The component identifier in the first of the two entities containing the linkage.
For polymer entities, this data item is a pointer to _pdbx_reference_entity_poly_seq.mon_id
in the PDBX_REFERENCE_ENTITY_POLY_SEQ category.
For non-polymer entities, this data item is a pointer to
_pdbx_reference_entity_nonpoly.chem_comp_id in the
PDBX_REFERENCE_ENTITY_NONPOLY category.