# Alpha helix

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A common motif in the secondary structure of proteins, the alpha helix (α-helix) is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier ($i+4 \rightarrow i$ hydrogen bonding). This secondary structure is also sometimes called a classic Pauling-Corey-Branson alpha helix (see below). Among types of local structure in proteins, the α-helix is the most regular and the most predictable from sequence, as well as the most prevalent.

## Contents

### Historical development

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 ~5.1 ångströms (0.51 nm).

Astbury initially proposed a kinked-chain structure for the fibers. He later joined other researchers (notably the American chemist Maurice Huggins) in proposing that: