P has a pI of 6.9, meaning it’s less negatively charged than Q at pH 8.0 due to replacing glutamate (acidic) with valine (neutral). In polyacrylamide gel electrophoresis at pH 8.0, P migrates faster toward the anode than Q.​

Key Concepts

The isoelectric point (pI) is the pH where a peptide has zero net charge. At pH > pI, the peptide is negatively charged and moves to the anode (positive electrode); at pH < pI, it’s positively charged and moves to the cathode. Glutamate contributes a negative charge (pKa ~4.3 for side chain), while valine is neutral.​

Peptide Analysis

Peptide P (pI 6.9) at pH 8.0 has pH > pI, so net negative charge. Q replaces Glu³ (negative) with Val (neutral), reducing negative charges and making Q less negative (higher effective pI than P). Thus, P has stronger negative charge, migrates faster to anode in electric field.​

Option Evaluation

• (A) P migrates faster to anode: Correct. P more negative, faster anodic mobility.​

• (B) P faster to cathode: Incorrect. Both negative at pH 8, move to anode, not cathode.​

• (C) Migrate together: Incorrect. Charge difference causes separation.​

• (D) Q faster to anode: Incorrect. Q less negative, slower to anode.​

Introduction: Unraveling Hexapeptide P and Q in Isoelectric Point Electrophoresis
In biotechnology exams like GATE BT, questions on hexapeptide P isoelectric point (pI) 6.9 test core concepts of peptide charge and migration in polyacrylamide gel electrophoresis at pH 8.0. Hexapeptide Q, a variant replacing glutamate with valine at position 3, shifts charge dynamics. This guide breaks down why P migrates faster toward the anode, optimizing for searches on hexapeptide electrophoresis pI analysis.

Isoelectric Point (pI) Fundamentals

The isoelectric point is the pH where a peptide’s net charge is zero. For hexapeptide P (pI 6.9), at pH 8.0 (> pI), it gains negative charge from deprotonated groups like Glu carboxylates. Replacing acidic glutamate (negative side chain) with neutral valine in Q reduces anions, making Q less negative.​

Electrophoresis Behavior at pH 8.0

In anionic PAGE, negative peptides move to the anode based on charge density. P’s extra negative charge from Glu accelerates it toward the anode faster than Q. This separation highlights how single substitutions alter pI and electrophoretic mobility.​

Why Option A is Correct

• P: More negative → higher anodic velocity.

• Q: Less negative → slower.​
  Common pitfalls include confusing cathode/anode or ignoring pH-pI relation.​