Cysteine (Cys, C) – Polar side chain: Mercaptomethyl group.

The mercapto, or sulfhydryl, group (-SH) is ionizable at higher pH (pK_a 8.3).  In proteins two cysteine side chains can join to form covalent disulfide cross-links between separate sections of a polypeptide chain (–CH₂–S–S–CH₂–).