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BIO NMDCAT Carbohydrates
nmdcat.online June 27, 2026

The blue color formed in the iodine test for starch is due to a charge-transfer complex. This test will yield a negative result (no blue color) with a sample of glycogen because glycogen’s

A. Lower molecular weight prevents complex formation
B. Much shorter average chain length of its outer branches forms an unstable, reddish-brown complex
✓
C. Absence of α-1,4 linkages prevents iodine binding
D. Covalent linkage to proteins blocks the iodine-binding sites

📝 Explanation

The iodine test requires a helix of sufficient length to stabilize the polyiodide chain. Glycogen's highly branched structure means its α-1,4 helical segments are very short. With iodine, it yields a reddish-brown color (not blue-black), which can be easily confused with a negative test if not careful.

📖 Additional Information

  • Lower molecular weight prevents complex formation
  • Much shorter average chain length of its outer branches forms an unstable, reddish-brown complex
  • Absence of α-1,4 linkages prevents iodine binding
  • Covalent linkage to proteins blocks the iodine-binding sites

The iodine test requires a helix of sufficient length to stabilize the polyiodide chain. Glycogen's highly branched structure means its α-1,4 helical segments are very short. With iodine, it yields a reddish-brown color (not blue-black), which can be easily confused with a negative test if not careful.

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