There are only three different bonds in the backbone structure of a protein. One of them is the amide (peptide) bond. As just discussed, there is no rotation about this bond. The other two bonds are the N-Cα and the Cα-Cacid bond there are labeled Phi (Φ) and Psi (Ψ), respectively. Theoretically, free rotation is allowed about both of these bonds. With one caveat this is true. At some positions around these bonds the atoms that are attached collide into each other and thus these angles are not as likely as others. Check out this interactive demonstration where you can see the ramifications of independent rotatation about Phi or Psi. While the angles are not important, what is important is that you should be able to see in the spacefilling model that multiple atoms try to take up the same space in some configurations. while in others there is much less spatial conflict. While the definition of the angles is not important, it is interesting to note that the picture of the angles that are allowed is decidedly asymmetrical. This goes back to the stereochemistry. Much of the spatial conflict is with some atom and the first carbon of the side chain. AND since the first carbon atom of the side chain (R group) is always on the same side, this leads to a consistent pattern in what is allowed and what is not.
One exception - one amino acid does not have any carbons in the sidechain (only a hydrogen). Which amino acid? and what affect do you predict this lack of carbon would have on this conflict of space mentioned above?