Alpha Helix

The alpha helix is the most abundant type of secondary structure in proteins and has been extensively documented. In brief, alpha helices are formed from stretches of consecutive amino acid residues with phi and psi angle pairs which correspond to the bottom left quadrant of the Ramachandran Plot. The mean phi and psi angles for alpha helices found in proteins are -62 degrees and -41 degrees, respectively. The alpha helix has 3.6 residues per turn, with a hydrogen bond between the CO of residue n and the NH of residue n +4. The closed loop formed by one of these hydrogen bonds and the intervening stretch of backbone contains 13 atoms (including the hydrogen). Hence the nomenclature for an alpha helix is 3.6(13)-helix, where the 3.6 is the number of residues per turn and 13 is the number of atoms in the hydrogen bonded loop. Some alpha helices are curved or show distinct kinks, for example those caused by the presence of proline residues. The example shown is a enterotoxin (CATH domain ID 1tiiC00).


Helix geometry in proteins.
Barlow DJ, Thornton JM
J Mol Biol201p601-19(1988 Jun 5)
The structure of proteins; two hydrogen-bonded helical configurations of the polypeptide chain.
Proc Natl Acad Sci U S A37p205-11(1951 Apr)
Packing of alpha-helices: geometrical constraints and contact areas.
Richmond TJ, Richards FM
J Mol Biol119p537-55(1978 Mar 15)
General architecture of the alpha-helical globule.
Murzin AG, Finkelstein AV
J Mol Biol204p749-69(1988 Dec 5)