A Ramachandran plot: torsion angles between amino acids. The conformations of peptides are defined by the values of ψ and φ for each amino acid residue. Allowable conformations are those that involve little or no steric hindrance between atoms of the amino acid side chain and nearby atoms of the peptide backbone. Shown here is the Ramachandran plot for Ala residues. Easily allowed conformations are in dark blue; medium blue signifies bond conformations that approach unfavorable values; light blue, conformations that are allowed if some flexibility is permitted in the torsion angles. Unshaded regions indicate conformations that are not allowed. With the exception of Gly and Pro residues, the plots for all other amino acid residues are very similar to this plot for alanine. The range of allowed φ and ψ values is characteristic for each type of secondary structure, as shown. Secondary structural elements are discussed in Section 4.2.