Alpha helix

a helix is the most common secondary structure.

The polypeptide backbone of an a helix is wound around an imaginary axis, with the R groups of the amino acid residues protrude outward from the helical backbone.

There is 3.6 a.a. per turn, and each turn extends 5.4 A in length which is maintained by the H-bond.

The H-bond is formed between the -C O  of residue (i) and the -N H  of residue (i+3).  

There are 2 H-bond per residue, and 3 or 4 H-bond per turn.

The ball-and-stick model of a right-handed a -helix, R group is represented by purple spheres.