(A) The Ramachandran plot is a 2D plot with the phi (φ) angle on the x-axis and the psi (ψ) angle on the y-axis.

(B) The allowed regions on the Ramachandran plot correspond to combinations of phi and psi angles that result in minimal steric clashes between the amino acid side chains and the polypeptide backbone.

(C) The disallowed regions on the Ramachandran plot are a result of angle combinations where the carbonyl oxygen of one amino acid would spatially clash with the C carbon of the adjacent residue

(D) The Ramachandran plot can be used to predict the secondary structure of proteins, as certain regions on the plot are characteristic of alpha helices, beta sheets, and other conformations.

Solution

At pH 5.5, the order of elution from the cation exchange column (negatively charged) is determined by the net positive charge of the amino acids. Here's the breakdown:

Alanine (Ala):

• The terminal amino group (NH3+) is protonated.
• The carboxyl group (COOH) is partially deprotonated.
• The R-group (non-polar) is neutral.
• Net positive charge: +1 2.

Arginine (Arg):

• The terminal amino group (NH3+) is protonated.
• The guanidino group in the R-group is protonated.
• The carboxyl group (COOH) is partially deprotonated.
• Net positive charge: +2 3.

Aspartic acid (Asp):

• The terminal amino group (NH3+) is protonated.
• The carboxyl group in the R-group is deprotonated.
• The terminal carboxyl group (COOH) is partially deprotonated.
• Net positive charge: +1

Since Aspartic acid (Asp) has the highest net positive charge (+1) among the three amino acids, it will elute first. Alanine (Ala) and Arginine (Arg) will elute next, as they have the same net positive charge (+1).

Therefore, the correct order of elution is option (C) Asp, Ala, then Arg.