"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.

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    to postulates or careful model building. ìs convenient to consider the infra-red evidence first. The frequencies mainly examined were the 0=0 vibra tions in the 6/uregion, and the ft-H vibrations in the 3/xregion. These show clearly that the great majority of the NH groups are forming hydro gen bonds. Moreover if the a-form Is dissolved,the character of the infra-red absorption is unchanged even at great dilutions'! This sug gests that the hydrogen bonds are formed within the structural unit, and that this unit persists in solution» Polarized infra-red studies, mainly by Elliott, showed that the dichroism of these bands in favourable specimens of the c-polypeptides was high, and occasionally very high, and in such a sense that the hydrogen bonds were (approximately) parallel to the fibre axis. This in itself would show that the polypeptide chain was not fully extended in the fibre direction. X-RAY FIBRE DIAGRAMS; The X-ray pictures obtained are fibre diagrams. This means that the crystallites which make up the specimen have been aligned so that the polypeptide chains are all roughly parallel, while the orientation of the crystallites about the fibre axis is random, or nearly so» The effect on the X-ray photograph is the same as if a single crystallite had been rotated about its fibre axis while the photograph was being taken. Thus all (or almost all) the information is superposed on one photograph, which is rarely an advantage. For example, when the lattice is exactly hexagonal in shape this means that every reflexion observed is in reality a number of different re flexions superposed, due to the random orientation of the crystallites. This naturally reduces the amount of information that one can obtain. The lack of exact parallelism between the fibre axes of the crys tallites, which draws out the spots into arcs; various forms of dis- % order in the crystallites, and the amorphous material inevitably pre® sent in the specimen all reduce the precision of the data, arid in general fibre diagrams compare poorly with X-ray photographs of single crystals. A further loss of information can occur if the specimen is not tilted to obtain the wide-angle reflexions near the meridian. This technique is almost as old as X-ray crystallography but it was neg- 11 lected in this field before the work of Perutz.