Amino acid which acts as a buffer at the physiological pH of 7.4 is:
The pK value of imidazolium group of Histidine is 6.1 and therefore effective as a buffer at the physiological pH of 7.4. The buffering capacity of plasma proteins and hemoglobin is mainly due to histidine residue. Also Know: Pauly's test is used to detect the amino acid group Imidazole (Histidine.) Ref: Textbook of Biochemistry By DM Vasudevan 5th Ed Page 20, 22
Glycine
Histidine
Lysine
Arginine