MOLECULAR AND M ACROMOLECULAR STRUCTURE 203 



seem, to be largely stereochemical : a polypeptide chain tends to assume 

 the tightly H-bonded helix unless other overriding side-chain inter- 

 action or interference is present to prevent it. When sufficiently long 

 runs of suitable residues exist the helix may form and may associate with 

 other helical segments to produce an a-crystallite which may be detected 

 by X-ray means. While many such forms may thus yield the definitive 

 wide-angle X-ray pattern they may differ greatly in their secondary and 

 tertiary structures. Some of these arrangements, interesting as special 

 problems, may not be immediately relevant to the keratin problem; 

 others, on the other hand, suggest structural possibilities which may be 

 found in the keratins themselves. 



An important example of an a-protein, important because of its origin 

 and the absence of complicating histological structures, is the material of 

 the bacterial flagellum to which Astbury has given the name " flagellin " 

 (1955). Bacterial flagella are thin ( ~ 100 A) whip-like threads attached 

 to bacteria and connected with mobility. 



Astbury, Beighton and Weibull (1955) have described the rich X-ray 

 pattern obtainable from preparations of flagella from Proteus vulgaris and 

 Bacillus subtilis (Fig. 86). It is clearly of the a-type with added reflections 

 indicating an admixture of the cross-/? configuration. Heating produces a 

 /3-form which may be oriented by pressing. The diagnostic 5*1 and 1*5 A 

 reflections are present. The numerous meridional reflections appear as 

 orders of a master spacing of 410 A which is similar to that observed in 

 muscle. The thickness of flagella would suggest that they are structural 

 analogues of the a-filaments of hair keratin. Their poor thermal stability 

 and composition show that they are not keratinized and they could there- 

 fore be related to the primitive non-stabilized a-type proteins of the 

 cytoplasm. Although they are located extracellularly, they are not simple 

 extrusions of the cell wall or capsule. They in fact take their origin from 

 a small basal granule located beneath the protoplast membrane to which 

 they remain attached even when the cell wall has been removed. Thus 

 they remain in direct communication with the cytoplasm and it is per- 

 missible then to regard them in this special sense as cytoplasmic filaments. 



Some a- and jS-proteins of insect origin 



Keratinization is not known in insects; their sclerotized proteins 

 are normally hardened by tanning with aromatic phenols (Chapter I). 

 The various dermal glands are, however, able to produce some variety 

 of protein-types among which are forms of a-type proteins whose study 

 has added importantly to our knowledge of molecular configurations and 

 their interconversions. Silk glands in Bombyx are labial glands and 

 their secretion while in the gland, according to infra-red evidence, has 

 random coil features. It transforms into the insoluble £-form during 



