A portion of secondary structure
that looks like a spiral staircase. More so than DNA
even, as it is single stranded. Think of the bannister
as the backbone, with poles joining one portion of the rail to the section directly above. This right handed helix
has a pitch of 0.54 nanometers
(10 to -9 M)and a 36 residue
helix turns 10 times. Therefore the rise is 0.15 nm per residue.
Left handed (pi) helices are also possible, but are less stable and therefore shorter. A three-ten helix is not uncommon; the name referring to the residues per turn (3) and the number of residues in the hydrogen bonded loop(10). In this sense, an alpha helix is a 3.6 - 13 helix.