310 helix

A 3₁₀ helix is a type of secondary structure found (rarely) in proteins.

tructure

The amino acids in a 3₁₀-helix are arranged in a right-handed helical structure. Each amino acid corresponds to a 120° turn in the helix (i.e., the helix has three residues per turn), and a translation of 2.0 Å (= 0.2 nm) along the helical axis. Most importantly, the N-H group of an amino acid forms a hydrogen bond with the C = O group of the amino acid "three" residues earlier; this repeated "i" + 3 → "i" hydrogen bonding defines a 3₁₀-helix. Similar structures include the α-helix ("i" + 4 → "i" hydrogen bonding) and the π-helix "i" + 5 → "i" hydrogen bonding).

Residues in 3₁₀-helices typically adopt (φ, ψ) dihedral angles near (−49°, −26°). More generally, they adopt dihedral angles such that the ψ dihedral angle of one residue and the φ dihedral angle of the "next" residue sum to roughly −75°. For comparison, the sum of the dihedral angles for an α-helix is roughly −105°, whereas that for a π-helix is roughly −125°.

The general formula for the rotation angle Ω per residue of any polypeptide helix with "trans" isomers is given by the equation

:$3\; cos\; Omega\; =\; 1\; -\; 4\; cos^\{2\}\; left(frac\{varphi\; +\; psi\}\{2\}\; ight).$

ee also

* alpha helix
* pi helix
* secondary structure

References

* Pauling L, Corey RB and Branson HR. (1951) "The Structure of Proteins: Two Hydrogen-Bonded Helical Configurations of the Polypeptide Chain", "Proc. Nat. Acad. Sci. Wash.", 37, 205.