page_title: Pi helix
hierarchical_section_title: Pi helix
caption_attribution_description: English: Close-up sideview of a "stick" model of a pi helix of poly-alanine using the dihedral angles φ=-55° and ψ=-70° and the Engh&amp;Huber bond geometry. Two hydrogen bonds are highlighted in magenta; the O-H distance is 1.65 Å (165 pm). The PDB file was made by me on 18 October 2006 using my own software and visualized by me using MOLMOL. I release this image under the GFDL.
context_page_description: A pi helix is a type of secondary structure found in proteins. Discovered by crystallographer Barbara Low in 1952 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. Because such insertions are highly destabilizing, the formation of π-helices would tend to be selected against unless it provided some functional advantage to the protein. π-helices therefore are typically found near functional sites of proteins.
context_section_description: A pi helix (or π-helix) is a type of secondary structure found in proteins.  Discovered by crystallographer Barbara Low in 1952 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. Because such insertions are highly destabilizing, the formation of π-helices would tend to be selected against unless it provided some functional advantage to the protein.  π-helices therefore are typically found near functional sites of proteins.
caption: 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.
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