly-iodine chain than that involved in stage C.