"Structure of polyglycine II"

Date:
1955
Reference:
PP/CRI/I/1/7
Catalogue details

Licence: In copyright

Credit: "Structure of polyglycine II". Source: Wellcome Collection.

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    {Reprinted from Nature, Vol. 176, pp. 780-781, Oct. 22, 1955) STRUCTURE OF POLYGLYCINE II By Dr. F. H. C. CRICK Medical Research Council Unit for the Study of the Molecular Structure of Biological Systems, Cavendish Laboratory, ERY recently Bamford and his colleagues 1 have described their work on a second form of poly- glycine, which they call polyglycine II. The purpose of this communication is to suggest a structure for this substance. Bamford et al. 1 have shown clearly that polyglycine can give two types of X-ray diagram according to the method of precipitation. Polyglycine I gives strong reflexions at 4 -4 A. and 3-45 A., and is thus some kind of ß-structure ; that is, the polypeptide chains are almost but not quite fully extended, and hydrogen bonds are formed between neighbouring chains (in approximately the same plane) to form sheets of parallel or anti-parallel chains. Polyglycine II, on the other hand, gives a very strong reflexion at 4-15 A., and a moderately strong one at 3-1 A. Only a powder diagram has so far been obtained. Bamford et al. 1 have shown that the X-ray picture cannot be easily explained in terms of any well- known configuration of the polypeptide chain. The structure we propose is simple. All the poly peptide chains are parallel and each one has a three fold screw axis. The chains are packed in a hexagonal array, each chain being hydrogen-bonded to each of its six neighbours. These hydrogen bonds lie roughly perpendicular to the screw axis, and, as implied above, run in several directions and not merely in one direction as in a ß-structure. This can be seen in Fig. 1, which is a projection of the structure looking down the screw axis. Another view of the structure is shown in Fig. 2. This is a projection along the cc-axis and perpendicular to the screw axis (the 2-axis), which runs^vertically. The easiest way to visualize the structure is to look at one of the residues at the bottom of the figure. This shows that the planar peptide group is inclined at about 35° to the fibre axis, and has its plane per pendicular to that of the paper. There is thus an in finite sequence of hydrogen bonds running from one ♦ Section on Physical Chemistry, National Institute of Mental Health, Bethesda 14, Maryland. and Dr. ALEXANDER RICH* Cambridge