Glycosidic bond in sucrose; why B-fructose?

[Necr0sis713]

I'm having trouble understanding how the bond in sucrose is beta on carbon 2. Is it flipped over or something? It looks like the fructose is turned over from its traditional "textbook image"

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[theonlytycrane]

It's flipped over in most images you'll see because the linkage between alpha(1) on glucose and beta(2) on fructose can be hard to draw otherwise.

 

[aldol16]

Just compare where the hydroxyl is on C2 and where the C6 -CH2OH is. If they're on the same side, it's beta and if they're on opposite sides, it's alpha.

 

[Necr0sis713]

Ok, thanks for clarifying. So in the image of sucrose, it's like the fructose has to be "flipped over" to do the bond, which is why it appears on the bottom of the ring?

 

[theonlytycrane]

Ok, thanks for clarifying. So in the image of sucrose, it's like the fructose has to be "flipped over" to do the bond, which is why it appears on the bottom of the ring?

Yep! I believe that by "bottom" you are referring to -OH down instead of up on the anomeric carbon (#2).

 

[Ischemia1032]

All of this is correct, changes in type of glucose can change bond orientation. For example, amylase can digest amylose and amylopectin but not cellulose not because of the chemical formula. The chemical formula is essentially the same. The difference is one has beta bonds and one has alpha bonds.