C cap
The term C cap (C-cap, Ccap) describes an
hydrogen-bonded
to the CO group of residue i-4 (or sometimes residue i-3). Because of this it is sometimes also described as the residue following the helix.
Certain motifs occur commonly at or around the C cap, notably the Schellman loop and the niche (protein structural motif).
The N cap is the corresponding amino acid residue at the other end of the helix