The localized foldings of alpha helixes and beta sheets are held together by hydrogen bonds. O groups in the polypeptide backbone. When the polypeptides adopts this conformation the R groups stick out from the central axis of the helix. There are 3.5 amino acids per turn of the helix. Proline is an alpha helix breaker.
Keratin is a protein that consists almost entirely of alpha helix.
If two amphipathic alpha helices from two different polypeptides come into contact, hydrophobic interactions will take place between the hydrophobic domains leading to the formation of a dimer. The leucine zipper is found in eukaryotic transcription factors that bind to DNA as dimers. O groups in the polypeptide backbone. They occur on widely separated domains of a polypeptide or between separate polypeptide chains. Small amino acids like glycine and alanine are favored in beta sheets.