Final answer:
Serine and threonine contain OH groups that have high pKa values, implying less favorable dissociation, hence their third pKa is not typically included in amino acid pH charts due to its irrelevance in biological systems. These hydroxyl groups are more crucial for protein function, such as phosphorylation, rather than for their acid-base properties at physiological pH.
Step-by-step explanation:
The question pertains to why the third pKa values for serine and threonine are not typically reported in amino acid pH charts, even though they both contain an OH functional group that could theoretically dissociate. The hydroxyl group in the side chains of serine and threonine does indeed have the potential to dissociate, but this occurs at a much higher pH than the range in which biological systems typically operate.
Hence, the third pKa is not of practical relevance for most biological studies, and therefore is often omitted from charts. Furthermore, the resonance stabilization that is possible in carboxylic acid conjugate bases is not available to the same extent in these hydroxyl-containing amino acids, which means their pKa values are much higher, and thus, the deprotonation of these groups is less favorable under physiological conditions.
Additionally, serine and threonine hydroxyl groups are important for the function of proteins, such as in the case of phosphorylation, where these groups participate in forming phosphoester bonds rather than acting as dissociable protons in biological systems. The role of these amino acids in protein structure and function is often emphasized over their less relevant acid-base properties in a physiological pH context.