Q.6 The result of an electrophoretic separation of a mixture of amino acids X, Y and Z at pH = 5.0 is represented as (Given the isoelectric points of X, Y and Z are 9.87, 3.22 and 5.43, respectively.)

The correct electrophoretic pattern at pH 5.0 is option (B), where amino acid Z stays at the origin, X moves toward the cathode (negative charge), and Y moves toward the anode (positive charge).

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Introduction

In this problem, the electrophoretic separation of a mixture of amino acids X, Y and Z at pH 5.0 is analyzed using their isoelectric points: X = 9.87, Y = 3.22 and Z = 5.43. Understanding how the charge of an amino acid changes relative to its isoelectric point underlies every question on electrophoretic separation in CSIR NET and other life‑science examinations.

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Step 1: Charge of each amino acid at pH 5.0

Rule for amino acids in electrophoresis:

• If pH < pI → amino acid carries net positive charge and migrates toward the cathode (−).

• If pH > pI → amino acid carries net negative charge and migrates toward the anode (+).

• If pH ≈ pI → net charge ≈ 0 and the molecule stays near the origin.

Apply this to X, Y and Z at pH 5.0:

1. Amino acid X (pI 9.87)

   • pH 5.0 is much less than 9.87 → X is protonated and positively charged.

   • Therefore X moves toward the cathode.

2. Amino acid Y (pI 3.22)

   • pH 5.0 is greater than 3.22 → Y is deprotonated and negatively charged.

   • Therefore Y moves toward the anode.

3. Amino acid Z (pI 5.43)

   • pH 5.0 is slightly less than 5.43, very close to its pI → net charge is nearly zero.

   • Z shows minimal movement and stays near the origin.

Thus the correct pattern must show:

• X at the cathodic side,

• Y at the anodic side,

• Z around the starting point.

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Step 2: Detailed analysis of each option

The figure shows four possible band positions for X, Y and Z on the electrophoretic strip. Interpret each choice in terms of direction of migration and compare with the charges deduced above.

Option (A)

• Pattern: X in the center, Z toward one end, Y toward the other end.

• Implication:

  • X is shown at the origin, so X would be neutral.

  • One of Z or Y is on the cathode side and the other on the anode side.

• Why it is incorrect:

  • X cannot be neutral at pH 5.0 because pH is far below its pI (9.87); X must be strongly positive and move toward the cathode.

  • Therefore any pattern keeping X at the origin contradicts the pH–pI rule.

Option (B) – Correct pattern

• Pattern (reading left to right): Z near the center (origin), X displaced toward one side, Y displaced toward the opposite side.

• Interpretation consistent with theory:

  • Z at the origin → approximately neutral, matching its pI (5.43) being close to pH 5.0.

  • X on the cathode side → positively charged at pH below its pI (9.87).

  • Y on the anode side → negatively charged at pH above its pI (3.22).

• Hence option (B) correctly depicts the electrophoretic separation of a mixture of amino acids X, Y and Z at pH 5.0.

Option (C)

• Pattern: Y and Z to one side, X to the other side, with none at the origin.

• Implied charges:

  • If X is isolated on one side, it would have opposite charge to both Y and Z.

  • This would mean Y and Z share the same net charge sign, which is not true from the pI analysis.

• Why it is incorrect:

  • At pH 5.0, Y must be negative, X positive, and Z nearly neutral; Z cannot migrate with Y as a charged species.

  • No amino acid is placed at or near the origin, so the near‑neutral behavior of Z is not represented.

Option (D)

• Pattern: A single band at one side labeled as a combination (X, Z) and another band labeled Y at the opposite side.

• Implied behavior:

  • X and Z would have identical charge and move together, while Y would be opposite.

• Why it is incorrect:

  • X (pI 9.87) is clearly positive at pH 5.0, whereas Z (pI 5.43) is almost neutral; they cannot migrate together as one band.

  • Z must remain near the origin, not at the same position as strongly charged X.

Therefore, only option (B) satisfies the required positions dictated by the relationship between pH and isoelectric point for all three amino acids.