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BIO NMDCAT Carbohydrates
nmdcat.online June 27, 2026

The iodine test for starch gives a blue-black color with the amylose component, but a red-violet or reddish-brown color with amylopectin. The reason for this color difference is that the shorter, branched chains of amylopectin form helices that accommodate

A. Longer polyiodide chains, resulting in a color shift
B. Shorter polyiodide chains, resulting in a different absorption spectrum
✓
C. No iodine, and the color is due to a different reaction
D. The same polyiodide chains, but at a different pH

📝 Explanation

The length of the polyiodide chain within the helix dictates the absorption wavelength. Amylopectin's branched structure limits the length of the continuous helical segments available, resulting in the inclusion of shorter polyiodide chains, which absorb light differently and produce a red-violet color.

📖 Additional Information

  • Longer polyiodide chains, resulting in a color shift
  • Shorter polyiodide chains, resulting in a different absorption spectrum
  • No iodine, and the color is due to a different reaction
  • The same polyiodide chains, but at a different pH

The length of the polyiodide chain within the helix dictates the absorption wavelength. Amylopectin's branched structure limits the length of the continuous helical segments available, resulting in the inclusion of shorter polyiodide chains, which absorb light differently and produce a red-violet color.

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