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Biochemistry

Learn the toughest concepts covered in Biochemistry with step-by-step video tutorials and practice problems by world-class tutors

9. Carbohydrates

Reducing Sugars

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Reducing Sugars

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in this video, we're going to begin our discussion on reducing sugars now, before we can fully understand reducing sugars its first helpful for us to be able to understand a settles and key towels. And so, in our previous lesson videos. Although we didn't really talk about a settles and key towels very much, we did talk about Hemi, a settles and Hemi key towels. And so really, everything that you see in this green box right here and in this blue box right here should look familiar to you guys from our previous lesson videos. And so really, the only new thing that we're showing you guys in this video is this yellow box that we see over here and so to refresh our memories. Of course, we know that alcohol groups can react with Aldo hide groups, toe form Hemi, a settles and alcohol groups can also react with key tone groups. Toe form Hemi Key tells now recall that the Hemi A settles and Hemi key towels are relatively unstable structures, and so if they're unstable, then they're going to be relatively, uh, reactive. If you're relatively unstable, you'll be relatively reactive. And so, essentially what we're saying here is that these Hemi acid tells and these Hemi key tales that air relatively unstable and relatively reactive, they're gonna be there for susceptible to reacting with other alcohol such as, uh, these purple alcohol groups that we see down below. And so when the Hemi acid towels and heavy Kittles react with these other alcohols, they conform. Mawr stable, a Seattle and key tell structures or groups. And so what you can see is that the Hemi asset tell that we have here, which is again relatively unstable and relatively reactive, can react with another alcohol group. And when it reacts with that alcohol group, it can produce this structure here, which is the ass it'll group. And so notice that it dropped this prefix Hemi, which means half. And so now we have a full ass. It'll group. And so what you'll notice about Theus? It'll group is that it has the O. R. Here and then, oh, are here and before we only had one of these Oh, ours. So it was half. And so here we have a full, uh 20 R groups branching off. Now notice that this arrow going to the right is actually a dehydration synthesis reaction, since a water molecule H 20 is being lost. Now, if we take a look down on the bottom half notice that the Hemi key tell is also going to be relatively unstable and relatively reactive and so it can react with another alcohol group is well. And that is, of course, going to generate this structure over here, which is the key towel group, and again notice that it dropped the prefix Hemi because it's no longer half of a keto notice. Over here we have 20 R groups branching off, whereas over here we only have 10 our group branching off. And so ultimately, what we're seeing is that through this reaction that we see here were able to replace the alcohol groups with O. R. Groups that are branching off. And so another thing to note here is that the reaction arrow going from left to right for both of these is larger than the reaction arrow going backwards from right to left. And so what this means is that the a settle and the key towel groups are going to be more stable and predominate. over the heavy ass. It'll and Hemi Keitel groups, which are again relatively unstable. And so keeping this in mind is going to be very important and very helpful in understanding reducing sugars. Uh, the difference between a settlement kettle groups and Hemi, a settle and heavy Keitel groups. Now, one thing important to note here is that notice that the only difference between an alga hide and a key tone is this hydrogen right here in this, our group notice that the only difference between ah hem ius it'll in a Hemi Keitel is again the same exact hydrogen and the same exact our group. And notice that the only difference between an ass it'll group and a key tail group is again the same hydrogen and the same our group. And so over time, you'll be able to get, uh, much better at distinguishing between Alto Hide, Emmy A settles and a Seattle groups, uh, in distinguishing them from key tone Hemi, Key Towel and Keitel groups just by looking for either ah, hydrogen atom branching off of the an America carbon or an additional our group branching off of the an America carbon. And so now that We understand the differences between Hemi ass settles and heavy key towels and a settles and key tails we can better be. We will be better prepared to understand reducing sugars, which we will talk more about in our next video. So I'll see you guys there.
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Reducing Sugars

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So, in addition to being able to distinguish between a settle Keitel, Hemi, a Settle and Hemi Keitel groups, there's also one more note that's going to be helpful and distinguishing between reducing sugars and non reducing sugars. And so this note is that all doses and key toeses can actually reversible e rearrange into each other, and they do so via the in denial rearrangement mechanism, which is a mechanism that you guys may have covered before in your old organic chemistry courses. But really, the main take away here of this note is that all doses can reverse Aly rearrange into ketosis, and key toeses can reverse Aly rearrange into all doses. So now, keeping that in mind As we move forward, we can go ahead and define reducing sugars, so a reducing sugar is just any sugar that's capable of being oxidized. And so, in other words, we could say that a reducing sugar is just a sugar that's capable of acting as a reducing agent and, of course, reducing agents get oxidized. Now, reducing sugars must either have their own free alga hide group or they must be able to rearrange to generate their own free alga hide group, and so reducing sugars are going to include the following two thanks. It's going to include all simple linear sugars, all doses and ketosis, since they're capable of reversible e rearranging into each other. And it's also going to include cyclic sugars but Onley cyclic sugars that have a free Hemi ass it'll or Hemi Kettle Group. And this is because the Hemi Asset Tellinhim acquittal groups are in equilibrium with their linear forms. And if they are going to be in their linear forms, that means that they're capable of either having or generating their own free alcohol group. And so non reducing sugars, on the other hand, are going to be the complete opposite of a reducing sugar. And so that means that non reducing sugars are not capable of act of being oxidized, and they're not capable of acting as a reducing agent, which is why they're non reducing and so non reducing sugars are going to include sugars that have full ass. It'll and key tell groups. And so this is why it's so important to be able to distinguish between a settle and kettle groups as well as Hemi, a subtle and Hemi Keitel groups because it will affect our ability to identify reducing sugars and non reducing sugars. And so, if we take a look at our example down below, we can appropriately label each of the following carbohydrates as either reducing sugars or non reducing sugars. And so taking a look at this sugar over here, what we can see is that we have a linear mono sacharow. And, of course, if it is a linear mono sack, right or sugar, that means that it's going to be a reducing sugar. And so here we have a linear mono sacha ride that has its own Aldo Hide group, its own free Aldo High Group. And that is more evidence to suggest that this is indeed a reducing sugar so we can write reducing here now, taking a look at this next sugar here notice that we have a cyclic sugar and so note that reducing sugars can be cyclic sugars. But on Lee, if they have their own free Hemi assets or Hemi Kettle Group and so notice Looking at the animator Carbon over here it is present in a Hemi, a Seattle group, and so you can see that it has its hydroxyl group. It has its O R group right here, and it has another our group. And then it has the hydrogen atom that's not being shown. And so this is a Hemi ass it'll and so because this has its own Hemi ass, it'll group this is going to be a reducing sugar as well. So down below, weaken right, reducing. Now, if we take a look at this final sugar over here, notice that we actually have a die Sacha ride. We have two sugars that air linked together CO violently. And so what you'll notice is that the an America carbon of this first sugar right here is right here. Since this is the only carbon atom bonded to to oxygen abs and the numeric carbon of this sugar over here is actually right here. And so what you'll note is that these neither of these sugars, this one or this one over here have an animated carbon that's part of a free Hemi Asset out or Hemi Kitale group. Instead, when we look at thes and America carbons, we'll see that they are present in a Seattle groups and key tell groups and So notice that right here with this animated carbon we have 20 R groups branching off. We have this one, and then we have this one branching off. And already we are at a settle. And so we know that it's in a settle because we have a hydrogen coming off at this position, however, noticed over here with this and Americ Carbon We have a, uh, Keitel because we have an oh on O R group branching off here we have another O R. Group branching off here. And instead of having a hydrogen atom coming up, we have another are grouped another carbon atom. And so here we have a key tell. And over here we have an a settle. And so again, if key towels and, uh, a settles our present, then this is going to be a non reducing sugar and so down below weaken right, non reducing. And so, as we move forward in our course, we're gonna be able to get more and more practice identifying, reducing and non reducing sugars. And really, the best way to get good at this is to get practice. So I'll see you guys in our next video
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Problem

Which of the following disaccharides are reducing sugars?

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Problem

Which chemical group below indicates that the sugar is a reducing disaccharide? Is “C” an acetal or ketal?

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Problem

Which of the molecules below have a reducing end?

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