code: def residue_distances(res_1_num, res_1_chain, res_2_num, res_2_chain, model):
      
  
      res1 = model[res_1_chain][res_1_num].child_list[-1]
      res2 = model[res_2_chain][res_2_num].child_list[-1]
      distance = res1 - res2
  
      return distance
docs: Distance between the last atom of 2 residues