Answer:


The residues have no more degrees of freedom than the rotation around the Cα-Cβ bond (the Cα sits in the mainchain and the Cβ is the first atom into the side chain. Since the Cα and the Cβ both are SP3 hybridized, you get the same rotational energy plot with three minima and three maxima that you already saw in several courses in the first year. Proteins are also ′part of organic chemistry′, they are just a bit bigger than what you have so far seen.