PROTEIN STRUCTURE 



the a-helix is distorted into a coiled coil with a pitch angle of 

 about 20 °, then theoretically such a coiled coil would give diffuse 

 meridional 5.1 and 1.5 A x-ray reflections. Pauling and Corey 

 have suggested a similar scheme shown in Figure 6 (81). The 

 packing together of such coiled coils would give ropelike struc- 

 tures with each a-helix a single strand in the final cable. Pauling 



50 A 



Fig. 6. (a) A representation of a compound helical configura- 

 tion of polypeptide chains, with the axis of the a-helix describing a 

 helical course, (b) A compound a-helix (at the left), a 7-strand a 

 cable (in the center), and a 3-slrand a rope (at the right). L. 

 Pauling and R. B. Corey, Nature, 777, 59 (1953). 



(77) considers that local variations in hydrogen bond length 

 along an cv-helix may be caused by side-chain interactions or 

 interactions between side chains and the carboxyl and amino 

 groups of the chain amide residue. Such minor discontinuities 

 would cause the helical axis to curve. Crick (31) on the basis of 

 model studies has shown that regular systematic "coiled-coil" 

 distortions of the helices would permit the side chain of one helix 



405 



