Amino acid with dissociation constant closest to physiological pH is:
Histidine has dissociaion constant close to physiologic pH of 7.4 Ref: Textbook of Biochemistry By DM Vasudevan, 3rd Edition, Page 15 ; Lippincott Biochemistry, 2nd Edition, Pages 9-10 ; Pulmonary Biology in Health and Disease By E. Edward Bittar, 2002, Page 277
Serine
Histidine
Threonine
Proline