Each backbone N—H group in an α helix normally forms a hydrogen bond to the carbonyl oxygen atom four residues back along the chain (toward the N-terminus; Fig. 6-7). Pro, which lacks a backbone N—H group, cannot do so and hence its presence in an interior position of an α helix destabilizes the helix through the loss of hydrogen bonding to the otherwise carbonyl acceptor. However, the four most N-terminal residues of an α helix cannot donate hydrogen bonds within the α helix. In addition, the pyrrolidine ring of a Pro residue in the interior of an α helix sterically interferes with the residue one turn towards the N-terminus. Pro residues in the N-terminal turn of an α helix do not have such steric clashes. Hence Pro residues can occupy the N-terminal turn of an α helix.