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In a handout the following is stated:

Amylose is insoluble in water, therefore a good storage compound e.g. in stroma of chloroplasts

This is with regard to the chemical structure of the molecule.

However it has left me wondering, what is it intrinsically about the structure of a molecule that makes it soluble/insoluble in water?

My thoughts at this stage are perhaps to do with whether the molecule is hydrophobic, and if that is the case - how is amylose hydrophobic?

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The availibity of -OH groups for hydrogen bonding with polar water molecules is reduced in amylose due to its coiled nature (resulting in part by H-bonds between the glucose monomers). Amylopectin is even less soluble due to the additional 1-6 glycosidic bonds on the branch chains, further reducing its H bonding potential and therefore reducing solubility in water.

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  • $\begingroup$ Welcome to Biology.SE! Your answer sounds reasonable, however, it is likely to receive a more favorable response if you include supporting references (primary literature is best, but a textbook available online would seem sufficient for this question). ——— You may also benefit from taking the tour and then consulting the help pages for additional advice on How to Answer effectively on this site. Thanks! 😊 $\endgroup$ – tyersome Oct 26 at 17:05
  • $\begingroup$ (+1) I think you are right about amylose: the linear polymer of glucose units in alpha (1-4) glycosidic linkage adopts a compact coiled structure which "encourages strong intramolecular hydrogen bonds and discourages access by solvent (ref and this answer). But amylopectin, due to its branched open structure that encourages hydrogen bonding with water, is soluble in (cold) water (see cited J Chem Ed ref). $\endgroup$ – user1136 Oct 26 at 17:37
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There are a couple of other questions that address this point indirectly: here and here.

In my answers to the first of these questions I mention that amylose is semi-crystalline because of its regular extended structure. I imagine that the semi-crystalline state is more thermodynamically favourable than the solution state, but suspect that a rigorous chemical answer might be quite complex.

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Depending on molecular weight of amylose and because of its helical structure formed by two macromolecules makes it partly soluble in water.

Heating amylose solution leads to formation of colloidal suspension of soluble fraction and remining insoluble higher molecular weight fraction of amylose does not dissolve. On cooling this suspension, certain portion of polysaccharide precipitates. This is because of the glycosidic linkages between these glucose molecules.

Reference
Cuevas et al. Structural differences between hot-water-soluble and hot-water-insoluble fractions of starch in waxy rice. Carbohydrate Polymers 81(3):524-532

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