The counterclockwise direction is indicated by the arrow. Both the torsion angle are usually given in the interval from -180 degrees to 180 degrees. In the figures above, the angle phi is negative. Here are some mathematical details, and a formula for computing the torsion angle. Maple workbook with procedures for computing bond and torsion angles.
Maple exercises to compute torsion and bond angles from a pdb file.
Protein backbone torsion angles. Protein sidechain torsion angles. The greek letters for the torsion angles for the side chains of a protein are given in the following table. The table indicates the bond direction around which the torsion angle is measured. The atoms in the sequence may vary for different sidechains.