Reducing sugars?

[freedyx3]

I would really appreciate if someone can tell me how do you distinguish between a reducing and nonreducing sugar and how do you ID each one.


thanks

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[PDPjester]

I'll tell you since no one is responding. I wasn't going to respond, but ... since you asked.

Ahhh let's see... the difference between reducing and non-reducing sugars. This subject actually came up during my graduate level course in biochemistry. The name "reducing sugars" was or were first given to those sugars that were capable of reducing ferric or cupric ions. that was the boring part, sorry. The reason why these sugars have this ability has to do with the infamous anomeric carbon (the hemiacetal carbon) on the sugars.

The anomeric carbon refers to the carbonyl carbon at the end of the sugar chain (please look at an aldose). When we draw a sugar structure, sometimes we draw it in straight chain or somthimes in a ring form. The true identity of sugar structure is (this is what my professor said, I've never actually seen a sugar molecule, I don't think he did either but he has a PhD in biochemistry and since I don't have one of those: I would say he's right on this) the true sturucture is somewhere in between, or it goes back and forth in equilibrium (but holds little bit more towards the cyclic form).

So when our sugar becomes a ring, the carbonyl C gets protonated, and the it becomes a chiral carbon, a hemiacetal in function group wise. (Depending on which side it gets protonated, we can get two orientations of a hemi-acetal sugar when in ring form. This is called anomers, and that's probably why we call this Carbon an anomeric carbon.) However, there are also full acetal sugars too, compared to hemi-acetals. I'm not exactly sure how one can form acetal sugars or if they only exist as is in nature. Anyways, let's go back to your question...

When monosacchride sugars are added to certain reagents (or vice versa), the carbonyl carbon oxidizes easily, as we learned in Orgo. There's somthing called Tollens Reagent, and it detects aldehydes in sugars. And the definition is: "any sugars that react with Tollens reagents to give silver mirror are called reducing sugars." Ok, but you have remember that Tollens reagent also react with ketose as well, because the basicity of the reagents open up the chain and promotes enediol rearrangements, and it isomerizes to an aldose.

Now, let's talk about a NON-reducing monosaccharides. Tollen reagents must react while the sugars in straight chain form, when there are free aldehydes or ketones. And as I mentioned, hemiacetal carbons go back and forth with straight chain and cyclic formation, thus sugars with hemiacetals will always be a reducing sugars. So the cyclic monosaccharides that can not be turned back to straight chains are the NON-reducing sugars, and these are the ones with FULL acetal, because they are stable under neutral and basic conditions. These sugars with acetal are called glycosides. That's about it with monosaccharides, however you can also have dissacchirides as reducing and nonreducing sugars.

Dissacchrides: When two reducing monosaccharide sugars form an O-glycosidic bond (a bond that forms a dissaccharide using a hydroxly group of one mono- and an anomeric carbon on the other), the anomeric carbon participating cannot be further oxidized. Now from this point we can have two cases:

1) if there is another anomeric carbon (which is not involved in a bond) then this sugar can be reduced further. This is what we call a Reducing Dissacchride. (Look at Maltose) Maltose is said to be a reducing sugar because it can be further oxidized.

2) if there is no more free anomeric carbon, because both are involved in the glycosidic bond, then this disaccharide is a NON-Reducing sugar. (Look at Sucrose). table sugar is a non-reducing sugar.

so remember, the reducing monosacchrides can be detected using Tollens reagent, and they are hemiacetal in form (both aldoes and ketoses), and they mutarotate. But the Nonreducing sugars are acetals in form and they do not mutarotate, and are called glycosides. And you can also have reducing and nonreducing sugar due to the arrangement of glycosidic bonds.

hope that helps...

 

[TXftw]

I'll tell you since no one is responding. I wasn't going to respond, but ... since you asked.

Ahhh let's see... the difference between reducing and non-reducing sugars. This subject actually came up during my graduate level course in biochemistry. The name "reducing sugars" was or were first given to those sugars that were capable of reducing ferric or cupric ions. that was the boring part, sorry. The reason why these sugars have this ability has to do with the infamous anomeric carbon (the hemiacetal carbon) on the sugars.

The anomeric carbon refers to the carbonyl carbon at the end of the sugar chain (please look at an aldose). When we draw a sugar structure, sometimes we draw it in straight chain or somthimes in a ring form. The true identity of sugar structure is (this is what my professor said, I've never actually seen a sugar molecule, I don't think he did either but he has a PhD in biochemistry and since I don't have one of those: I would say he's right on this) the true sturucture is somewhere in between, or it goes back and forth in equilibrium (but holds little bit more towards the cyclic form).

So when our sugar becomes a ring, the carbonyl C gets protonated, and the it becomes a chiral carbon, a hemiacetal in function group wise. (Depending on which side it gets protonated, we can get two orientations of a hemi-acetal sugar when in ring form. This is called anomers, and that's probably why we call this Carbon an anomeric carbon.) However, there are also full acetal sugars too, compared to hemi-acetals. I'm not exactly sure how one can form acetal sugars or if they only exist as is in nature. Anyways, let's go back to your question...

When monosacchride sugars are added to certain reagents (or vice versa), the carbonyl carbon oxidizes easily, as we learned in Orgo. There's somthing called Tollens Reagent, and it detects aldehydes in sugars. And the definition is: "any sugars that react with Tollens reagents to give silver mirror are called reducing sugars." Ok, but you have remember that Tollens reagent also react with ketose as well, because the basicity of the reagents open up the chain and promotes enediol rearrangements, and it isomerizes to an aldose.

Now, let's talk about a NON-reducing monosaccharides. Tollen reagents must react while the sugars in straight chain form, when there are free aldehydes or ketones. And as I mentioned, hemiacetal carbons go back and forth with straight chain and cyclic formation, thus sugars with hemiacetals will always be a reducing sugars. So the cyclic monosaccharides that can not be turned back to straight chains are the NON-reducing sugars, and these are the ones with FULL acetal, because they are stable under neutral and basic conditions. These sugars with acetal are called glycosides. That's about it with monosaccharides, however you can also have dissacchirides as reducing and nonreducing sugars.

Dissacchrides: When two reducing monosaccharide sugars form an O-glycosidic bond (a bond that forms a dissaccharide using a hydroxly group of one mono- and an anomeric carbon on the other), the anomeric carbon participating cannot be further oxidized. Now from this point we can have two cases:

1) if there is another anomeric carbon (which is not involved in a bond) then this sugar can be reduced further. This is what we call a Reducing Dissacchride. (Look at Maltose) Maltose is said to be a reducing sugar because it can be further oxidized.

2) if there is no more free anomeric carbon, because both are involved in the glycosidic bond, then this disaccharide is a NON-Reducing sugar. (Look at Sucrose). table sugar is a non-reducing sugar.

so remember, the reducing monosacchrides can be detected using Tollens reagent, and they are hemiacetal in form (both aldoes and ketoses), and they mutarotate. But the Nonreducing sugars are acetals in form and they do not mutarotate, and are called glycosides. And you can also have reducing and nonreducing sugar due to the arrangement of glycosidic bonds.

hope that helps...

Your answer kills it! Thanks for the explanation, even though I may be 13 years too late

 

[AwfulEverything]

wait... do we see this crap again in dental school?

 

[PocketRocket]

I got really excited when I thought this was a diet thread lmao

 

[TXftw]

wait... do we see this crap again in dental school?

I was seeking this information for my undergrad Biochem class. Not sure if we'll see this in dental school, hopefully not.

 

[akog]

wait... do we see this crap again in dental school?

That was my thought exactly

 

[Daneosaurus]

You do see this in dental school. You do NOT need to know it at the level of the above explanation.


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