Q.13 What is the role of sodium dodecyl sulfate (SDS) in protein electrophoresis
(A). Add net negative charge to the protein
(B). Seperate lipids on the basis of their molecular weight
(C). Modify native conformation ofa protein
(D). Allows direct destaining of the proteins
1. (A). (B) and (D) only
2. (A) and (C) only.
3. (C) and (D).
4. (B) and (D) only.
SDS plays dual roles in SDS-PAGE by denaturing proteins and imparting uniform negative charge for size-based separation. The correct answer is option 2: (A) and (C) only.
SDS-PAGE Fundamentals
SDS-PAGE separates proteins by molecular weight using polyacrylamide gels under an electric field. SDS, an anionic detergent, binds proteins at ~1.4 g/g ratio, masking native charges and linearizing structures for consistent migration.
Option Analysis
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(A) Add net negative charge: Correct. SDS coats proteins uniformly negative (~1 SDS per 2 amino acids), overriding intrinsic charge so separation depends solely on size.
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(B) Separate lipids by molecular weight: Incorrect. SDS-PAGE targets proteins; lipids require different methods like TLC. SDS solubilizes but doesn’t separate lipids here.
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(C) Modify native conformation: Correct. SDS disrupts hydrophobic interactions and H-bonds, unfolding proteins to primary structure with heating/BME aid.
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(D) Allows direct destaining: Incorrect. Destaining uses diffusion in methanol/acetic acid baths; SDS doesn’t facilitate this—stains like Coomassie bind post-SDS.
Option 1: Includes wrong B, D.
Option 2: A+C only—matches SDS roles precisely.
Option 3: C+D—misses charge, includes wrong D.
Option 4: B+D—both wrong.
The role of sodium dodecyl sulfate (SDS) in protein electrophoresis is crucial for SDS-PAGE, where it adds net negative charge to proteins and modifies native conformation for molecular weight separation. This guide solves the exact GATE question with detailed breakdowns for competitive exam success.
SDS Mechanisms
SDS’s amphipathic nature (hydrophobic tail, anionic head) binds polypeptide backbones, yielding ~0.5-1.4 g SDS/g protein. This denatures via hydrophobic collapse and electrostatic repulsion, creating rod-like SDS-protein complexes with uniform charge/mass ratio (migration ∝ log MW⁻¹).
| Function | Description | Correct? |
|---|---|---|
| Net Negative Charge | Masks pI; all migrate to anode | Yes |
| Denature Conformation | Unfolds to linear form | Yes |
| Lipid Separation | Not applicable (proteins only) | No |
| Destaining Aid | Destaining diffusion-based | No |
Why Option 2?
Combines primary SDS functions: charge uniformization (A) enables size-based sieving; denaturation (C) ensures shape independence. B confuses with lipid TLC; D misattributes Coomassie protocols.
GATE Prep Tips
Memorize SDS ratio, binding isotherm, and adjuncts (β-ME for disulfides, boil 95°C). Practice variants: Native-PAGE (no SDS, charge/shape-based), Tricine-PAGE (small peptides). Visualize: SDS = “clothesline” straightening proteins for ladder-like bands.
