2 PROTEINS 327 



incorrect, sense to real solutions of substances of low molecular 

 weight, Nageli's original definition was retained by botanical 

 cvtologists, for to this very day the enclosures of the aleurone granules 

 in the seeds of Ricinns (Pfeffer, 1872), Momordica (Zimmermann, 

 1922), Telfairia (Leuthold, 1935), etc. are called crystalloids. 



The crystal lattice of globular proteins are often cubic or hexagonal ; 

 witness the occurrence of cubic or rhombohedral crystal shapes in the 

 crystallized reserve proteins of vegetable seeds. The globular ele- 

 mentary units of the molecular lattice (see p. 26) are so big as to 

 produce a large spaced lattice (Fig. 90a, p. 136), into the meshes of 

 which swelling agents and dyestuffs can penetrate. The swelling of 

 the rhombohedral protein crystalloids is anisotropic, being, as 

 Nageli (1862) had already discovered, different parallel to the crystal 

 axis from what it is perpendicular to it. Up to 1939 only seven of all 

 the many crystallizing globular proteins had been examined by X-ray 

 crystallography, these being pepsin, insulin, excelsin, lactoglobulin, 

 haemoglobulin, chymotrypsin and tobacco seed globulin (Crowfoot, 

 1939, 1941). For, in spite of repeated attempts, it was long before any 

 success crowned the efforts to obtain X-ray photographs of mono- 

 crystals of the crystalloids. Thus, for example, the crystalloids of the 

 seed globulin excelsin of the spruce have threefold symmetry, and 

 those of pepsin hexagonal, but this fact was in no way revealed by 

 the X-ray photograph of a single crystal. On the contrary, until a 

 short while ago all monocrystal photographs of globular proteins, and 

 particularly in the case of the well "crystallized" pepsin (Astbury and 

 LoMAx, 1934), only produced Debye-Scherrer rings with lattice 

 spacings of 4.6 and 11.5 A, which unexpectedly proved to have the 

 backbone thickness and the side chain spacing of polypeptide chains. 

 In view of the large molecular weight of the crystallized proteins, it 

 was anticipated that, instead of such spacings, there would be very 

 large periods which would produce interference dots quite near the 

 centre of the photograph. Although some such large lattice spacings 

 had been found in insulin (Clark and Corrigan, 1932) and in pepsin 

 (Fankuchen, 1934), Bernal and Crowfoot (1934) were the first to 

 be entirely successful in obtaining monocrystal X-ray diagrams. The 

 secret of their success lay in the fact that they irradiated the pepsin 

 crystalloids (hexagonal bipyramids 2 mm in height) in their mother 

 liquor. In this way they discovered a wide-meshed crystal lattice, the 



