Learning Objectives state the charge on amino acid side chains using the Henderson Hasselbach equation and the approximate charge by inspection at any given pH draw mechanisms and identify products for the reaction of nucleophilic side chains Lys and Cys with common chemical modification agents and extend this understanding to reactions of His. draw mechanisms for disulfide exchange reactions for sulfhydryls using them and oxidation numbers to explain redox reactions of cysteine/cystine. Template:HideTOC Thumbnail: Structure of human hemoglobin. The proteins α and β subunits are in red and blue, and the iron-containing heme groups in green. From PDB: 1GZX​. (GNU; Proteopedia Hemoglobin).