Well, it turns out that in the hemoglobin molecule, we have several groups that can actually bind H plus ion. So one of the group is basically the terminal residue, the amino group on the terminal residue of the alpha subunits. And the other group are the histidine residues found on the beta 146 position and the alpha 122 position. So these groups on the hemoglobin molecule can actually bind H plus ions. And by binding H plus ions, those ions can then participate in forming stabilizing salt bridges. And to see what we mean by that, let's take a look at the following diagram.