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I have stumbled upon the following past exam question:

4 decapeptides A, B, C, D, made only of glycine residues, have different conformations, and the phi and psi angles of individual peptides are indicated on the Ramachandran plot (unfortunately I cannot provide a specific plot). Order these peptides by decreasing length.

How should I solve this, what information useful in solving this problem can I deduce from the Ramachandran plot?

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When φ = ψ = 180° the peptide is in the fully extended conformation i.e. it is the 'longest' it can be.

If it isn't possible to deduce a length order based on this information then I guess you will have to do some geometry to calculate the sums of the distances between the α C atoms using the known values for all of the bond angles - sounds laborious to me!

This site tabulates values of φ/ψ for typical structures (helix, sheet, turn) so it may be possible to decide if some of peptides are adopting these.

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The Ramachandran plot describes secondary structure. If you see a lot of beta sheets in your plot you will have a more compact molecule than one which is wholly alpha helix. Gly can be almost anywhere in any quadrant. Look for residues with secondary structure to determine tertiary structure.

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