Beta linkage in Sucrose

[regeneration]

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Hi,

How can I determine that fructose is making a Beta linkage with glucose in Sucrose? I understand that the anomeric carbon on fructose is carbon 2, but to me it looks like the OH coming off of the anomeric carbon 2 is pointing DOWN which would make it an alpha bond.

http://cellbiologyolm.stevegallik.org/sites/all/images/sucrose.png

 

[cheesier]

Classification of the bond depends on your perspective, so looking at fructose, the OH on the anomeric carbon is down and the hydroxyl substituent on carbon 5 of fructose is pointing downward as well. They're cis so fructose is beta, and therefore you could name the bond a b(2-1) linkage. In reality, that's not very descriptive since glucose is alpha, so you would usually say that sucrose has an ab(1-2) bond.

 

[regeneration]

So to clarify, you would determine alpha for the glucose by seeing if the OH is trans on the 1 & 5 glucose carbons and you would determine beta for the fructose by seeing if the OH is cis on the 2 & 5 fructose carbons? So the anomeric and D/L determining carbons are what determine if a bond is alpha or beta?

 

[cheesier]

So to clarify, you would determine alpha for the glucose by seeing if the OH is trans on the 1 & 5 glucose carbons and you would determine beta for the fructose by seeing if the OH is cis on the 2 & 5 fructose carbons? So the anomeric and D/L determining carbons are what determine if a bond is alpha or beta?

Correct 👍 To determine what the bond is, look at the individual carbohydrates linked together and determine their configurations.

 

[adrakdavra]

on the mcat beta will be pointing up and alpha down with the fishes