Question

Amino acids are bifunctional having - $COOH$ and $- NH _{2}$ groups. Depending upon the position, they may by $\alpha$ amino acids, $\beta, \gamma$, or $\delta$ - amino acids. There are about $20 \alpha$ amino acids, commonly found in proteins. They are soluble in water, but insoluble in organic solvents. It is due to formation of zwitterions. Thee show the properties of $- NH _{2}$ group as well as - $COOH$ group. Every amino acid has its own characteristic isoelectric point.
What is the structure of the a bove amino acid in a solution of $pH =9.87 ?$

• A. $H _{2} N - CH _{2}- CH _{2}- CH _{2}- CH _{2}- \underset{\overset{|}{{\oplus}NH_3}}{CH}- COOH$
• B. $H _{3} \overset{\oplus}{N} - CH _{2}- CH _{2}- CH _{2}- CH _{2}- \underset{\overset{|}{{\oplus}NH_3}}{CH} - COOH$
• C. $H _{3} \overset{\oplus}{N} - CH _{2}- CH _{2}- CH _{2}- CH _{2}- \underset{\overset{|}{{\oplus}NH_3}}{CH} - COO ^{-}$
• D. $H _{3}\overset{\oplus}{N} - CH _{2}- CH _{2}- CH _{2}- CH _{2}- \underset{\overset{|}{{\oplus}NH_3}}{CH} - COO ^{-}$

Answer 1

Answer: (D)

Since the given amino acid has two $- NH _{2}$ group and one $COOH$ group,
the isoelectric point is obtained as the average value of $pK _{ a }$ of both $- NH _{2}$ groups only.
Thus $pI =\frac{8.95+10.79}{2}=\frac{19.47}{2}=9.87$