Linkage, D/L carbohydrate questions (quick)

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SuperOrchestra

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Hey, I just got worked over on a TBR passage dedicated to D/L and Carbohydrate linkage. So i've scoured the boards and my books, and still not sure If I understand everything....


1.) D is when the highest number stereocenter carbon has the hydroxyl pointing right, and L is the opposite... but I want to double check one thing.

When the sugar is in ring form can I just simply see if the absolute configuration of the corresponding carbon in the ring structure is R or S to determine if its D(R) or L(S)

2) linkage. I was a little thrown off but.
For linkage do we just consider the sugar of whose oxygen is bound to the anomeric carbon that is doing the linking? So if that hydroxyl group is lets say Alpha, then the link will then be Alpha 1,4 or 1,2 etc....

I spent entirely way too long looking at this today, and just want to know if I can put it to rest.
 
Hey, I just got worked over on a TBR passage dedicated to D/L and Carbohydrate linkage. So i've scoured the boards and my books, and still not sure If I understand everything....


1.) D is when the highest number stereocenter carbon has the hydroxyl pointing right, and L is the opposite... but I want to double check one thing.

When the sugar is in ring form can I just simply see if the absolute configuration of the corresponding carbon in the ring structure is R or S to determine if its D(R) or L(S)
If the CH2OH group is sticking up in ring form, it's probably safe to say you have a D sugar. Yes?

2) linkage. I was a little thrown off but.
For linkage do we just consider the sugar of whose oxygen is bound to the anomeric carbon that is doing the linking? So if that hydroxyl group is lets say Alpha, then the link will then be Alpha 1,4 or 1,2 etc....

I spent entirely way too long looking at this today, and just want to know if I can put it to rest.
You can name sugars linked by a glycosidic bond from either end. If you started with the sugar with the acetal, you would name that 1, and you number the other carbon based on its position relative to its own anomeric carbon. But you can do it either way.
 
You are right. What defines a glycosidic linkage as alpha or beta is the original position of the OH in the anomeric carbon; the other OH (of the other monosaccharide) can be above or below the ring and it does not change the fact that the linkage is alpha or beta, according to the anomeric carbon that is part of the linkage.

An interesting glucosidic linkage is the one of sucrose: since the anomeric carbon of an alpha D glucose (the C1 ) is linked to the anomeric carbon of a Beta D fructose (C2), this linkage is called an alpha1-Beta2 O glycosidic linkage. In that case it is necessary to mention both alpha and beta, since both are anomeric carbons.

I hope it helps.

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