Biomolecules Te...

An amino acid is characterized by two $$pka$$ values the one corresponding to 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 zwitterion 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. The acidic amino acids have acidic side chains basic amino acids have basic side chains. $$pI$$ for an acidic amino acid is an average of $$pka_1$$ and $$pka_3$$. $$pI$$ of a 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	$$2.17$$	$$9.04$$	$$12.48$$