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.