Pi helix

Discovered by crystallographer Barbara Low in 1952[1] and once thought to be rare, short π-helices are found in 15% of known protein structures and are believed to be an evolutionary adaptation derived by the insertion of a single amino acid into an α-helix.

The majority of π-helices are only 7 residues in length and do adopt regularly repeating (φ, ψ) dihedral angles throughout the entire structure like that of α-helices or β-sheets.

A long left-handed π-helix is unlikely to be observed in proteins because, among the naturally occurring amino acids, only glycine is likely to adopt positive φ dihedral angles such as 55°.

This mis-characterization results from the fact that naturally occurring π-helices are typically short in length (7 to 10 residues) and are almost always associated with (i.e. flanked by) α-helices on either end.

Such helical bulges have previously been referred to as α-aneurisms, α-bulges, π-bulges, wide-turns, looping outs and π-turns, but in fact are π-helices as determined by their repeating i + 5 → i hydrogen bonds.

[2] One of the most notable group of proteins whose functional diversification appears to have been heavily influenced by such an evolutionary mechanism is the ferritin-like superfamily, which includes ferritins, bacterioferritins, rubrerythrins, class I ribonucleotide reductases and soluble methane monooxygenases.

Side view of a standard π-helix of L - alanine residues in atomic detail. Two hydrogen bonds to the same peptide group are highlighted in magenta; the oxygen-hydrogen distance is 1.65 Å (165 pm). The protein chain runs upwards, i.e., its N-terminus is at the bottom and its C-terminus at the top of the figure. Note that the sidechains point slightly downwards , i.e., towards the N-terminus.
Top view of the same helix shown above. Four carbonyl groups are pointing upwards towards the viewer, spaced roughly 87° apart on the circle, corresponding to 4.1 amino-acid residues per turn of the helix.
A short, 7 residue π-helix (orange) is embedded within a longer, α-helix (green). The "bulge" of the π-helix can be clearly seen, and was created as the result of a single amino acid that has been inserted into an α-helix. PDB code 3QHB.