Pee spiral

Pi-helix (or π-helix) is a kind of secondary structure that is sometimes formed in proteins.
Normal structure
Amino acids in a normal π-helix are arranged in a right-handed helical structure. Each amino acid corresponds to a 87 ° helix turn (ie, the helix has 4.1 residues per turn), and has a length of 1.15 Å (= 0.115 nm) along the helix axis. An important feature of the structure is that the amino group of one amino acid forms a hydrogen bond with the C = O group of amino acids five residues earlier, these repeated i + 5 → i hydrogen bonds determine the π-helix. Similar structures include the 310-helix (i + 3 → i hydrogen bond) and the α-helix (i + 4 → i hydrogen bond). The four upward-facing carboxyl groups are approximately 87 ° at each other in a circle, corresponding to 4.1 amino acid residues in a spiral rotation. angle (-55 °, -70 °). More generally, they accept double-sided angles such that the angle ψ of one residue and the angle φ of the next are approximately −125 °. For comparison, the sum of the corresponding angles for the 310-helix is about −75 °, while for the α-helix it is about 105 °. The general formula for the angle of rotation Ω for the remainder of any polypeptide helix with transisomers is given by the equation: cos
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{ displaystyle 3 cos Omega = 1-4 cos ^ {2} left [ left ( phi + psi right) / 2 right ]}

Left-handed structure
In principle, a left-hand version of a π-helix is possible, changing ak dihedral angle (φ, ψ) at (55 °, 70 °). This pseudo-"mirror" spiral has approximately the same number of residuals per revolution (4.1) and a spiral pitch of 1.5 Å. However, this structure is not a true mirror image because the amino acid residues still have left chirality. The long left-handed π-helix is unlikely to be observed in proteins, because among natural amino acids only glycine is able to take a positive angle φ.

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.
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