Editing Beta sheet (section)

===Geometry===
The majority of β-strands are arranged adjacent to other strands and form an extensive [[hydrogen bond]] network with their neighbors in which the [[amine|N−H]] groups in the backbone of one strand establish [[hydrogen bond]]s with the [[carbonyl|C=O]] groups in the backbone of the adjacent strands. In the fully extended β-strand, successive side chains point straight up and straight down in an alternating pattern. Adjacent β-strands in a β-sheet are aligned so that their C<sup>α</sup> atoms are adjacent and their side chains point in the same direction. The "pleated" appearance of β-strands arises from tetrahedral chemical bonding at the C<sup>α</sup> atom; for example, if a side chain points straight up, then the bonds to the C′ must point slightly downwards, since its bond angle is approximately 109.5°. The pleating causes the distance between C{{su|p=α|b=''i''}} and C{{su|p=α|b=''i'' + 2}} to be approximately {{cvt|6|Å|nm|lk=on}}, rather than the {{cvt|7.6|Å|nm}} expected from two fully extended ''[[Cis-trans isomerism|trans]]'' [[peptide bond|peptide]]s. The "sideways" distance between adjacent C<sup>α</sup> atoms in [[hydrogen bond|hydrogen-bonded]] β-strands is roughly {{cvt|5|Å|nm}}.
[[File:Ramachandran plot general 100K.jpg|thumb|left|200px| Ramachandran (''φ'',&nbsp;''ψ'') plot of about 100,000 high-resolution data points, showing the broad, favorable region around the conformation typical for β-sheet amino acid residues.]]
However, β-strands are rarely perfectly extended; rather, they exhibit a twist. The energetically preferred [[dihedral angle]]s near (''φ'',&nbsp;''ψ'') = (–135°,&nbsp;135°) (broadly, the upper left region of the [[Ramachandran plot]]) diverge significantly from the fully extended conformation (''φ'',&nbsp;''ψ'') = (–180°,&nbsp;180°).<ref>{{cite book |title=Biochemistry | vauthors = Voet D, Voet JG |year=2004 |edition=3rd |publisher=Wiley |location=Hoboken, NJ |isbn=0-471-19350-X |pages=[https://archive.org/details/biochemistry00voet_1/page/227 227–231] |url=https://archive.org/details/biochemistry00voet_1|url-access=registration }}</ref> The twist is often associated with alternating fluctuations in the [[dihedral angle]]s to prevent the individual β-strands in a larger sheet from splaying apart. A good example of a strongly twisted β-hairpin can be seen in the protein [[BPTI]].

The side chains point outwards from the folds of the pleats, roughly perpendicularly to the plane of the sheet; successive amino acid residues point outwards on alternating faces of the sheet.