Helical or Coiled Forms

Left-handed a helices are characterized by torsional angles for the backbone with t) = +57° and il/ = +47°, whereas right-handed a helices have (f = —57° and il/ = —47° (Pauling and Corey, 1951 Ramachandran and Sasisekharan, 1968). Thus far, among small peptides, only Z-Aib-LPro-Aib LAla-OMe (Shamala et a/., 1977) and Boc-LPro-Aib-LAla-Aib-OBzl (Smith 

Peptides containing only proline residues, Aoc-LPro-LPro-LPro-OH (Kartha et a/., 1974b), and Boc-LPro-LPro-LPro-LPro-OBzl (Matsuzaki, 1974), form about one turn of a helix resembling the poly-LPro II helix (Ramachandran and Sasisekharan, 1968). In the poly-LPro II helix the torsional angles are 0 = —80° and 0 = +155°, whereas in the tri- and tetraproline molecules 0 is near —65° and 0 is near 153°. It is interesting 

A double-stranded, left-handed p helix with antiparallel chains has been characterized in the crystal of the linear octapeptide Boc-(LVal-DVal)4-OMe (Benedetti et al, 1979). In Fig. 30, the two molecules in the helix are related by a twofold rotation axis, which is perpendicular to the helix axis. The conformation of each chain can be described by l (between —148° and — 112°) and ip (between 98° and 122°) for the l residues, and l (between 80° and 129°) and ij/ (between —163° and —152°) for the d residues. Fourteen intermolecular NH 0=C bonds link the two antiparallel chains with 

N -O distances of 2.7-3.0 A. The dimer forms a cylinder with a hydrophilic inner channel having a diameter of 5.1 A and a hydrophobic outer layer of isopropyl groups. 

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