Which compound has a pKa of 12.5?

The compound with a pKa of 12.5 is arginine. This specific pKa value reflects the ionization of the guanidinium group present in arginine, which is a basic amino acid. Arginine's side chain contains a positively charged guanidinium group, which can accept a proton, contributing to the higher pKa.

In the context of biological systems, such a high pKa indicates that at physiological pH (around 7.4), the guanidinium group remains predominantly protonated, making arginine positively charged. This characteristic plays an important role in protein structure and function, particularly in interactions with negatively charged molecules or other amino acids.

Understanding the pKa values of amino acids helps illuminate their behavior in biological systems, particularly how they can act as proton donors or acceptors, influencing enzymatic activity and molecular interactions.