D and L sugars, Alpha and Beta Anomers

[90210]

I've probably asked four different people this same question, but it seems nobody really knows for sure. My question is this:

Is there a correlation between a D or L sugar and Alpha or Beta anomers? In other words, are D sugars always Beta anomers (and vice versa)?

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[Lexicon0]

I've probably asked four different people this same question, but it seems nobody really knows for sure. My question is this:

Is there a correlation between a D or L sugar and Alpha or Beta anomers? In other words, are D sugars always Beta anomers (and vice versa)?

I'm not 100% sure on this but I'm inclined to say the two aren't related. Since D and L mainly refer to the stereochemistry of the highest numbered stereocenter and anomers arise from nucleophilic attack of the carbonyl carbon, I would assume that D/L designation doesn't really affect what anomer is formed.

 

[GreenRabbit]

I can tell you with 100% certainty that D/L and alpha/beta are not related. D/L are stereoisomers in open-chain form. alpha/beta are anomers of a ring sugar.

D-glucose can form both alpha and beta D-glucopyranose. Likewise, L-glucose can form both alpha and beta L-glucopyranose.

 

[90210]

Thanks both of you for clearing that up. What is it exactly then about Biological Systems that doesn't allow us to digest L-sugars then (aside from the fact that we lack the enzymes). In other words, how are sugars being digested ... is it via breaking the glycosidic linkage? If so, then why can't we digest an L-sugar if it could produce a B1,4 linkage just like a D-sugar, since they are both isomerizing into both anomeric forms.

 

[GreenRabbit]

Glycosidic linkages connect sugar residues in di- and polysaccharides, they aren't the reason why we can't digest L-monosaccharides such as L-glucose and L-fructose. These sugars can't be digested because the enzymes required for their entry into glycolysis, hexokinase and fructokinase respectively, work on D- but not L- sugars.

 

[90210]

Glycosidic linkages connect sugar residues in di- and polysaccharides, they aren't the reason why we can't digest L-monosaccharides such as L-glucose and L-fructose. These sugars can't be digested because the enzymes required for their entry into glycolysis, hexokinase and fructokinase respectively, work on D- but not L- sugars.

Perfect explanation. Thank you!! 🙂

 

[Isoprop]

D/L are stereoisomers in open-chain form

Not quite. Just wanted to clear up--you D and L stereoisomers is still "retained" in the ring form. In the Haworth projections,

For example;

This is an alpha, L sugar. It's L because the methoxy group is pointing down. It's alpha because the anomeric carbon has the OH group pointing in the opposite direction.

Otherwise, this is pretty much spot on.

 

[GreenRabbit]

Yeah, good addition. I can see how what I said could be misinterpreted.

 

[90210]

Not quite. Just wanted to clear up--you D and L stereoisomers is still "retained" in the ring form. In the Haworth projections,

For example;

This is an alpha, L sugar. It's L because the methoxy group is pointing down. It's alpha because the anomeric carbon has the OH group pointing in the opposite direction.

Otherwise, this is pretty much spot on.

I'm not sure I understand what you mean. How could you tell the methoxy group is pointing down? From my understanding the sugar is being tilted in Haworth's Projection so that the wedges are pointing towards you.

 

[Isoprop]

I'm not sure I understand what you mean. How could you tell the methoxy group is pointing down? From my understanding the sugar is being tilted in Haworth's Projection so that the wedges are pointing towards you.

How do I know it's pointing down? Because it's not pointing up? Here, I'll circle it for you.

Compare that with alpha D-glucose.