Protein folding is accelerated by constraints placed upon the folding proteins that restrict the number of possible conformations that are sampled during the folding process. One such limitation is the restriction of typically observed Phi and Psi dihedral angles at the alpha carbon .
Basically, the asymmetry of the ramachandran plot is due to clashes between Cbeta and the main chain (See https://en.wikipedia.org/wiki/Ramachandran_plot). For glycine, the ramachandran plot is symmetrical)
Dear Shahid Aziz ,
As indicated by Annemarie Honegger the constrains you mentioned are described by the Ramachandran plot, see for example:
https://swissmodel.expasy.org/course/text/chapter1.htm
It states: “L-amino acids cannot form extended regions of left-handed helix, but occasionally individual residues adopt this conformation. These residues are usually glycine but can also be asparagine or aspartate, where the side chain forms a hydrogen bond with the main chain and therefore stabilises this otherwise unfavourable conformation.”
And “Disallowed regions generally involve steric hindrance between the side chain atoms and main chain atoms. Glycine has no side chain and therefore can adopt phi and psi angles in all four quadrants of the Ramachandran plot. Hence it frequently occurs in turn regions of proteins where any other residue would be sterically hindered.”
Do realize that quite often ‘distortions’ compared to the ‘idealized' helices have nothing to do with violation of the dihedral angles according to Ramachandran but with other factors (see for more details the section: 1.3.3 Distortions of alpha-helices).
Best regards.
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