![]() Other regions are less favorable and are poorly populated in good-quality structures (yellow color). These are called allowed or energetically most favorable regions. For example, in the Ramachandran plot for the higher resolution structure on the image on the right, it is easy to see that the values of the torsion angles are clustered within the red regions (α- and β-regions). The higher resolution of the X-ray data usually results in a higher quality three-dimensional structure. The upper red region cluster corresponds to β-structure torsion angles, and the lower red region cluster corresponds to α-helical structures. Both horizontal and vertical axes start from -180 and extend to +180. The horizontal axis on the plot shows φ values, while the vertical shows ψ values. This is due to the better quality of the high resolution structure (see text below). On the other hand, at the lower resolution, many dots are found in energetically less favorable (yellow) regions. It can be seen that the dots for the high resolution structure (1het) are nicely clustered in the energetically most favorable red regions of α-helices, β-sheets, and left-handed α-helix L-α. The image on the left (PDB code 6adh) corresponds to a structure at around 2.9 Å resolution, while on the right, the structure (1het) is at 1.15 Å resolution (much higher resolution). For those who may be interested in details of the distribution of torsion angles in a Ramachandran plot, I can recommend the paper by Hovmöller et al., 2002, which provides an excellent discussion of the subject.Ī Ramachandran plot showing the distribution of the torsion angles of a protein refined at two different resolutions. However, for actual experimental structures, these values were found to be different. Theoretically, the average phi and psi values for α-helices and β-sheets were predicted to cluster around -57, -47, and -80, +150, respectively. What is the reason for this? This depends on the quality of the structures and is discussed below on this page. The dots are much better clustered in the right image than in the left. We can also see that the distribution of the torsion angles is different in the two images. ![]() The two regions are well separated and occupy different plot areas (marked as α and β). In the images, the regions corresponding to φ and ψ angles of α-helices and β-sheets are marked. Each dot in the plot corresponds to an amino acid, with its φ and ψ angles. In the Ramachandran plot, we can view the distribution of torsion angles in a protein structure (image below). A special way for plotting and visualization of protein torsion angles was introduced by Ramachandran and co-authors and has since then been called the Ramachandran plot.
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