How would you classify the link between the monosaccharides in cellobiose?

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Accepted Answer

Hi, everybody. Let's take a look at the next problem, consider the structure of lactose. And then that is drawn for us, what type of bond connects the monosaccharide units in lactose? So we see we have our two monosaccharide units and we remember what lactose is that will be one unit of glucose plus one unit of galactose. So on the left here, we have our glucose ring, six membered ring, oxygen positioned in the top, right as is conventional. And what we need to do, we look at this uh glu that we look at this glucose ring and we see that it's connected, there's a bond from one of its carbons to an oxygen and a bond from that oxygen to another carbon. And we know that there was originally a hydroxyl group on the carbon and the glucose. And this is a hemi acetyl because this carbon, we're gonna go ahead and call it carbon one because we know it's the anomic carbon. So it had the oxygen in the ring and then it originally had a hydroxyl group there. And the kind of bond you have when you have a hydroxyl group, that's part of a hemi acetyl or a hemi kyl group like this carbon one and then a hydroxyl group on another molecule. So in this case, the hydroxyl group on our galactose molecule is a glycolytic bond. So glucose and galactose joined by a glycolytic bond. So now we just need to identify which carbons on each molecule are joined by that glyco cytic bond. And is this an alpha or beta linkage? And when it comes to naming the bonds with alpha and beta, you look at the hydroxyl group where it was on the first molecule. So if that hydroxyl group was below the ring and the original sugar say below ring and first sugar, then it's alpha. And if it's above the ring and the first sugar, then it's the beta form, the bond was a beta gly acidic bond. So in this case, we can see that that original hydroxyl group on the glucose when label glucose on the left galactus, on the right. So that original hydroxyl group went up from the plane of the ring. You can see by the way, the bond is drawn in this sort of zigzag from going up and then down to the oxygen and then down and up to the next molecule. So this is going to be a beta gly acidic bond I like like aci bond there. And the last thing we need to do is make sure we know which carbons are connected here. So the uh glucose is going to be easy. We have already labeled this as carbon one because that's our anomic carbon. So in a six member ring like this, then the pinos form carbon one is going to be the carbon with those two oxygens attached to it. Now, it can be a little trickier on the other sugar because that's usually sort of turned around. So what do we mean by that? Well, let's look for the anomic carbon in our galactose carbon with two oxygens. Is that carbon on the right side? So that's going to be carbon one. And then we go around away from the oxygen that's within the ring. So it's number are carbons 23. And we see that carbon four is the one that is involved in the glyco bond. So the type of bond we see in lactose is known as a beta beta and then we have open parentheses one and then an arrow pointing to four. So the bond is from 1 to 4 and then closed parentheses, glycolytic bond. So not too bad to figure that one out, you just have to be kind of careful numbering the carbons in your molecule that's on the right. And it may be backwards from the way you usually see it or just the numbering is different. So our answer here, the type of bond that connects the monosaccharide units in lactose is beta 14 gly acidic bond. See you in the next video.

How would you classify the link between the monosaccharides in cellobiose?

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Key Concepts

Glycosidic Bond

Disaccharides

Cellulose