At physiological pH buffering action in blood is caused by

Maximal buffering capacity occurs at pH equal to pka of buffer. Therefore, to work as a best buffer at physiological pH amino acid should have pka value close to physiological pH (7.4). Amino acids can have buffering action due to three ionizable groups :- 1) a-carboxyl group- Different amino acids have pka value of c-carboxyl group between 3.5-4. So carboxyl group of amino acids has maximum buffering capacity between pH 3.5-4. 2) a-amino group :- Different amino acids have pka value of c-amino group between 8.0-9.0. Thus, a-amino group has maximum buffering capacity between pH 8.0-9.0. 3) Special ionizable group (in some amino acids) - Among special ionizable group of amino acids, imidazole group of histidine has pka value 6.5-7.4, which is closest to physiological pH. Hence, histidine (due to imidazole group) has maximum buffering capacity at physiologial pH.