In the Classroom edited by
JCE Featured Molecules
William F. Coleman Wellesley College Wellesley, MA 02481
The Chemistry of Popcorn: Polymers of Glucose March Featured Molecules The featured molecules this month are all polymers of glucose, and relate to the two papers on the chemistry of popcorn (see pages 414–416B). 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
cellobiose (glucose dimer with a β-1-4 linkage)
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 and other molecules are available at the JCE Digital Library Web site: http://www.JCE.DivCHED.org/JCEWWW/Features/ MonthlyMolecules/2006/Mar
maltose (glucose dimer with an α-1-4 linkage)
40-mer of amylose— all glucose linkages α-1-4 10-mer of cellulose— all glucose linkages β-1-4 (not optimized)
amylopectin fragment with one branch (an α-1-6 linkage)
10-mer of cellulose— all glucose linkages β-1-4 (optimized)
www.JCE.DivCHED.org
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Vol. 83 No. 3 March 2006
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Journal of Chemical Education
413
