Q.20 In the formation of agarose gel, which one of the following bonds is involved?
(1) Van der Waals interaction
(2) Intra-chain hydrogen bonding
(3) Inter and Intra-chain hydrogen bonding
(4) Disulphide bond
The Chemistry Behind Agarose Gel Formation
Agarose, a polysaccharide from red seaweed, forms the matrix for DNA/RNA electrophoresis. When dissolved in hot buffer and cooled, it gels via specific intermolecular forces. This MCQ tests that: In the formation of agarose gel, which one of the following bonds is involved? (1) Van der Waals interaction (2) Intra-chain hydrogen bonding (3) Inter and Intra-chain hydrogen bonding (4) Disulphide bond.
Correct Answer: (3) Inter and Intra-chain hydrogen bonding
Hydrogen bonds create the double helix and network for gel solidity.
Mechanism of Agarose Gelation: Hydrogen Bonds at Work
Agarose monomers (D-galactose + 3,6-anhydro-L-galactose) polymerize into long chains:
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Heating (>85°C): Chains separate, solution viscous.
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Cooling (35-40°C): Intra-chain H-bonds form left-handed double helices (two chains coil).
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Further Cooling (<30°C): Inter-chain H-bonds link helices into 3D network, trapping water (99% hydrated gel).
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Pore size (100 nm-500 nm) sieves DNA by size.
Hydroxyl (-OH) and ether groups form H-bonds with water or chains. Concentration tunes pores: 0.5% large pores (big DNA), 2% small (small fragments).
Explanation of All Options: Bonds in Context
Why hydrogen bonds dominate agarose (neutral polysaccharide):
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(1) Van der Waals interaction: Weak, non-specific. Minor role in helix packing but insufficient alone for gelation. Seen in synthetic polymers.
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(2) Intra-chain hydrogen bonding: Partial credit—forms helices within chains but misses inter-chain links for network.
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(3) Inter and Intra-chain hydrogen bonding: Complete ✓. Intra: helix formation; Inter: aggregation into gel matrix.
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(4) Disulphide bond: Absent—no sulfur/cysteine in agarose. Covalent, seen in proteins (e.g., SDS-PAGE).
| Bond Type | Strength | Role in Agarose | Gel Formation? |
|---|---|---|---|
| Van der Waals | Weak (~1 kcal/mol) | Minor packing | No |
| Intra-chain H-bond | Moderate (~5 kcal/mol) | Helix formation | Partial |
| Inter + Intra H-bond | Moderate | Full 3D network ✓ | Yes |
| Disulfide | Strong covalent | None (no S) | No |
Practical Tips for Lab Success
Optimal gelling: 1% agarose in TAE/TBE buffer, microwave, cool slightly before pouring. Avoid bubbles—tap rack. Ethidium bromide intercalates DNA for UV visualization.
Diagram of structure:
Chain 1: -Gal-AGal-Gal-AGal-
↑ Intra H-bonds → Double helix
Chain 2: -Gal-AGal-Gal-AGal-
Inter H-bonds → Network → Gel pores
Resources: Sigma-Aldrich protocols or Electrophoresis journal reviews.
