Editing Protein structure (section)

===Secondary structure===
[[File:Alpha helix.png|thumb|100px|An α-helix with hydrogen bonds (yellow dots)]]
{{Main|Protein secondary structure}}
[[Secondary structure]] refers to highly regular local sub-structures on the actual polypeptide backbone chain. Two main types of secondary structure, the [[alpha helix|α-helix]] and the [[beta strand|β-strand]] or [[beta sheet|β-sheet]]s, were suggested in 1951 by [[Linus Pauling]].<ref name="Pauling1951">{{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 = 205–211 | date = April 1951 | pmid = 14816373 | pmc = 1063337 | doi = 10.1073/pnas.37.4.205 | doi-access = free | bibcode = 1951PNAS...37..205P }}</ref> These secondary structures are defined by patterns of [[hydrogen bonds]] between the main-chain peptide groups. They have a regular geometry, being constrained to specific values of the dihedral angles ψ and φ on the [[Ramachandran plot]]. Both the α-helix and the β-sheet represent a way of saturating all the hydrogen bond donors and acceptors in the peptide backbone. Some parts of the protein are ordered but do not form any regular structures. They should not be confused with [[random coil]], an unfolded polypeptide chain lacking any fixed three-dimensional structure. Several sequential secondary structures may form a "[[supersecondary structure|supersecondary unit]]".<ref name="ChiangYS2007">{{cite journal | vauthors = Chiang YS, Gelfand TI, Kister AE, Gelfand IM | title = New classification of supersecondary structures of sandwich-like proteins uncovers strict patterns of strand assemblage | journal = Proteins | volume = 68 | issue = 4 | pages = 915–921 | date = September 2007 | pmid = 17557333 | doi = 10.1002/prot.21473 | s2cid = 29904865 }}</ref>