644 GOEBEL, DIMPFL, BRANT
Mucromolecules
The Conformational Energy of Maltose and Amylose Carol V. Coebel, William L. Dimpfl, and David A. Brant* Depurtment of Chemistry, University of Culifornia, Ircine, Califimia 92664. Received April 30, 1970
ABSTRACT: The conformational energies of helical amylose and its dimeric skeletal unit maltose have been calculated using structural models based upon the crystal structures of cyclohexaamylose and methyl &maltoside. Potential functions which account for intramolecular van der Wads, coulombic, and hydrogen-bonded interactions have been employed. Intramolecular hydrogen bonding is found greatly to enhance the stability of certain helical conformations favored also on the basis of van der Waals interactions. Coulombic interactions appear to make negligible contributions to the conformational energy within the domain of sterically allowed conformations. Conformations corresponding to crystalline amylose V occur in the immediate domains of the least energetic right- and left-handed helices. Conformations corresponding to native amylose B occur at somewhat higher energies, presumably owing to neglect of intermolecular interactions. Left-handed V amylose is found to be less energetic than its right-handed counterpart by 2.6 kcal molkl, a difference near the probable limits of uncertainty in the present calculations of helix energy. Facile conversions of amylose V to amylose B possessing the same chirality are described, and it is argued on the basis of results obtained that reversal of chirality in V to B transitions is improbable.
A
pproximate conformational energy calculations have had demonstrable value in helping to provide interpretation of polymer properties in terms of the structural characteristics of the chains. The properties of several polymers in dilute solution have been interpreted using estimates of the conformational energy of appropriate skeletal segments. l-' When applied t o regularly ordered helical polymer chains, conformational energy calculations have successfully predicted the stable crystalline forms.8- l 7 Motivation for the present paper is found within the context of a general program which seeks to relate the structure and properties of polymers of the polysaccharide class. l8-2" Specifically, an interpre-
*
To whom correspondence should be addressed.
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