_pdbx_nmr_chem_shift_reference.carbon_shifts_flag ? _pdbx_nmr_chem_shift_reference.details "External DSS sample" _pdbx_nmr_chem_shift_reference.entry_id 1ABC _pdbx_nmr_chem_shift_reference.id 1 _pdbx_nmr_chem_shift_reference.label chemical_shift_reference_