"X-ray analysis and protein structure"

Date:
1957
Reference:
PP/CRI/I/1/15
Catalogue details

Licence: In copyright

Credit: "X-ray analysis and protein structure". Source: Wellcome Collection.

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    solvent either one or the other can be precipitated from solution at will. Thus m-cresol usually gives the a-form, while the /3-form is precipitated from formic acid. The two forms give quite different X-ray patterns and they can also be distinguished by means of their infrared absorption spectra (as shown by the extensive studies of Elliott and his coworkers, 1956; see the review by Doty and Geiduschek, 1953). a. a-Polypeptides. The best X-ray photographs of polypeptides in the a-form have been obtained from poly -l -alanine (Bamford et al, 1954; Brown and Trotter, 1956) and from poly -7 -methyl -l -glutamate (Bamford et al., 1952, 1953). These photographs have very characteristic features, and so far all polypeptides in the a-form have given similar X-ray patterns, though with varying degrees of perfection. The most detailed studies of them are those carried out by Bamford and his colleagues at Messrs. Courtaulds Ltd (Bamford et al, 1956) and described in their recent book. The main features are a strong meridional reflection of 1.5 A., discovered by Perutz (1951) ; a strong layer line of reflections with layer line spacing 5.4 A.; together with a strong reflection, spacing about 10 A. (depending on the side chain), on the equator. It now seems certain that the configuration of the «-polypeptides is based on the a-helix of Pauling et al, (1951). This is a folded configura tion of the main chain; the positions of the atoms in the side chains beyond C ß are not specified by it. A diagram of the a-helix is given in Fig. 7. The polypeptide chain backbone follows an approximately helical path having a pitch of 5.4 A. and containing about 3.6 amino acid residues per turn. The translation per residue in the fiber axis direction is thus 5.4/ 3.6 = 1.5 A. The C a carbon atoms, to which the side chains are attached, are all at a radius of 2.3 A. The whole structure is held together by bydrogen bonds running from the NH of one peptide group to the CO of another peptide group on the next turn of the helix. The arguments in favor of the a-helix have already been rather fully set out elsewhere (Crick, 1954) and will be only very briefly recapitulated here. From the X-ray pattern it is possible to deduce unambiguously the parameters of the nonintegral screw axis: these are a rotation of about 100° and a translation of 1.5 A. From the density of the specimen and the dimensions of the unit cell it can be shown that the asymmetric unit consists of a single amino acid residue. The positions of the strong reflec tions, together with the fact that a-polypeptides can be stretched into a /3-form, show that there is only one polypeptide chain per lattice point, rather than two or more intertwined. Only two structures can be built to this specification: one, the a-helix, has the same parameters as those observed; the other (described by Bamford et al., 1952) is very much less satisfactory stereochemically.