What is the pKa value of histidine's R-group?

The pKa value of histidine's R-group is approximately 6.1. This significance stems from the unique structure of histidine, which contains an imidazole side chain. At physiological pH (around 7.4), histidine can exist in both protonated and deprotonated forms. The imidazole group has a pKa close to 6.1, allowing it to readily donate or accept protons, which is crucial for its role in enzymatic reactions and as a buffer in biological systems.

At a pH lower than its pKa, the imidazole group is predominantly protonated, while at a pH higher than this value, the group is more likely to be deprotonated. This property makes histidine particularly important in enzyme active sites where it can participate in proton transfer, making it an effective catalyst in many biochemical reactions.

Understanding the pKa of histidine is essential for grasping its chemistry and biological functions, especially in protein structure and function.