Final answer:
The sulfhydryl group of cysteine is a) protonated at a pH below its pKa.
Step-by-step explanation:
At a pH below its pKa(side chain), the sulfhydryl group of cysteine is protonated. This is due to the acidic nature of the thiol (-SH) group found in the side chain of cysteine. The thiol group is easily oxidized under certain conditions, which can lead to the formation of a disulfide (S-S) bond. Proteins that contain cysteine can form these disulfide linkages, which have important roles in the folding and stability of protein structures. The speciality of the sulfhydryl group of cysteine lies in this capability of forming a disulfide bond, thereby distinguishing it from other side chains. When the pH is below the pKa of a group, it tends to retain its protons, becoming protonated.