Question

An electric current is passed through an aqueous solution of a mixture of alanine (isoelectric point $6.0$) glutamic acid $(3.2)$ and arginine $(10.7)$ buffered at $pH6$. What is the fate of the three acids ?

• A. Glutamic acid migrates to anode at $pH6$. Arginine is present as a cation and migrates to the cathode. Alanine in a dipolar ion remains uniformly distributed in solution
• B. Glutamic acid migrates to cathode and others remain uniformly distributed in solution
• C. All three remain uniformly distributed in solution
• D. All three move to cathode

Answer 1

Answer: (A)

Amino acids exists as Zwitter ions which contain both positive and negative charge but overall electrically neutral. At their isoelectric point, amino acids donot migrate to any electrode and remain uniformly distributed in the solution. Thus, alanine (isoelectric point $6.0$ ) does not move to any electrode.
At a pH lower than their isoelectric point, they exist as cation and move towards cathode. Thus, arginine (isoelectric point 10.7) exist as cation.
At a $pH$ higher than their isoelectric point, they exist as anion and move towards anode. Thus, glutamic acid (isoelectric point $3.2$ ) at $pH6.0$ moves towards anode.
$\underset{\text{Low pH (cation)}}{H_3\stackrel{+}{N}-\stackrel{\underset{|}{R}}{C}H-COOH}\stackrel{H^{+}}{\leftarrow }\underset{\text{Zwitter ion(neutral)}}{\stackrel{+}{N}{H}_3-\stackrel{\underset{|}{R}}{C}H-CO{O}^{-}}$
$\stackrel{O{H}^{-}}{\to }\underset{\text{High pH (anion)}}{H_{2}N-\stackrel{\underset{|}{R}}{C}H-CO{O}^{-}}$