Question

If glutamic acid is placed in an electric field, its migration depend on pH of the solution. Which of the following options are correct regarding alanine? LtpH = 3.2 there is no net migration of alanine

• A. towards a particular electrode
• B. lambda*tpH = 9.8 alanine migrates towards anode
• C. At pH = 3.2 . Alanine migrates towards cathode
• D. At pH = 6 Alanine migrates towards anode

Answer 1

Answer: (B), (C)

B and C

Answer 2

Amino acids are amphoteric molecules with both acidic (-COOH) and basic (-NH₂) groups. Their net charge depends on the pH of the solution. This charge dictates their migration in an electric field.

The given pKa values for alanine are $pKa_1 = 3.2$ (for the carboxyl group) and $pKa_2 = 9.8$ (for the amino group). The isoelectric point (pI) is the pH at which the amino acid has no net charge (is in its zwitterionic form). It is calculated as the average of the two pKa values: $pI = \frac{pKa_1 + pKa_2}{2} = \frac{3.2 + 9.8}{2} = 6.5$

• If pH < pI, the amino acid is net positively charged and migrates towards the cathode (negative electrode).
• If pH > pI, the amino acid is net negatively charged and migrates towards the anode (positive electrode).
• If pH = pI, the amino acid is neutral (zwitterionic) and shows no net migration.

Analyzing the options:

• Option A: At pH = 3.2, which is equal to $pKa_1$, the net charge is positive. Therefore, alanine migrates towards the cathode, not shows no net migration. Incorrect.
• Option B: At pH = 9.8, which is equal to $pKa_2$, the net charge is negative. Therefore, alanine migrates towards the anode. Correct.
• Option C: At pH = 3.2, the net charge is positive. Positively charged species migrate towards the cathode. Correct.
• Option D: At pH = 6, which is below the pI (6.5), the predominant species is the zwitterion ($H_3N^+-CH(CH_3)-COO^-$), which has a net charge of 0. Thus, alanine shows no net migration, not migrates towards the anode. Incorrect.