I have a dataset of pdb files which are all complex structures. From them, I have to select only those which are protein-RNA complexes. I code in python. I am trying to use a keyword or pattern to select the RNA bound files. Using only 'RNA' as keyword won't help as that will also include complexes of RNA polymerases or RNA binding proteins which doesnt have RNA bound to it. Kindly help me, if anybody has done it or can give an idea.