How do I extract the coordinates of all the residues of any pdb file in matlab, and then use the value of the coordinates to find the torsion angles(psi,phi)? In the end, I want to display the output in terms of a file including the residue name,psi,phi angle. (Not necessarily in Matlab, I just want to understand how to figure out which atoms coordinate values are to be used exactly to find torsion angle. It would help if you give an example.)