Question 37:
When wool sweaters are washed in hot water, they shrink. In contrast, when silk clothes are washed, they do not shrink under the same conditions:
Choose the correct answer from the options given below:
Wool sweaters shrink in hot water due to α-helix to β-sheet conversion in keratin, while silk’s stable β-sheets resist this, making option (B) the key correct statement—though (A) and (B) together best explain the contrast, with multi-select (B) fitting standard exam logic.
Question Solution
Wool keratin features α-helical chains that unfold and rearrange into β-sheets under hot water agitation and alkali (from detergent), causing felting/shrinking. Silk fibroin, already β-sheet stacked tightly with small amino acids (Gly, Ala, Ser), maintains stability without conformational shift. Heat disrupts wool’s weaker H-bonds/disulfide links more than silk’s inter-chain bonds.
Option Breakdown
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(A): Correct—β-woolen (keratin α-helix) converts to β-conformation (not native α-helical), explaining shrink.
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(B): Correct—silk β-sheets are more stable (compact, crystalline) than wool’s dynamic α-helices.
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(C): Incorrect—wool chains do not remain native (α-helix); they convert, causing shrink.
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(D): Incorrect—contradicts (B); silk β-sheets are more stable than wool’s structure.
Answer: (B) (A), (B) and (D) only? Wait—no, (D) false. Actual best: (A) and (B), but per options, (B) as primary. Standard match is (A) only or multi with (B); context picks (B) for stability focus.
Wool sweaters shrink in hot water but silk clothes do not because wool’s α-helical keratin converts to β-sheets, while silk’s β-pleated fibroin stays stable—key for GATE Life Sciences protein folding questions.
Protein Structures Compared
Wool keratin: Mostly α-helices (coiled, elastic, 50% bulky amino acids like Cys for disulfide bonds). Silk fibroin: β-pleated sheets (flat, strong, 85% small Gly/Ala/Ser for tight packing).
Hot water + agitation → Wool α-helices unfold → form new β-bridges → fibers mat/shrink. Silk β-sheets: Already maximal H-bonded, resist change.
| Fiber | Native Structure | Hot Water Effect | Stability |
|---|---|---|---|
| Wool | α-helix | Converts to β-sheet; shrinks | Lower |
| Silk | β-pleated sheet | No change; stable | Higher |
Exam Tips
GATE focuses: α → β in keratin = felting; silk β inert. Mnemonic: “Wool Wriggles (α flexible), Silk Stiff (β rigid).” Practice: Explain via H-bonds/disulfide disruption.
