204 KERATIN AND KERATINIZATION 



spinning. The detailed structure of /3-fibroin illustrates clearly how the 

 insolubilization is produced by multiple H-bonding between the carbonyl 

 and imino groups of the peptide bonds. Nevertheless the transformation 

 is not fully understood; there is evidence both for an unfolding of the 

 chains effected by stretching and for an aggregation process which does 

 not obviously involve chain unfolding (p. 129). 



Other glands, the colleterial glands, accessory to the female genital 

 system, produce various fibrous secretions, which serve to support or 

 protect the eggs. One example, the egg-bearing stalk of Chrysopa, is 

 important as a naturally-occurring example of a cross-/3 system (see p. 201), 

 i.e. the polypeptide chains in a /S-form are oriented at right angles 

 to direction of stretching of the fibre. Such a configuration is not un- 

 common in tissue proteins under experimental conditions (see p. 200) but 

 never achieves the perfection of orientation found in these egg stalks. 

 Another extremely interesting structure based on an a-form was discovered 

 by Rudall (1955-6) in the egg case of a mantis which forms from the 

 secretion of the colleterial glands. The dried case is tanned and insoluble. 

 However, when freshly formed it may be dispersed by tryptic digestion 

 and is seen to be composed of masses of long, very thin ribbons (5Qa X 

 1-2//) which (electron-microscopically) are crossed by regularly-spaced lines 

 (120 A apart) making an angle of about 20° with the length of the ribbon. 



When first secreted from the colleterial gland the protein is a viscous 

 mass of globules which change first into long fibrils and then into ribbons. 

 The air-dried secretion before this change gives a somewhat diffuse X-ray 

 pattern but the definitive a-spacings : 1*5 A and 5*18 A are present. The 

 usual, strong, equatorial reflections at 10 A are missing, the lateral spacing 

 being represented by diffractions near 14*5 and 8*3 A. This pattern could 

 be given by rods of diameter 16*5 A packed in hexagonal array. This same 

 secretion after changing into ribbons yields a remarkably different pattern : 

 the main wide-angle meridional spacings at 1*5 A and 5*1 A are present, 

 but photographs taken at right angles to the length of the ribbons and 

 parallel to their face are of the a-type, and the pattern is dominated by a set 

 of row lines which are orders of 17*5-1 8*5 A (depending on hydration 

 (Plate 3A)). 



Parallel to the long axis of the ribbons the diffractions from planes 

 perpendicular to the surface of the ribbons contain one set indicating a 

 period of 31 or 62 A, leading to a picture of rods 10-3 A in diameter in 

 hexagonal packing (Fig. 87). 



On stretching the ribbons in water a £-form with axial periodicity of 

 3*33 A is produced which returns to the a-form on releasing the tension. 

 The /^-structure is still double-oriented, i.e. the " backbone spacing " 

 (4*7 A) is oriented perpendicular to the ribbon surfaces, which would seem 

 to imply that the a ±5: /? transformation is effected with the maintenance of 



