310 helix

A 3₁₀ helix is a type of secondary structure found in proteins and polypeptides. Of the countless protein secondary structures present, the 3₁₀-helix is the fourth most common type observed; following α-helices, β-sheets and reverse turns. 3₁₀-helices constitute nearly 10-15% of all helices in protein secondary structures, and are typically observed as extensions of α-helices found at either their N- or C- termini. Because of the α-helices tendency to consistently fold and unfold, it has been proposed that the 3₁₀-helix serves as an intermediary conformation of sorts, and provides insight into the initiation of α-helix folding.

Right around the 1950s, Max Perutz, who was the head of the Medical Research Council at the University of Cambridge, first wrote a paper documenting the elusive 3₁₀-helix. Of those cited in his work, two of his most famous colleagues during these experiments were James Watson and Francis Crick. In his paper, his two main observations regarding the theoretical configurations of polypeptides were as follows:

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, and has 10 atoms in the ring formed by making the hydrogen bond. 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 long 3₁₀-helices adopt (φ, ψ) dihedral angles near (−49°, −26°). Many 3₁₀-helices in proteins are short, so deviate from these values. More generally, residues in long 3₁₀-helices 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°.

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