def get_ca_pos_from_residues(df, res):
    """Look up alpha carbon positions of provided residues."""
    ca = df[df['atom_name'] == 'CA'].reset_index().set_index(
        ['pdb_name', 'model', 'chain', 'residue'])
    nb = ca.reindex(res)
    nb = nb.reset_index().set_index('index')
    return nb