Proline is a particular amino acid, since it's side chain forms a closed pentagon cycle with the backbone. It can be recognized easily because it looks like a handle on the backbone.
- This closed cycle enables only a little freedom of move and usually bends the backbone. For example it can be found in a short strand that link two strands of a sheet.
- The cycle formed forbid any hydrogen bond of the backbone: thus it is almost never found in helices or sheets.
This amino acid is hydrophobic.