Polyproline helix

In proteins, a left-handed poly-Pro helix is formed when sequential residues all adopt (φ,ψ) backbone dihedral angles of roughly $(-75^{\circ },145^{\circ })$.

Poly-Pro II helix (trans peptide bonds)

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Helical pitch, 3 residues per turn

Surprisingly highly populated region of Ramachandran plot

Discuss mirror image helix (seen with glycine).

Poly-Pro I helix (cis peptide bonds)

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Much more rare. Seen mainly with proline-rich segments (duh ;)

Structural roles

Good "molecular ruler"

References

• Adzhubei AA and Sternberg MJE. (1993) "Left-handed Polyproline II Helices Commonly Occur in Globular Proteins", J. Mol. Biol., 229, 472-493.