The ID of the second of the three atoms that define the angle.
The second atom is taken to be the apex of the angle.
An atom with this ID must exist in the component of the type
specified by _chem_comp_link.type_comp_1 (or
_chem_comp_link.type_comp_2, where the appropriate data item
is indicated by the value of _chem_comp_angle.atom_2_comp_id).