"The structure of the synthetic a-polypeptides"

Date:
[1953]-1954
Reference:
PP/CRI/H/1/18
Catalogue details

Licence: In copyright

Credit: "The structure of the synthetic a-polypeptides". Source: Wellcome Collection.

    4/69
    rather than one with a crystallographic axis, is that the structure is dominated by the interaction between successive turns of the same helix, rather than by interactions between adjacent helices. The screw axis applies only to the residues of one chain. It does not relate neigh boring helices to each other. c ~helix was derived by Pauling, Corey and Branson by enumerating a series of postulates, and then searching systematically for all those configurations which satisfied them. These postulates were: (1) normal bond distances and angles for chemical bonds (Z) the planarity of the peptide (amide) bond (3) good hydrogen bonding (4) equivalence The great strength of this type of method really lies in the first postulate. Largely due to the careful work of the crystallographers at the California Institute of Technology there has been built up over the past fifteen years an accurate body of information on the probable bond distances and angles of the polypeptide chain. This has been ob tained by studying small molecules, such as amino acids and peptides. A structure which deviated from these tances by more than about 7° or from the angles by more than a few degrees would be regarded as very unlikely. The second and third postulates also derive from this work. The short length of the C-N bond (1.32 A), indicating partial double-bond character, and the fact that in all the crystals so far examined the peptide group, II, £ \ l! c N o // / \ H C has been planar, or very nearly so, strongly supports this postulate« Similarly a set of empirical rules for NH...0 hydrogen bonds can be nee should not be far given (Donohue). For example the N 0 disti O from Z, 85 A, and the NH bond should point approximately towards the oxygen atom The fourth postulate is of a radically different type. i y 'equi valence' one means that an atom in one residue has an identical atomic