So deoxy hemoglobin has a cavity between the two beta subunits to which the two three BPG can comfortably bond to via electrostatic forces. So between the beta two and the beta one, we have residues. We have amino acids that contain side chains that have positive charge. And these positively charged sections of beats of a beta two and beta one subunits can bond electrostatically to the negatively charged phosphate groups of 23 BPG. And this space, this cavity inside the hemoglobin, only exists when no oxygen are actually bound to the heme groups found within our hemoglobin. So once again, note that 23 BPG can only bond to deoxiform of hemoglobin because when oxygen actually binds onto the hemoglobin, it creates a conformational change that squeezes in, that narrows in that space in the middle.