Physiological pH is near neutral. It would appear that only histidine is of physiological relevance. However, pKa values can be shifted significantly by neighboring charged groups in complex molecular structures.
Reactions of amino acids
Free amino acids (excluding proline) share similar chemical reactivities due to the common amino and carboxyl groups.
Different amino acid side chains have different chemical reactivities. Therefore,
reactivities of different proteins reflects the composition of the unique sequence of amino acids in their structure.
Some common carboxyl-group reactivities: