An amino acid is characterized by two pKa values the one correspondingto the more acidic site is designated as pKa_1 and the other corresponding to the less acidic site is designated as pKa_2 The isoelectric point also called isoionic point (pI) is the pH at which concentration of zwitter ion is maximum . pI is the average of pKa_1 and pKa_2.Generally the value of pI is slightly less than 7. some amino acids have side chain with acidic or basic groups.These amino acids have pKa_3 value also for the side chain.Acidic amino acid have acidic side chains and basic amino acids have basic side chains . pI for acidic amino acid is average of pKa_1 and pKa_3.pI for basic amino acid is the average of pKa_2 and pKa_3. {:(S.NO,"Amino acid",p^(Ka1),p^(Ka2),p^(Ka3)("side chain")),(I,"Aspartic acid",1.88,9.6,3.65),(II,"Glutamic acid",2.19,9.67,4.25),(III,"Lysine",2.18,8.95,10.53),(IV,"Arginine",2.17,9.04,12.48):} In the table given above the acidic amino acids are