are drawn on some of the peptide bonds to emphasize that in an α-helix the planar peptide bonds rotate about the axis of the helix. When viewing the helix on end, observe the open center of the helix. The shows the axis of the α-helix rotating in the y-plane. Two common examples of secondary structure are illustrated below. The different secondary structures can be distinguished by their range of φ and ψ values with the values of different secondary structures mapping to different regions of the Ramachandran plot. Secondary structures of a peptide are segments of the peptide that have ordered and repetitive structure, and the repetitive structure is due to a repetitive conformation of the residues and, ultimately, repetitive values of φ and ψ. 2 Plot regions limited by steric hindrance.The scene on the right is the Ramachandran plot of ribonuclease H. By making a Ramachandran plot, protein structural scientists can determine which torsional angles are permitted and can obtain insight into the structure of peptides. Many of the angle combinations, and therefore the conformations of residues, are not possible because of steric hindrance. The torsional angles of each residue in a peptide define the geometry of its attachment to its two adjacent residues by positioning its planar peptide bond relative to the two adjacent planar peptide bonds, thereby the torsional angles determine the conformation of the residues and the peptide. Plotting the torsional angles in this way graphically shows which combination of angles are possible. by plotting the φ values on the x-axis and the ψ values on the y-axis, as for the image at left. In sequence order, φ is the N(i-1),C(i),Ca(i),N(i) torsion angle and ψ is the C(i),Ca(i),N(i),C(i+1) torsion angle. The Ramachandran plot is a plot of the torsional angles - phi (φ)and psi (ψ) - of the residues (amino acids) contained in a peptide. Ramachandran plot and contours from 100,000 high-quality general-case datapoints
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