If we're above a PH of 4.1, this will predominate. And what that means is, at the normal physiological PH, because that is higher than 4.1, these ions will predominate over these neutral species. And so, because they're going to have a full negative charge, they can actually form many ionic bonds. Now, let's move on to Histidine. Histidine has PKA of 6.0, which is actually relatively close to the normal physiological PH. And that's exactly what makes histidine very special.