What is the pKa of the R-group of cysteine?

The pKa value of the R-group of cysteine is approximately 8. Cysteine contains a thiol group (-SH) in its side chain, which can undergo protonation and deprotonation depending on the pH of the environment. The pKa value represents the pH at which half of the cysteine residues are in the deprotonated thiolate form (–S^-) and half are protonated.

At physiological pH (around 7.4), the majority of cysteine exists in the deprotonated form, making its thiol group reactive. The value of around 8 indicates that at lower pH levels, the thiol group is more likely to be protonated (–SH), while at higher pH levels, it tends to lose a proton and become negatively charged.

Understanding the pKa of cysteine is important in biochemical contexts, such as enzyme catalysis and protein folding, where the redox reactions involving the thiol groups can significantly influence the protein's structure and function.