Since I recollected an explanation regarding iodine-starch color from
long ago [graduate school days] I looked back in the original?? 
Lehninger BIOCHEMISTRY. Page 229 has a reference to the blue color
formed with iodine and amylose [and red-violet for amylopectin].
However, I recall a more detailed explanation from that era [I was
assaying starch content of seeds using a perchloric acid / iodine
staining photometric technique -- always loved living dangerously and
wonder how I actually did!] If I recall correctly [I don't know the
source -- it may have been a CRC publication], the explanation was
basically that the iodine fit into the spaces between adjacent loops
in the helical coils of the starch molecule, possibly shifting the
coil dimensions and thus causing the color change in the starch-iodine
complex. At any rate, if you draw an amylose helix and insert I2
atoms between the coils, you'd pretty much have what I recall seeing
in that diagram in a long forgotten source.
I wish someone would verify or correct this old explanation.
Department of Biology
Wingate, NC 28174
"William S. Cohen" wrote:
> What is the chemical basis for the staining of starch by
> Iodine/Potassium Iodide?
> Dr. William S. Cohen
> Associate Director
> School of Biological Sciences
> University of Kentucky
> 101 Morgan Building
> Lexington, KY 40506-0225
> Voice:  257 1030
> Fax: 257-1717
> E-mail:wscohen at uky.edu(Note-New Address)