def _get_binding(pdb_filename):
    """
    Get binding mode of pdb file.  x if none.

    e.g. 11as_r_u.pdb would give u
    """
    if '_u' in pdb_filename:
        return 'u'
    elif '_b' in pdb_filename:
        return 'b'
    else:
        return 'x'
