Amino-acid Preferences
One might expect that larger side chains would result in more restrictions and consequently a smaller allowable region in the Ramachandran plot. In practice this does not appear to be the case; only the methylene group at Cβ has a large influence. Glycine has only a hydrogen atom for its side chain, with a much smaller van der Waals radius than the CH3, CH2, or CH group that starts all other amino acids. Hence it is least restricted, and this is apparent in the Ramachandran plot for glycine (see Gly plot at left) for which the allowable area is considerably larger. In contrast, the Ramachandran plot for proline, with its 5-membered-ring side chain connecting Cα to backbone N, shows only a very limited number of possible combinations of ψ and φ (see Pro plot at left).
Read more about this topic: Ramachandran Plot
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