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. 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?