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The vast majority of the peptide bonds in proteins are trans, though the peptide bond to the nitrogen of proline has an increased prevalence of cis compared to other amino-acid pairs. [9] The side chain dihedral angles are designated with χ n (chi-n). [10]
The ω angle at the peptide bond is normally 180°, since the partial-double-bond character keeps the peptide bond planar. [3] The figure in the top right shows the allowed φ,ψ backbone conformational regions from the Ramachandran et al. 1963 and 1968 hard-sphere calculations: full radius in solid outline, reduced radius in dashed, and ...
Protein torsion angles are calculated for phi, psi, omega (which corresponds to the peptide bond) and chi1 (the first side chain torsion angle) using standard IUPAC definitions. These values are listed under four different column headers: PHI, PSI, OMEGA and CHI1. All torsion angles are reported in degrees.
The key requirement is that the sum of the ψ i angle of residue i and the φ i+1 angle of residue i+1 remain roughly constant; in effect, the flip is a crankshaft move about the axis defined by the C α-C¹ and N-C α bond vectors of the peptide group, which are roughly parallel.
The first approach uses discrete variables for representing the coordinates or the dihedral angles of the protein structure. The variables are originally all continuous values and, to transform them into discrete values, a discretization process is typically applied. The second approach uses continuous variables for the coordinates or dihedral ...
Thus, the γ-turn has two forms, a classical form with (φ, ψ) dihedral angles of roughly (75°, −65°) and an inverse form with dihedral angles (−75°, 65°). At least eight forms of the beta turn occur, varying in whether a cis isomer of a peptide bond is involved and on the dihedral angles of the central two residues.
The peptide backbone dihedral angles (φ, ψ) are about (–140°, 135°) in antiparallel sheets. In this case, if two atoms C α i and C α j are adjacent in two hydrogen-bonded β-strands, then they form two mutual backbone hydrogen bonds to each other's flanking peptide groups; this is known as a close pair of hydrogen bonds.
For comparison, the sum of the dihedral angles for a 3 10 helix is roughly −75°, whereas that for the π-helix is roughly −130°. The general formula for the rotation angle Ω per residue of any polypeptide helix with trans isomers is given by the equation [15] [16] 3 cos Ω = 1 − 4 cos 2 φ + ψ / 2