Some basics of protein structure

Torsion angles:

Many aspects of the geometry of proteins have a fixed character. Bond lengths, bond angles and the planarity of certain groups are all geometric characteristics with very limited freedom. Nevertheless, proteins can fold in a near infinite number of different structures which is caused by two facts:

• There are 20 different protein side chains (actually 19 + no side chain for glycine), and each has a different influence on the local backbone.
• Two of the three backbone torsion angles (1 in proline) have the freedom to adapt different values, and thus to adjust themselves to better match with the own side chain and the rest of the protein.

What is a torsion angle?

If you look from the N (blue) via the two carbons (green) to the oxygen (red) you see that a clockwise rotation of about 120 degrees is needed to rotate the N on top of the O . This is a clock-wise rotation which in general is called negative so it is actually called -120 degrees. The funny thing is, that if you look from O to N, and try to rotate the O on top of the N the rotation is also -120 degrees. So, torsion angles are order-independent.