Cys - C
- Primarily acs as an acidic at neutral pH, but is usually not classified as an acidic aa.
- -SH group does ionize.
- Uncommon to find on the surface of proteins (even though it has an ionizable side group), because 1) sulfur (unlike oxygen) has a low propensity to form hydrogen bonds, and 2) the thiol group can react with another cysteine’s thiol group in an oxidation reaction that forms a disulfide bond.