The α-helix is a right-handed helical conformation with 3.6 amino acid residues per turn. Its hydrogen bonds are confined between consecutive turns of the helix. Backbone atoms are close packed.
The favorable main chain contributions to helix stability are constant from one residue to the next because all have identical backbone torsion angles (except proline).
In an α-helix, the first four N-H groups and last four C=O groups lack intrahelical hydrogen bonds. Helix capping refers to provision of hydrogen bond partners by side chains at or near the ends of the helix. These interactions are important for the stability of protein secondary and tertiary structure.
Positioning a Pro after position four of an α-helix results in exposure of two backbone carbonyl oxygen atoms and the loss of two intrahelical hydrogen bonds. For example, a 12-residue α-helix containing a proline in its interior will have only 6 H-bonds within the helix, instead of 8.