Editing Alpha helix (section)

== Discovery ==
{{Multiple image
| width = 180
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| direction = vertical
| image1 = alpha helix neg60 neg45 sideview.png
| caption1 = Side view of an α-helix of [[alanine]] residues in [[atom]]ic detail. Two hydrogen bonds for the same peptide group are highlighted in magenta; the H to O distance is about {{cvt|2|Å|nm}}. The [[protein]] chain runs upward here; that is, its [[N-terminus]] is at the bottom and its C-terminus at the top. Note that the sidechains (black stubs) angle slightly downward, toward the N-terminus, while the peptide oxygens (red) point up and the peptide NHs (blue with grey stubs) point down.
| image2 = alpha helix neg60 neg45 topview.png
| caption2 = Top view of the same helix shown above. Four [[carbonyl]] groups are pointing upwards toward the viewer, spaced roughly 100° apart on the circle, corresponding to 3.6 [[amino acid|amino-acid]] residues per turn of the helix.
}}

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 ≈{{convert|5.1|Å|nm|abbr=off|lk=on}}.

Astbury initially proposed a linked-chain structure for the fibers. He later joined other researchers (notably the American chemist [[Maurice Loyal Huggins|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 [[Beta sheet|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]]<ref>{{cite journal | vauthors = Kendrew JC, Dickerson RE, Strandberg BE, Hart RG, Davies DR, Phillips DC, Shore VC | s2cid = 4167651 | title = Structure of myoglobin: A three-dimensional Fourier synthesis at 2&nbsp;Å resolution | journal = Nature | volume = 185 | issue = 4711 | pages = 422–7 | date = February 1960 | pmid = 18990802 | doi = 10.1038/185422a0 | author-link = John Kendrew | bibcode = 1960Natur.185..422K }}</ref> 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.<ref>{{cite journal | vauthors = Neurath H | author-link = Hans Neurath | year = 1940 | title = Intramolecular folding of polypeptide chains in relation to protein structure | journal = Journal of Physical Chemistry | volume = 44 | pages = 296&ndash;305 | doi = 10.1021/j150399a003 | issue = 3}}</ref> Neurath's paper and Astbury's data inspired [[Hugh Stott Taylor|H. S. Taylor]],<ref>{{cite journal | vauthors = Taylor HS | author-link = Hugh Stott Taylor | year = 1942 | title = Large molecules through atomic spectacles | journal = Proceedings of the American Philosophical Society | volume = 85 | issue = 1 | pages = 1&ndash;12 | jstor = 985121}}</ref> [[Maurice Loyal Huggins|Maurice Huggins]]<ref>{{cite journal | vauthors = Huggins M | author-link = Maurice Loyal Huggins | year = 1943 | title = The structure of fibrous proteins | journal = Chemical Reviews | volume = 32 | pages = 195&ndash;218 | doi = 10.1021/cr60102a002 | issue = 2}}</ref> and [[William Lawrence Bragg|Bragg]] and collaborators<ref>{{cite journal | vauthors = Bragg WL, Kendrew JC, Perutz MF | s2cid = 93804323 | author-link1 = William Lawrence Bragg | author-link2 = John Kendrew | author-link3 = Max Perutz | year = 1950 | title = Polypeptide chain configurations in crystalline proteins | journal = Proceedings of the Royal Society of London, Series A | volume = 203 | pages = 321&ndash;? | doi = 10.1098/rspa.1950.0142 | issue = 1074|bibcode = 1950RSPSA.203..321B }}</ref> to propose models of [[keratin]] that somewhat resemble the modern α-helix.

Two key developments in the modeling of the modern α-helix were: the correct bond geometry, thanks to the [[crystallography|crystal structure determinations]] of [[amino acid]]s and [[peptide]]s and Pauling's prediction of ''planar'' [[peptide bond]]s; and his relinquishing of the assumption of an integral number of residues per turn of the helix. The pivotal moment came in the early spring of 1948, when Pauling caught a cold and went to bed. Being bored, he drew a polypeptide chain of roughly correct dimensions on a strip of paper and folded it into a helix, being careful to maintain the planar peptide bonds. After a few attempts, he produced a model with physically plausible hydrogen bonds. Pauling then worked with Corey and Branson to confirm his model before publication.<ref>{{cite journal | vauthors = Pauling L, Corey RB, Branson HR | title = The structure of proteins; two hydrogen-bonded helical configurations of the polypeptide chain | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 37 | issue = 4 | pages = 205–11 | date = April 1951 | pmid = 14816373 | pmc = 1063337 | doi = 10.1073/pnas.37.4.205 | author-link = Linus Pauling | bibcode = 1951PNAS...37..205P | author-link2 = Robert Corey | author-link3 = Herman Branson | doi-access = free }}</ref> In 1954, Pauling was awarded his first Nobel Prize "for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances"<ref>{{Cite web | url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1954/ |title = The Nobel Prize in Chemistry 1954}}</ref> (such as proteins), prominently including the structure of the α-helix.