Starch-iodine complex

Nancy Harrison vulpia at sonic.net
Tue Apr 7 11:46:57 EST 1998


Jon Monroe wrote:
> 
> >What is it? I have searched thru our chemical references and
> >cannot find the formula for the starch-iodine complex. I am
> >probably looking in the wrong place: under "starch" and "iodine".
> >Inquiring junior college minds want to know! -NHarrison
> 
> Nancy and other inquiring minds;
> 
> My understanding is that amylose (the nearly unbranched form of starch)
> forms a helix with six glucose units per turn (see Salisbury and Ross,
> Plant Physiology 4th ed. page 247).  Iodine gets into the middle of the
> "tube" and in that environment appears blue or purple.  Amylopectin (the
> branched form of starch) can be purple or red with iodine stain.
> 
> You can see an beautiful example of this in a paper by Kakefuda and Duke
> (Plant Physiology 75: 278-280, 1984) in which they ran various starch
> hydrolases on a nondenaturing polyacrylamide gel, then electrophoretically
> transferred the enzymes through a second polyacrylamide gel containing
> starch (like a western transfer).  The enzymes partially chew "holes" in
> the starch as they go and after staining the second gel with iodine the
> holes appear blue, brown, grey or red.  It is a very nice technique but one
> must wear a mask or not talk when setting it up because salivary amylases
> can speckle the second gel with white spots!
> 
> Jon
> 
> -----------------------------------------------------------------
>   Jonathan Monroe                voice:  540-568-6649 (office)
>   Department of Biology                  540-568-6045 (lab)
>   James Madison University       fax:    540-568-3333
>   Harrisonburg, VA 22807-0001    e-mail: monroejd at jmu.edu
>     http://www.jmu.edu/biology/biofac/jmonroe/jmonroe.html
> -----------------------------------------------------------------
Thanks, Jon, and all who responded to my starch-iodine query.
Yesterday I took myself over to the Chem dept and got an
illustrated mini-lecture on the subject: yes, starch in aqueous
solution forms helices like "slinky toys". The iodine has to be
I2KI or something with both elemental iodine and iodide ion.
These latter form a linear structure like a broomstick. The
broomsticks find their way into the coils of the "slinky" thus
causing a change in light absorption, and the blue-black color.
The point for the students, who ask why the "blue-black" stuff
doesn't diffuse out of the starch bag, is not that the complex
is to big - it just doesn't exist as a compound, and the iodine
and starch are still separate entities in the bag!
  Amazing stuff - what you can learn every day with the great
help of the plant-edders! -Nan



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