MOLECULAR AND MACROMOLECULAR STRUCTURE 165 



fruits if a single dominant component is sufficiently crystalline to 

 yield a recognizable diffraction pattern (Chapter I). The method, as it 

 were, cuts through the tangle of secondary structures and minor constit- 

 uents and yields immediate information concerning the arrangement of 

 the atoms in the crystalline regions. Further, since it leaves the material 

 unharmed the same sample may be used for other tests. Fortunately all 

 the keratinized mammalian tissues, as was discovered by Astbury and his 

 associates (see Chapter I), give substantially the same pattern, proving that 

 they contain crystallites of similar molecular structure. Feather keratin 

 gives a /S-type pattern, one of the most detailed yielded by any protein 

 fibre. 



The formal analysis of the X-ray diffraction patterns of a well-crystalline 

 material can lead to the exact placing of the atoms (other than hydrogen) 

 in the structures and a growing number of organic compounds of biological 

 importance have been determined in this way. The proteins, whose 

 molecules may contain thousands of atoms, offer enormous difficulties 

 but great success has been achieved in recent years (Perutz, 1959). 



The low-angle pattern. The total X-ray pattern yielded by biological 

 fibres is separated conventionally into two parts (Fig. 3, p. 11) described 

 as the wide-angle pattern and the low-angle pattern. The dividing line is 

 arbitrary, but reflections corresponding to a Bragg spacing of less then 

 20 A are referred to the wide-angle pattern, those corresponding to 

 longer spacings belong to the low-angle pattern. As described in Chapter 

 I, the number of wide-angle patterns is small — whole groups of fibrous 

 proteins being characterized by the same pattern. In contrast to this 

 simple situation among the wide-angle patterns, a considerable variety of 

 small-angle patterns are found even among proteins which are classified 

 together on account of their similar wide-angle patterns. These low-angle 

 X-ray patterns of the protein fibres are often of great complexity, and 

 indicate the existence of elaborate structures of macromolecular dimensions 

 which are as yet ill-understood. The relation of these structures to the 

 smaller molecular formations responsible for the large-angle patterns we 

 have just been considering, is also obscure. The " long spacings", which 

 may be either meridional or equatorial in placing in the X-ray patterns, can 

 be measured with some accuracy. If the reflection is sharp, a lattice 

 spacing corresponding to it is usually calculated by means of the Bragg 

 equation. When they are diffuse this procedure is difficult to apply; but 

 the totality of the pattern is characteristic of the protein and may undergo 

 changes when the material is chemically altered. In this respect the 

 low-angle pattern is more characteristic of a particular protein than are the 

 wide-angle patterns. 



Sketches of two low-angle a-patterns are given in Figs. 68 and 69, and in 

 Table 9 are listed the long spacings of the keratins as measured by 



