identifying anomeric carbon (and alpha/beta orientation) on ring structures

[thebillsfan]

One of the explanations to a question I missed is that alpha anomers are defined as those in which the hydroxyl group on the anomeric carbon is oriented axially.

What? I thought alpha anomers were when the hydroxyl is pointing down, whether thats axial or equatorial. this explanation is basically saying that you can have an AXIAL, downward-pointing hydroxyl and still be an alpha sugar. Is that right? If that's the case, then what's the whole deal with up=beta down=alpha?

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[thebillsfan]

And this explanation also says that maltose, a disaccharide, can mutarotate...how is that possible? maltose is locked in a ring conformation thanks to the alpha linkage.

 

[crazybob]

your book might have some typos. for simple sugars, alpha is a result of being "attacked" from above so the hydroxyl group is down. beta is a result of being "attacked" from below so the hydroxyl group is up. http://en.wikipedia.org/wiki/Anomer for sugars in general, alpha is for the same configurations at two carbons, and beta is for different configurations at two carbons.

mutarotate - http://en.wikipedia.org/wiki/Mutarotation changing from the alpha to beta. it's done by enzymes with a complex mechanism.

what did your book say about maltose? did it say mutarotating as in change one of the sugars from alpha to beta? or change the linkage from alpha to beta? it could separate by an enzyme and then rejoin with a different linkage.

 

[thebillsfan]

alright, so i was right...alpha is NOT due to axial. if you have an equatorial pointing slightly downwards, that'd still be alpha?

for maltose:

the question said (paraphrase): results of benedict's test before hydrolysis of maltose was positive. this proves that:

correct answer: maltose contains a free hemiacetal group.

explanation: maltose is a disacc but differs from sucrose in that it is a reducing sugar. maltose is made up of two gluc sub units but only one of the units is a glucoside. the other sub unit is present in the hemiacetal form hence it can mutarotate. mutarotation is characterized by (blah blah blah)...therefore, choice d is the corect answer as maltose contains one free hemiacetal group. the other choices are incorrect since benedicts test has nothing to do with the glycosidic bond.

WHAT? benedict's test has EVERYTHING to do with the glycosidic bond, right?

My other questions from this are:
why isn't sucrose also a reducing sugar?
isn't only ONE of the subunits in sucrose also a glucoside? that would make it a reducing sugar according to this explanation

 

[crazybob]

http://en.wikipedia.org/wiki/Reducing_sugar http://answers.yahoo.com/question/index?qid=20071027205153AADSf5X

http://en.wikipedia.org/wiki/Benedict's_reagent

for simple sugars, if the -OH group is down, then it's alpha. if the -OH group is up, it's beta. up and down is something you determine by looking at both axial and equatorial positions at the same carbon.

 

[dummymummy]

I believe that the anomeric carbon is the only carbon on the ring directly connected to two oxygen"s", and down is alpha and up is beta with regards to position.

 

[sleepy425]

My other questions from this are:
why isn't sucrose also a reducing sugar?
isn't only ONE of the subunits in sucrose also a glucoside? that would make it a reducing sugar according to this explanation

no, neither subunit of sucrose can mutarotate. it has to do with the way the glucose and the fructose are linked. Maltose is a 1,4 linkage of two glucose monomers. That means that the glucose monomer that has the "1" part of the linkage is a glucoside, while the glucose monomer with the "4" part of the linkage is free at its anomeric carbon (go look at the structure and identify which two carbons would be the anomeric carbons in the free monomers). Sucrose is a 1,2 linkage. Specifically, it is linked between the C-1 of the glucose monomer and the C-2 of the fructose monomer. In free glucose, C-1 is the anomeric carbon, and in free fructose, C-2 is the anomeric carbon. Thus, both of the monomers' anomeric carbons are tied up in the linkage, so neither can exist as the free aldehyde or ketone in solution.

 

[maroon95]

My other questions from this are:
why isn't sucrose also a reducing sugar?
isn't only ONE of the subunits in sucrose also a glucoside? that would make it a reducing sugar according to this explanation

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sucrose (disaccharide) is not a reducing sugar
the glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the nonreducing end of the other. The effect of this inhibits further bonding to other saccharide units. Since it contains no anomeric hydroxyl groups, it is classified as a nonreducing sugar


http://en.wikipedia.org/wiki/Sucrose

 

[thebillsfan]

sweet thanks. also, i get confused w alpha beta linkages. i know that going from left to right, lets say you have a disaccharide. if the linkage from the left sugar to the right sugar is pointing downwards, we called that a beta linkage. i've seen problems, however, that required you to identify the linkage from the RIGHT sugar to the LEFT sugar. is this also beta since it's still (obviously) pointing up? or would you rotate it in your head so that the right sugar is now on the left side and flipped over or somethign? ahh