PROTEIN STRUCTURE 



In the TT-helix (Figure 5) the hydrogen bonds are between one 

 amide group and the fourth amide group beyond. The initial 

 vertical tilt is less in the 7r-helix, P/n — 1 . 1 4 A, and the horizontal 

 coinponent of the CCoN angle is consequently larger than in the 

 a-helix. 



The TT-helix satisfies the Pauling-Corey criteria if these are 

 modified to permit a 4° angular distortion in the tetrahedral 

 GC2N (a-carbon) angle. Deviations of this magnitude have 

 been observed elsewhere for instance in threonine (100) — and 

 lead to only a slight reduction in relative stability per mole per 

 residue. 



The a-helix is wrapped up more tightly than the 7r-helix. 

 It is possible to go from one configuration to the other with only 

 limited loss of local hydrogen bond stabilization. The a-heli.x 

 has greater over-all configurational stability than the 7r-helix 

 and than all other inembers of the "a" series. It is the most 

 stable coiled peptide chain structure known. 



There is another family of helical configurations to which 

 the Pauling-Corey 7-helix belongs. This "7" series may be 

 represented by the formula 



H O 



N(C(HR) . CO . NH)„_iC . (HR)G 



with 3(/i — 1) + 5 atoms in the hydrogen-bonded loop. For 

 the 7-helix itself n = 5. The two series of helices arc quite 

 distinct. In the "a"' series, as Figure 5 shows, the CpCj line is 

 inclined upwards in such a way that the CO and NH bond 

 directions are nearly vertical and parallel to the helical axis. 

 If the C1-C2 direction is horizontal, the CO and NH bonds are 



Fig. 5. (a) Perspective drawing of the right-handed ir/f-heHx. The 

 Hnk-end carbon atoms are numbered consecutively. The /3-carbon and 

 hydrogen atom positions of L-amino acid residues are shown attached to 

 C2 (a) and C4 (a), {h) Plan view of T^-helix. The hydrogen positions 

 are shown with broken lines. Nitrogen positions are omitted at the 

 two crossover points. B. W. Low and H. J. Grenville-Wells, Proc. Natl. 

 Acad. Sci. U. S., 39, 785 (1953). 



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