def construct_res_tuple(res):
    """
    Build the BigDFT fragment tuple given the residue of the structure

    Args:
       res(Residue): A residue Class on the Biopython package

    Returns:
       tuple: the (chain,fragname,fragno) tuple
    """
    chain = res.full_id[2]
    if len(chain.lstrip(' ')) == 0:
        chain = 'A'
    resname = res.resname
    position = res.full_id[3][1]
    return chain, resname, position