MadSci Network: Biochemistry
Query:

Re: When iodine tests for starch is it continous??

Date: Thu Dec 30 12:29:29 2004
Posted By: Alvan Hengge, Faculty, Chemistry and Biochemistry, Utah State University
Area of science: Biochemistry
ID: 1101259253.Bc
Message:

Lee,

The iodine test for starch should be usable in the way you describe, as a continuous assay for the presence of starch. Starch gives a blue-black color with the IKI reagent, while glycogen gives a brown-blue color. Other polysaccharides and monosaccharides yield no color change; the test solution remains the characteristic brown-yellow of the IKI reagent. It is thought that starch and glycogen form helical coils. The iodine molecules and iodide ions can fit into the helices to form a starch-iodine or glycogen-iodine complex. Starch in the form of amylose and amylopectin is less branched than glycogen, and as a result, the helices of starch are longer than glycogen, allowing them to binding more iodine. The result is that the color produced by a starch-iodine complex is more intense than that obtained with a glycogen-iodine complex. Other polysaccharides, and monosaccharides, do not provide helical coils to bind the iodine and thus produce no color change.

Pertinent to your question, if you add an enzyme to cleave the polysaccharide bonds in starch, you will break up the helical structure and release the iodide into solution, which should reverse the color change.

A web site that contains a graphical image of the process may be found at: www.elmhurst.edu/~chm/vchembook/548starchiodine.html


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