What is the pKa value of the R-group of Lysine?

The pKa value of the R-group of lysine is indeed around 10.5. This value is significant because it indicates the pH at which the side chain amine group of lysine is half protonated and half unprotonated. Generally, lysine has a basic side chain that can accept protons, which makes it positively charged under physiological pH conditions (around 7.4). The pKa value facilitates understanding lysine's behavior in biological systems, particularly in enzymatic reactions and protein interactions, as it will affect the overall charge and reactivity of the lysine residue. The value of 10.5 shows that below this pH, the side chain is predominantly protonated (positively charged), while above this pH, it becomes neutral, which is essential for the roles that lysine plays in protein structure and function.