PROTEIN STRUCTURE 



\l/) and vertical components of the angle N1C2C2 is equal to 110°, 

 the tetrahedral carbon angle. 



Tilting the residues in this way gives CO and NH bond 

 directions closely parallel to the helical axis. Upon detailed 

 examination it is seen that the a-helix corresponds to the chemical 

 sequence 



O H 



II I 



C— (NH.CH(R).CO)3N 



read in either direction, with 13 atoms in the hydrogen-bonded 

 loop so formed and with 3.6 residues per unit turn. It may thus 

 be designated 3.613. Minor changes in the wrap-up angle and 

 pitch will change the number of residues in the helix from 3.6 

 to 3.7 without affecting its over-all agreement with the detailed 

 chemical criteria set out above. Thus if n = 3.7, P = 5.4 A, P/n 

 = 1.47 A. In the a-helix with 3.60 residues per turn and a 

 vertical increment per unit residue P n = 1.47, P = 5.3. The 

 tetrahedral cv-carbon bond angle is 108.0°. This helix has 18 

 residues in 5 turns. Hence the plane of the Cj - Co residue is 

 also the plane of the Cis - C19. 



A helix structure described by Low and Bay butt (69,70) 

 is shown in Figure 5. This corresponds to the sccjuence 



O H 



C— (NH.C(HR).CO)4N 



with 16 atoms in the hydrogen-bonded loop so formed. It thus 

 belongs to the "a" series if this be characterized as a sequence 



O H 



C— (NH.C(HR)CO)„N 



with (3n-{-4) atoms in the hydrogen-bonded loop. 



Comparison of Figure 4 and Figure 5 shows that the two 

 helices are closely related as adjacent members of the "a" series. 

 In the a-helix each amide group forms hydrogen bonds with the 

 third amide group beyond it in either direction along the chain. 



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