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The Chemistry of Popcorn: Polymers of Glucose
The featured molecules this month are all polymers of glucose, and relate
to the two papers on the chemistry of popcorn: Popping
Popcorn Kernels: Expanding Relevance with Linear Thinking, and Popcorn—What's
in the Bag? Cellulose, a polymer consisting of glucose molecules linked
by β-1-4 glycoside linkages, is largely found as a structural material.
The dimer is called cellobiose. A 10-mer is given in two forms. Prior to energy
minimization the structure is both straight and relatively flat. Students should
compare that structure to the energy minimized form (minimized using the Amber
force field parameterized for polysaccharides) to see which structural features
are changed and which are retained. The helical structure of amylose clearly
illustrates the structural differences that arise from having all linkages
be α-1-4. The dimer with an α-1-4 linkage is maltose. Using the
Jmol rendering of the larger amylose molecule students should easily be able
to determine why iodine molecules are entrained in amylose, producing the blue-purple
color that is familiar as the starch-iodide test. The color arises from electronic
transition in the resultant charge-transfer complex. Students could explore
the amylopectin structure to find and identify the branch linkage, and to comment
on the structural effect of the branch region in this small fragment (in larger
polymers of this system the α-1-6 branch linkages occur every 20-30 glucose
units).
Viewing Requirements
In addition to the static images, two fully manipulable versions
(Jmol, MDLChime)
of these molecules appear below.
 Download
Chime (registration required)
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