In the alpha helix, first proposed by Linus Pauling and Robert Corey, the alternating a -carbons and peptide linkages follow a zigzag path tracing a regular spiral. The tightly coiled polypeptide main chain forms the inner part of the rod and the amino acid side chains extend outward from the axis of the spiral. The structure is stabilized by a regularly space series of hydrogen bonds and wand der Walls forces set up between atoms in the main chains. Although an alpha helix may turn in either direction, the alpha helices of almost all natural proteins wind to the right. Thus, amino acids spaced three and four apart in the linear sequence are spattially quite close to one another in an alpha helix.
Most proteins contain alpha-helical segments in least some regions.
Where such segments occur, the portion of the amino acid chain containing them is rigid and rodlike. Globular proteins usually contain several short alpha-helical segments running in different directions. Proteins that take the form of elongated fibers typically contains one or more alpha-helical segments that run the length of the molecule with few or no breaks. Such alpha helical coiled coils are found in keratin in hair, myosin and tropomyosin in muscle, and epiderminin skin, and fibrin in blood clots. The Beta Strand Pauling and Corey proposed second regular arrangement of amino acid chains, which they termed the beta strand.