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I know that amylose and amylopectin are major constituents of starch. I also know the chemical structures of amylose and amylopectin. However, I want to determine how amylose and amylopectin bond together. I think that the OH from the amylose bonds with the HO of the amylopectin to form an oxygen bond and a water. Am I correct, or does the OH chain of amylose bond with the O chain of amylopectin?

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  • $\begingroup$ What are you talking about? They're two molecules. When and why would such polymerisation supposed to occur? $\endgroup$ – Mithoron Aug 28 '16 at 0:32
  • $\begingroup$ Starch is composed of amylopectin and amylose molecules bonded together. My question is how and where are they bonded together. Is it a dehydration synthesis with the OH and HO groups? $\endgroup$ – DrPepper Aug 28 '16 at 0:35
  • $\begingroup$ Different molecules can't be connected with covalent bonds as they would become one molecule. If you have a sugar or mix of different sugar molecules, they're connected with hydrogen bonds. $\endgroup$ – Mithoron Aug 28 '16 at 0:40
  • $\begingroup$ Ok. Thank you. So where would be the hydrogen bond connecting these two molecules? Which two parts of the respective molecules would be connected with each other? $\endgroup$ – DrPepper Aug 28 '16 at 3:30
  • $\begingroup$ I suggest h bonds will form in all different parts between hydroxy groups and ether groups and between hydroxy and hydroxy. Water will form a hydration shell and will therefore function as some sort of lose connector. $\endgroup$ – AstronAUT Aug 29 '16 at 2:58
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Actually you were close, below I have tried to discuss how this occurs:

Glycosidic bonds between monosaccharide units are the basis for the formation of oligosaccharides and polysaccharides such as starch.

It is possible for a sugar hydroxyl group (ROH) bonded to the anomeric carbon (the anomeric carbon is the carbonyl carbon of the open chain form of the sugar and is the one that becomes a chiral centre in the cyclic form) to react with another hydroxyl (R’-OH) to form a glycosidic linkage (R’-OR).

(A glycosidic linkage is not an ether)

Glycosidic linkages can take various forms; the anomeric of one sugar can be bonded to any one of the -OH groups on a second sugar to form an alpha- (as in starch) or beta-glycosidic linkage.

An example of a glycosidic bond (showing formation of maltose):

a

C-1 of one glucose is linked by a glycosidic bond to the C-4 oxygen of the other glucose when an ⎯OH (alcohol) of one glucose molecule (right) condenses with the intramolecular hemiacetal of the other glucose molecule (left), with elimination of H2O and formation of a glycosidic bond.

The reversal of this reaction is hydrolysis—attack by H2O on the glycosidic bond. In animal digestion (first phase), it is aided by α (1,4)-glucan 4-glucanohydrolase (salivary α-amylase)

Here is another comparison of the α 1,4 glycosidic linkage and α1,6 glycosidic linkage:

b

Having discussed about the glycosidic bond, we can refer back to starch: Starch is composed of two components, α-amylose and amylopectin. α-Amylose is composed of linear chains of D-glucose in α (1,4) glycosidic linkages. The linear linkages in amylopectin are α (1,4), whereas the branch linkages are α (1,6) glycosidic bonds:

c

Edit

It is also possible for polysaccharide units to form hydrogen bond interactions between neighbouring units, although these are minor and serve to stabilise the helical structure of amylose chains (around up to 6 residues per turn) but the major bonds are glycosidic bonds

I am sure you now have a clue on how they are bonded. Hope this helps.

References:

Biochemistry (Campbell and Farell)

Lehninger Principles of Biochemistry

Biochemistry (Grisham)

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