Polyproline helix

Similarly, a more compact right-handed polyproline I helix (PPI, poly-Pro I) is formed when sequential residues all adopt (φ,ψ) backbone dihedral angles of roughly (-75°, 160°) and have cis isomers of their peptide bonds.

The PPII helix is defined by (φ,ψ) backbone dihedral angles of roughly (-75°, 150°) and trans isomers of the peptide bonds.

The "mirror image" PPII backbone dihedral angles (75°, -150°) are rarely seen, except in polymers of the achiral amino acid glycine.

[6] The PPII helix is not common in transmembrane proteins, and this secondary structure does not traverse lipid membranes in natural conditions.

In 2018, a group of researcher from Germany constructed and experimentally observed the first transmembrane PPII helix formed by specifically designed artificial peptides.

Traditionally, PPII has been considered to be relatively rigid and used as a "molecular ruler" in structural biology, e.g., to calibrate FRET efficiency measurements.

[9][10] Further studies using terahertz spectroscopy and density functional theory calculations highlighted that polyproline is in fact much less rigid than originally thought.

Top view of a twenty-residue poly-Pro II helix, showing the three-fold symmetry.
Side view of a poly-Pro II helix, showing its openness and lack of internal hydrogen bonding.
Top view of a twenty-residue poly-Pro I helix, showing its non-integer number of residues per turn.
Side view of the poly-Pro I helix, showing its greater compaction.