The α-helix is not the only helical structure in proteins. Other helical structures include the 3_10 helix, which is stabilized by hydrogen bonds of the type (i, i+3), the π-helix, which is stabilized by hydrogen bonds of the type (i, i+5), and the left handed L-α helix. The 3_10 helix has a smaller radius than the α-helix, while the π-helix has a larger radius. The first detailed analysis of the occurrence of the π-helix in proteins, based on the analysis of entries in the Protein Data Bank (PDB), was published by Fodje & Al-Karadaghi, 2002
. Generally, all these helices are relatively rare when compared to the α-helix.
In addition to the "simple" helical structures mentioned here, there are also so-called coiled-coil structures, in which two or more α-helices build higher-order helical structures together.The β-sheet
The second major secondary structure element in proteins is the β-sheet. β-sheets consist of several β-strands, stretched segments of the polypeptide chain kept together by a network of hydrogen bonds between adjacent strands. An example of a β-sheet, with the stabilizing hydrogen bonds between adjacent strands (dotted lines), is shown in the image below (left):