Alpha Helix

**Semantic type:** Spatial Concept

**Definition:** A right-handed spiral conformation; the a-helix has a pitch of 5.4 A and has 3.6 amino acid residues per turn; every main chain C=O and N-H group is hydrogen-bonded to a peptide bond 4 residues away; the peptide planes are roughly parallel with the helix axis and the dipoles within the helix are aligned, i.e. all C=O groups point in the same direction and all N-H groups point the other way; side chains point outward from helix axis and are generally oriented towards its amino-terminal end. (from expasy.org)