Such an arrangement is stable because hydrogen bonds can form between C=O of aminoacid n and N-H of aminoacid n+4 (i.e., between the 1st and the 5th, the 2nd and the 6th, etc.). For simplicity, only the peptide N-Cα-C=O backbone is shown in the figure. The helix rises about 5.4 Å per turn. The angular distance between aminoacids is close to 100º, so that about 3.6 aminoacids are present in each turn of the helix:
direct themselves outwards
is stabilized by many hydrogens bonds. Helix-destabilizing residues (e.g. like can break the helix, or bend it (as in this case)
Observation of the
(with sidechains) allows one to predict where in the protein it will be. Spin the structure in order to appreciate how one of the faces of the helix contains polar, hydrophylic residues, while the other mostly contains hydrophobic residues.
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