Because this is so close to the normal physiological PH of our cells, this amino acid that contains this side chain group can actually readily exchange our h atoms. It can donate h atoms, and it could also accept h atoms on this nitrogen. Now, what about cysteine? Well, cysteine is that amino acid that can participate in forming disulfide bridges. And not only that, it has a PK of 8.3. And what that means is, if we're at 8.3, if our PH is at zero point, 350 percent will exist in this form, and the remaining will exist in this negatively charged form.