So basically, this is the histidine residue and this is the aspartate residue. And at a relatively high PH, so let's say at a normal PH of 7.4, the PH is not low enough to actually add an H plus ion onto this nitrogen. And so this nitrogen will be missing an H plus ion and no bond will be formed between these two groups. 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.