Final answer:
The sulfhydryl (SH) group of cysteine is deprotonated when the pH is above its pKa, leading to a negatively charged state. Option b.
Step-by-step explanation:
At a pH above its pKa, the sulfhydryl group of cysteine is deprotonated. This occurs because the thiol (SH) group, like any other acidic proton, is more likely to lose a proton as the pH rises above its pKa.
The sulfhydryl group is no different. As the pH value rises above the pKa of that group, the thiol group (SH) loses its hydrogen (H+), resulting in the formation of a thiolate anion (S-). This leads to the amino acid having a negatively charged sulfhydryl group at higher pH levels.
The pKa of the sulfhydryl group in cysteine is typically around 8.3, which means below this pH, the sulfhydryl (SH) group would generally be protonated (contain a hydrogen), while above this pH level it becomes deprotonated.
Therefore, when the pH is above the group's pKa, the resulting form would carry a negative charge, thus option b) Deprotonated is the correct answer.