Chemistry: Pa Uling and Corey the Structure of Synthetic Polypeptides by Linus Pauling

In a preliminary communication last year' we stated that there are only two helical configurations of polypeptide chains in which the residues are all equivalent and intramolecular hydrogen bonds are formed, and in which the interatomic distances, bond angles, and other structural features, especially the coplanarity of the conjugated amide system, are as required by earlier work in these Laboratories on amino acids, simple peptides, and other substances related to protein's. These two helical configurations were described in detail in a later paper2 and it was mentioned that there is evidence that they occur in a keratin, a myosin, supercontracted keratin and myosin, and other fibrous proteins, and also constitute an important structural feature of hemoglobin and other globular proteins.3 In the following paragraphs we discuss evidence that one of the helical structures is assumed also by synthetic polypeptides. Oriented films and fibers of several synthetic polypeptides have been prepared and examined by x-ray diffraction by Bamford, Hanby, and Happey,4 and the oriented films have been investigated with polarized infrared spec-troscopy by Ambrose and Elliott.5 The best photographs were given by poly-,y-methyl-L-glutamate and poly-,y-benzyl-L-glutamate. The x-ray data indicate an identity distance in the direction of the fibers of 5.50 A for the first substance, and a slightly larger value for the second substance. The dichroism observed for the N-H stretching and bending infrared absorption bands and for the C==O stretching band indicates that these groups are oriented nearly parallel to the fiber axis, and the conclusion is accordingly drawn that intramolecular hydrogen bonds are formed nearly parallel to this axis. These authors have interpreted all of their data as providing strong support for the a,, structure shown in figure 1. The all structure must, however, be rejected. In the structure as discussed by Bamford and coworkers the amide group is not assigned a planar configuration. There is, in fact, very strong theoretical and experimental evidence that the carbon-nitrogen bond has a large amount of double-bond character, and that the four atoms adjacent to these two atoms must form a coplanar system with them. The carbon-nitrogen distance is 1.32 A, which is 0.15 A less than the single-bond distance between these atoms.