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Concept
Alpha helix
Summary
An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix).
The alpha helix is the most common structural arrangement in the secondary structure of proteins. It is also the most extreme type of local structure, and it is the local structure that is most easily predicted from a sequence of amino acids.
The alpha helix has a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid that is four residues earlier in the protein sequence.
The alpha helix is also commonly called a:
Pauling–Corey–Branson α-helix (from the names of three scientists who described its structure).
3.613-helix because there are 3.6 amino acids in one ring, and there are an average of 13 residues per helical turn, with 13 atoms being involved in the ring formed by the hydrogen bond.
In the early 1930s, William Astbury showed that there were drastic changes in the X-ray fiber diffraction of moist wool or hair fibers upon significant stretching. The data suggested that the unstretched fibers had a coiled molecular structure with a characteristic repeat of ≈.
Astbury initially proposed a linked-chain structure for the fibers. He later joined other researchers (notably the American chemist Maurice Huggins) in proposing that:
the unstretched protein molecules formed a helix (which he called the α-form)
the stretching caused the helix to uncoil, forming an extended state (which he called the β-form).
Although incorrect in their details, Astbury's models of these forms were correct in essence and correspond to modern elements of secondary structure, the α-helix and the β-strand (Astbury's nomenclature was kept), which were developed by Linus Pauling, Robert Corey and Herman Branson in 1951 (see below); that paper showed both right- and left-handed helices, although in 1960 the crystal structure of myoglobin showed that the right-handed form is the common one. Hans Neurath was the first to show that Astbury's models could not be correct in detail, because they involved clashes of atoms.
Official source
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