Alpha/Beta for sucrose

[ilovemedi]

I'm confused at the labeling of alpha/beta linkage on SUCROSE. I've searched the threads, and still am realllly confused. According to TBR, alpha = when anomeric oxygen and last carbon are opposite (one up, one down); and beta = same side.

For the sucrose above, the glucose on the left is alpha because the anomeric carbon's O is opposite side of the CH2OH. The issue is fructose -- where is the anomeric carbon? Can someone label or let me know what i should be looking for to determine alpha or beta, or where the heck the anomeric oxygen if so fructose. there appears to be an anomeric H, but the O is gone..

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[pm1]

I'm confused at the labeling of alpha/beta linkage on SUCROSE. I've searched the threads, and still am realllly confused. According to TBR, alpha = when anomeric oxygen and last carbon are opposite (one up, one down); and beta = same side.

For the sucrose above, the glucose on the left is alpha because the anomeric carbon's O is opposite side of the CH2OH. The issue is fructose -- where is the anomeric carbon? Can someone label or let me know what i should be looking for to determine alpha or beta, or where the heck the anomeric oxygen if so fructose. there appears to be an anomeric H, but the O is gone..

wouldn't the anomeric carbon be the one on the left of fructose? doing the linkage? It's just a guess.. Just because that is the only carbon that is linked to two oxygens. I also learned that alpha points down and beta up, but I guess because the fructose is inverted from the usual it is beta even though it is pointing down?

I am also curious about this, hopefully someone will shine some light on this

 

[N1st]

I'm confused at the labeling of alpha/beta linkage on SUCROSE. I've searched the threads, and still am realllly confused. According to TBR, alpha = when anomeric oxygen and last carbon are opposite (one up, one down); and beta = same side.

For the sucrose above, the glucose on the left is alpha because the anomeric carbon's O is opposite side of the CH2OH. The issue is fructose -- where is the anomeric carbon? Can someone label or let me know what i should be looking for to determine alpha or beta, or where the heck the anomeric oxygen if so fructose. there appears to be an anomeric H, but the O is gone..

Alpha means the anomeric carbon and the configurational carbon (the one which makes the compound D or L) have different configurations (that is, S,R or R, S).

Beta means they have the same configurations (S,S or R, R)

It has nothing to do with up or down, it is the absolute configurations.

For glucose, when C1 (the anomeric carbon) has the OH pointing down, that happens to be alpha. Fructose's anomeric carbon is C2. C2 is the one participating in the glycosidic bond.

Here is a picture where I circled the two anomeric carbons in green, and the two configurational carbons in red:

 

[ilovemedi]

WOAH. For fructose, the anomeric carbon also pariticipates in the linkage? So it's kinda like glucose but flipped to the other side?

Also when you say "For glucose, when C1 (the anomeric carbon) has the OH pointing down, that happens to be alpha" - there is no OH with C1, there is only an "--O--"

 

[N1st]

WOAH. For fructose, the anomeric carbon also pariticipates in the linkage? So it's kinda like glucose but flipped to the other side?

Also when you say "For glucose, when C1 (the anomeric carbon) has the OH pointing down, that happens to be alpha" - there is no OH with C1, there is only an "--O--"

Yeah by OH I mean what would have been an OH if the glucose were on its own. You are correct, that is where the - O - currently is.

EDIT: and yes, fructose has an anomeric carbon too. That's why they call this an alpha, beta linkage, because glucose's anomeric carbon is alpha, and fructose's is beta.

 

[Cool Stud]

I'm still too confused!! can u show how the structure of fructose is inverted in sucrose!!

 

[phattestlewt]

Alpha means the anomeric carbon and the configurational carbon (the one which makes the compound D or L) have different configurations (that is, S,R or R, S).

Beta means they have the same configurations (S,S or R, R)

It has nothing to do with up or down, it is the absolute configurations.

For glucose, when C1 (the anomeric carbon) has the OH pointing down, that happens to be alpha. Fructose's anomeric carbon is C2. C2 is the one participating in the glycosidic bond.

Here is a picture where I circled the two anomeric carbons in green, and the two configurational carbons in red:

Quick Correction,

TBR is right in saying that its the LAST carbon, not the penultimate (second to last carbon) which means that those red circles should be on the carbons above (for the glucose) and below (for the fructose).

You want to compare the anomeric OH with the last CH2OH to determine alpha beta configs

 

[Fonsi]

Does somebody knows if both structures (alpha and Beta fructose) are present in the nature? or there is a specific structure, so to say in the fruits or for the disaccharide sucrose