BARBARA W. LOW AND JOHN T. EDSALL 



inclined to the vertical and when the residue is tilted in the 

 opposite direction (downwards), this angle is increased. Helical 

 chains of the "a" and the "7" series are not interconvertible 

 without unwrapping, nor can a region of one succeed a region of 

 the other along the same axis. Although the 7-helix satisfies the 

 initial Pauling-Corey criteria, excluding the a-carbon single 

 bond orientation effects, it is less stable than the a, and some 

 other "a" series, configurations. It appears unlikely that the 7 

 structure or any of the other, even less stable, members of this 

 general series are present in protein structure. 



The helices (Figures 4 and 5) are shown with /3-carbon 

 atoms in the correct configuration for L-amino acid residues. 

 Bijvoet, Peerdeman, and van Bommel (13) have established the 

 absolute configuration of D( + )-tartaric acid and demonstrated 

 the correctness of the Fischer convention. The vr-helix shown 

 has a right-handed screw ttr. The mirror image of a right- 

 handed screw is a left-handed screw; the mirror image of an 

 L-amino acid residue is a D-amino acid residue. The mirror 

 image of a left-handed helix of L-amino acid residues, is thus a 

 right-handed helix of D-amino acid residues. These are true 

 enantiomorphs. A left-handed helix of L-amino acid residues is 

 different from a right-handed helix of L-amino acid residues. In 

 the latter (Figures 4 and 5) the aC — ^C bond has a vertical com- 

 ponent in the same direction as the vertical component of the 

 NH bond, whereas in the former the two are opposed. There 

 are thus two alternative helical chain configurations possible 

 with L-residues. These two helices are sometimes described in 

 terms of the two possible ;5-carbon atom positions. This 

 terminology depends on the enantiomorphous relationship 

 discussed above. X-ray diffraction effects cannot normally 

 distinguish between mirror image structures. A left-handed 

 helix of D-amino acid residues is thus the equivalent of a right- 

 handed helix of L-amino acid residues. 



There are three extended (jS) polypeptide chain structures 

 which satisfy the initial Pauling-Corey chemical criteria and 

 which give the favored stable orientation about the single bonds 



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