Editing Alpha helix (section)

=== Stability ===
{{See also|Stapled peptide}}
Helices observed in proteins can range from four to over forty residues long, but a typical helix contains about ten amino acids (about three turns). In general, short [[polypeptide]]s do not exhibit much α-helical structure in solution, since the [[entropy|entropic]] cost associated with the folding of the polypeptide chain is not compensated for by a sufficient amount of stabilizing interactions. In general, the backbone [[hydrogen bond]]s of α-helices are considered slightly weaker than those found in [[beta sheet|β-sheets]], and are readily attacked by the ambient water molecules. However, in more hydrophobic environments such as the [[cellular membrane|plasma membrane]], or in the presence of co-solvents such as [[trifluoroethanol]] (TFE), or isolated from solvent in the gas phase,<ref>{{cite journal | vauthors = Hudgins RR, Jarrold MF | year = 1999 | title = Helix Formation in Unsolvated Alanine-Based Peptides: Helical Monomers and Helical Dimers | journal = Journal of the American Chemical Society | volume = 121 | pages = 3494&ndash;3501 | doi = 10.1021/ja983996a | issue = 14}}</ref> oligopeptides readily adopt stable α-helical structure.  Furthermore, crosslinks can be incorporated into peptides to conformationally stabilize helical folds.  Crosslinks stabilize the helical state by entropically destabilizing the unfolded state and by removing enthalpically stabilized "decoy" folds that compete with the fully helical state.<ref>{{cite journal | vauthors = Kutchukian PS, Yang JS, Verdine GL, Shakhnovich EI | title = All-atom model for stabilization of alpha-helical structure in peptides by hydrocarbon staples | journal = Journal of the American Chemical Society | volume = 131 | issue = 13 | pages = 4622–7 | date = April 2009 | pmid = 19334772 | pmc = 2735086 | doi = 10.1021/ja805037p }}</ref> It has been shown that α-helices are more stable, robust to mutations and designable than β-strands in natural proteins,<ref>{{cite journal | vauthors = Abrusan G, Marsh JA | title = Alpha helices are more robust to mutations than beta strands | journal = PLOS Computational Biology | volume = 12 | issue = 12 | pages = e1005242 | date = 2016 | pmid = 27935949 | doi = 10.1371/journal.pcbi.1005242 | bibcode = 2016PLSCB..12E5242A | pmc=5147804 | doi-access = free }}</ref> and also in artificially designed proteins.<ref>{{cite journal | vauthors = Rocklin GJ et al. | title = Global analysis of protein folding using massively parallel design, synthesis, and testing | journal = Science | volume = 357 | issue = 6347 | pages = 168–175 | date = 2017 | pmid = 28706065 | doi = 10.1126/science.aan0693 | bibcode = 2017Sci...357..168R | pmc=5568797}}</ref>

[[Image:Helix electron density myoglobin 2nrl 17-32.jpg|thumb|right|200px|An α-helix in ultrahigh-resolution electron density contours, with oxygen atoms in red, nitrogen atoms in blue, and hydrogen bonds as green dotted lines (PDB file 2NRL, 17–32). The N-terminus is at the top, here.]]