Gamma helix (or γ-helix)[2][3] is a type of secondary structure in proteins that has been predicted by Pauling, Corey, and Branson,[1][4] but has never been observed in natural proteins.[3] The hydrogen bond in this type of helix was predicted to be between N-H group of one amino acid and the C=O group of the amino acid six residues earlier (or, as described by Pauling, Corey, Branson, "to the fifth amide group beyond it"). This can also be described as i + 6 → i bond and would be a continuation of the series (310 helix, alpha helix, pi helix and gamma helix). This theoretical helix contains 5.1 residues per turn.[1]However, a fully developed gamma helix has characteristics of a structure that has 2.2 amino acid residues per turn, a rise of 2.75Å per residue, and a pseudo-cyclic (C7) structure closed by intramolecular H-bond. Depending on the amino acid's side chain (R) involved in this main-chain reversal motif, two stereoisomers can occur with their Cα-substituent located either in the axial or in the equatorial position relative to the H-bonded pseudo-cycle.[5]
