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  Home > JCE Print > Journal of Chemical Education > Issues > 2006  > March  >
In the Classroom
JCE Featured Molecules
The Chemistry of Popcorn: Polymers of Glucose
William F. Coleman
Department of Chemistry, Wellesley College, Wellesley, MA 02481
Cover
March 2006
Vol. 83 No. 3
p. 413
Full Text
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).

Fully manipulable (Chime and Jmol) versions of these molecules are available at the JCE Digital Library Web site.

Image of molecule.

Figure 1. cellobiose (glucose dimer with a β-1-4 linkage)

Image of molecule.

Figure 2. maltose (glucose dimer with an α-1-4 linkage)

Image of molecule.

Figure 3. 10-mer of cellulose—all glucose linkages β-1-4
(not optimized)

Image of molecule.

Figure 4. 40-mer of amylose—all glucose linakges α-1-4

Image of molecule.

Figure 5. 10-mer of cellulose—all glucose linkages β-1-4
(optimized)

Image of molecule.

Figure 6. amylopectin fragment with one branch
(an α-1-6 linkage)
More Information
*  Citation
 Coleman, William F. J. Chem. Educ. 2006 83 413.
*  Keywords
 Computer-Based Learning; Internet / Web-Based Learning; Molecular Modeling; Molecular Properties / Structure
*  History
 Created:
Last Updated:		 1/31/2006
2/10/2006
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