MOLECULAR AND MACROMOLECUL AR STRUCTURE 183 



occurrence of this particular spacing has become diagnostic for the 

 a-helix. Astbury et al. (1959) proved that the 1-5 A reflection arose from 

 the same structure as the 5-1 A by showing that both reflections were 

 similarly altered by the small (2%) reversible extensions of fibres. 



Very detailed comparisons have been made between the observed X-ray 

 patterns of certain synthetic polypeptides and that predicted from the 

 a-helix by Bamford et al. (1956), Yakel (1953) and Brown and Trotter 

 (1956) and they have demonstrated that the a-helix forms the structural 

 basis of these polypeptides. The patterns given by fibres of these materials 

 are often of remarkable perfection and far superior to those of the natural 

 fibrous proteins. It seems probable that the study of a synthetic polypeptide, 

 based more closely on the naturally-occurring sequence of residues in 

 the crystalline regions of the keratins may lead most easily to further 

 advances in the understanding of the natural structures. 



When the patterns of the a-synthetic polypeptides (see Bamford, Elliott 

 and Hanby, 1956) are compared with those of keratin and muscle, the 

 resemblance is striking and leaves little doubt that the natural structures 

 are based on the helix. Differences are equally striking, and these have 

 been emphasized by Bamford and Hanby (1951). The main characteristics 

 of the a-pattern are the strong meridional arcs of 5*15 A and 1*5 A, and a 

 group of spacings centred around 10 A at or near the equator (Astbury and 

 Woods, 1933; Macarthur, 1943). When these observational facts are 

 compared with the pattern to be expected from a crystal of hexagonally- 

 packed (Fig. 78) a-helices parallel to the axis two difficulties appear: 



(1) The a-helix would give a strong layer line at 54 A but the intensity 

 on the meridian would be zero. In fact we find the strong 5*15 A 

 arc on the axis. 



(2) If the centre of the broad equatorial reflection (9-8 A) is taken as the 

 (10-0) reflection of a simple hexagonal lattice the calculated density 

 for a-keratin is too low. 



Coiled coils and a-filaments 



An attempt to resolve these difficulties by suggesting that the whole 

 helix (minor helix) might be twisted into a super helix (major helix) or 

 coiled coil, has been made by Pauling and Corey (1953a), and by Crick 

 (1952). Both proposals involve tilting the a-helices to form the super-helix 

 or a coiled coil (Fig. 77) with a pitch angle of about 18° giving a projection 

 on the axis with a periodic variation in density at intervals of 54 cos 18° = 

 5*1 A. According to Crick the reason for the deformation may be found in 

 the difficulty of packing the side chains projecting from the non-integral 

 helix. If a simplified model in which the side chains are represented by 

 knobs is taken and, on a piece of paper wrapped around it, the position of 

 the knobs is marked, a pattern is found on unwrapping into which a second 



