But as we lower the PH and increase the concentration of the H plus ions, now we approach the PKA value of this residue, which is about seven. And so what that means is we have enough H plus ions so that the H plus ions can protonate this nitrogen. And by protonating this nitrogen, we basically create this nitrogen hydrogen bond. And now the hydrogen, which bears a partial positive charge, can interact with the negatively charged oxygen atom of the side chain of the aspartate 94 amino acid. And this forms a salt bridge. A salt bridge is basically a stabilizing electric interaction between these two adjacent atoms.