Q.41 The net charge on the following peptide at pH 7.0 is _____ .
Val-Asp-Asn-Lys-Ser-Ile
The net charge on the peptide Val-Asp-Asn-Lys-Ser-Ile at pH 7.0 is +1.
The net charge on peptide at pH 7 determines protein behavior in biochemical assays, crucial for CSIR NET Life Sciences preparation. This guide breaks down the calculation for Val-Asp-Asn-Lys-Ser-Ile using standard pKa values and Henderson-Hasselbalch principles.
Ionizable Groups
Peptides have charges from N-terminal amino (~pKa 9.0, protonated +1 at pH 7), C-terminal carboxyl (~pKa 2.0, deprotonated -1 at pH 7), and side chains where pH > pKa means deprotonated (acidic: -1; basic: 0) or pH < pKa means protonated (basic: +1).
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Val (V): No charged side chain.
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Asp (D): Carboxyl side chain pKa ~3.9, deprotonated -1 at pH 7.
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Asn (N): Neutral amide side chain, charge 0.
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Lys (K): Amino side chain pKa ~10.5, protonated +1 at pH 7.
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Ser (S): Hydroxyl side chain pKa >13, neutral 0.
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Ile (I): Hydrocarbon side chain, neutral 0.
Charge Calculation
Sum the charges: N-terminal (+1) + C-terminal (-1) + Asp side chain (-1) + Lys side chain (+1) = +1.
| Group | pKa | State at pH 7 | Charge |
|---|---|---|---|
| N-terminal NH3+ | ~9.0 | Protonated | +1 |
| C-terminal COOH | ~2.0 | Deprotonated | -1 |
| Asp side chain | 3.9 | Deprotonated | -1 |
| Lys side chain | 10.5 | Protonated | +1 |
| Others (Val, Asn, Ser, Ile) | N/A | Neutral | 0 |
| Net | +1 |
CSIR NET Exam Tips
No options provided, but common distractors include forgetting terminals (-1 total), miscounting Asp/Lys (0 or +2), or assuming neutral side chains charged. Practice with pI tables: acidic residues lower pI, basic raise it. Verify using peptide calculators for confirmation.
