Mark M. Green,' Glenn Blankenhorn, and Harold Hart' Michigan State University East Lansmg 48824
Textbook Errors, 123
Which Starch Fraction is WaterSoluble, Amylose or Amylopectin?
It is really hard to believe that the question of the solubility of a substance in water should be a matter for debate or textbook error. It ought to be a fact. Yet this is not so with amylose and amylopectin, the two major components of starch. Organic chemistry texts correctly state that starch consists mainly of these two components, and further, all agree regarding the structure of each; yet they disagree about which one is water-soluble, and in fact most texts are either wrong or a t best misleading. How could this be? We came upon this curious situation in the following way. One of us (MMG) was teaching a unit on carbohydrates to undergraduate premedical students. It was explained that starch can be separated into two types of molecules. The amylose component of starch consists of glucose units joined mainly by a-glycosidic links through the 1,4 positions to give an essentially linear polymer, coiled into a spiral with six glucose units per turn (Fig. In contrast, amylopectin has a highly branched structure. The branches consist of about 20-25 glucose units linked a-D-(1 4) as in amylose, joined to other branches by a-D-(6 1) linkages (Fig. 2). One theme which had been emphasized in the course was that molecular structure influences gross chemical properties. Consequently i t was not surprising that the highly branched, open structure of amylopectin allowed easy access to hydrogen-bonding solvent molecules and accounted for its solubility in water. On the other hand, the tightly packed, linear, and coiled amylose structure encourages strong intramolecular hydrogen bonds and discourages access by solvent water molecules. Thus from their structures
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Figule 1. Amylose: malnly a linear a D ( 1 4) glucan (taken from Fernier. R. M.. "Monosaccharide Chemistry." Penguin Baoks Ltd., England. 7972).
J., and Collins. P.
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' To whom correspondence should he sent.
A small degree of branching may be naturally present; structural alterations may also be introduced as a consequence of the procedures used to separate amylose from amylopectin. :'Glycogen's structure is similar to that of amylopectin, hut with an average of only 12 glucose residues per branch, and with molecular weight differences. We first phoned Professor Roy Whistler (Purdue), an expert on polysaccharides, who acquainted us with the subtleties of the questions and who also checked this paper for accuracy. We appreciate his kindness. "he most succinct and up-to-date review of the properties of starch and glycogen we found was the chapter by [Greenwood, C. T., in "The Carbohydrates," (Editors: Pigman, W., and Horton, D.L Academic Press. New York. 2nd Ed.. Vol. IIB. 1970.. DD. - ~. . .. 4715131. This review ineludes 168 references, and though written by an expert who is an active contributor to the field, is nevertheless clear and easy for non-experts to read. "he physical properties of amylose and amylopedin in solution have been reviewed by [Foster, J. F., in. "Starch. Chemistry and Technology," (Editors: Whistler, R. L., and Paschall, E. F.), Academic Press, New York,Vol. 1, 1965, p. 3491. 'For reviews, see [Schoch, T. J., Aduon. Carbohyd. Chem., 1, 247 (1945)l; [Muetgeert, J., Aduon. Carbahyd. Chem., 16, 299 ~~
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' Although this method is w r y old, it was exploited primnrily by K. I i . hlewr and el,-uorkers:see [hleyer, K . H., Hrtnr~no.W.. and Hernfdd. 1'. Hvlv ( ' h i m . A r l n . 23. R i f , 1194u.l. A temperatme of 70 'C, just above the gelatinization temperature of corn starch is used. The granules swell and the amylase fraction is leached into the hot water. Meyer is responsible for the terms amylose and amylopectin.
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Figure 2. Arnylopectin; a-D
