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Common Sugars: Nomenclature & Structure – Simple Carbohydrates

by Kevin Ahern, PhD
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    00:00 sugar unit.

    00:01 Now there's a lot of nomenclature that's associated with the common sugars and I want to go through that nomenclature, because these are commonly used to describe the sugars and their properties.

    00:11 The first terms I want to introduce are those of aldose and ketose. To introduce that I present to you two molecules, the molecules of glucose and fructose, two common sugars that we find in nature. When we compare the structures of these two molecules we see that for the lower four carbons of each of these molecules, they have identical structures.

    00:31 They have the top one with the hydroxide on the left, the next two hydroxides on the right, and they terminate in a CH2OH. However the first two carbons of these molecules are different.

    00:43 We see for example that glucose has terminates in an aldehyde group at the very top, whereas fructose is not aldehyde but instead has a ketone group on its carbon number two.

    00:54 This means that glucose is what we describe as an aldose, meaning an aldehyde sugar, and fructose is a ketose, meaning a ketose sugar. I should also mention that whenever we name sugars, we put the letters OSE at the end of the name to indicate that it is some form of a carbohydrate.

    01:13 The various sugars that exist, exist in a variety of forms, shapes and sizes. In this case we compare the different sizes of the individual sugars. In this case I picked four different aldoses. The first of these glyceraldehydes has three carbons and we describe it as a triose. The second one has four carbons and we describe it as a tetrose. The third one, ribose, a common sugar within nucleotides has five carbons and is a pentose. And the last one, glucose, has six carbons and is a hexose. We can combine the names, we can describe glucose for example as an aldohexose.

    01:56 In addition to size, there are differences between the individual sugars with respect to stereochemistry. Now relating back to organic chemistry, we remember that any carbon that is attached to four different atoms can have those four atoms arranged in three-dimensional space in two different ways. So what I'm showing you on the screen are two different forms of the simple sugar known as glyceraldehyde. You can see that the central carbon in this case is asymmetric; it has four different molecules attached to it. The L-configuration is used to designate when the hydroxide is on the left portion of the molecule, as seen here, and the D-configuration is used to describe the molecule when it has the hydroxide on the right part. Now we will see for higher order sugars, that is larger sugars, they have multiple asymmetric centers. The D and L designation is only given to describe next to the bottom sugar when it's written in the linear form as we shall see. Now D and L glyceraldehyde have another property associated with them, and they are mirror images of each other, and this, we will see, gets a special name of its own.

    03:11 Now as I said some sugars have multiple asymmetric centers. So glucose for example has four different asymmetric centers as shown here. Each of those carbons can exist in different configurations, and whenever we have a different configuration, we no longer have glucose, so the naming is followed not by the stereochemistry, but by individual common names that are given to the sugars. With four asymmetric centers, an aldose can have 16 different names associated with them.

    03:45 This sugar is D-glucose because next to the last carbon at the very bottom, has a hydroxide on the right side as noted for glyceraldehyde, so it’s the next of the last carbon that designates the D versus the L configuration. If we were to take the mirror image of D-glucose, we would have what's known as L-glucose. Now the L designation does not come about just simply from flipping the hydroxide on the bottom carbon, instead the mirror image of D-glucose flips every single one. So when we look at D-glucose, we see starting at carbon number 2, the hydroxides on the right, then on the left, and then the right and then the right. But if we look at L-glucose, each of those is flipped, starting at carbon number 2, it's left, right, and then left, left. The term that we give to describe sugars that are mirror images of each other is the term enantiomer. It is important to note that every D sugar has, as its mirror image the L sugar of the same name. So D-glucose enantiomer is L-glucose, D-glyceraldehyde enantiomer was L-glyceraldehyde. Similarly for fructose, D-fructose enantiomer is L-fructose.

    05:01 Another important term to understand is the term diastereomers. Diastereomers, as we'll see in the structure here, relate to sugars that have different configurations and the same basic chemistry but are not mirror images of each other. What you see here is D-glucose and a related sugar called D-gulose. D-glucose and D-gulose have similar structures.

    05:24 For example they are both aldoses as you can see by their aldehyde groups at the top. You can also see that their hydroxides at carbon number two, both are on the right side. At the bottom, there's hydroxide on the right side followed by a CH2OH. However, when we compare the structures of the carbons at positions three and four, we see that they're not the same. And as a result of this, D-glucose and D-gulose are diastereomers. Diastereomers, the term, relates to sugars that have different configurations and the same chemistry, meaning aldoses in this case, but are not mirror images of each other. And you can see the difference highlighted right there in their different configurations.

    06:08 Now a diastereomer, a special type of diastereomer is a molecule that's called an epimer. So an epimer is a diastereomer that has only one single carbon whose configuration is different. So if we compare for example D glucose and D-galactose, we see that they have identical structures with respect to their hydroxide groups, except for the carbon that's shown in orange.


    About the Lecture

    The lecture Common Sugars: Nomenclature & Structure – Simple Carbohydrates by Kevin Ahern, PhD is from the course Biochemistry: Basics.


    Included Quiz Questions

    1. Ribose is an aldo-hexose
    2. D-glucose is the mirror image of L-glucose
    3. Glucose is an aldo-hexose
    4. Fructose is a keto-hexose
    1. Two sugars that are mirror images of each other
    2. Two sugars that differ in configuration of one carbon
    3. Two sugars that differ in configuration of their anomeric carbons
    4. An aldose-ketose pair, such as fructose and glucose
    1. Epimers
    2. Aldose-ketose pairs
    3. Mirror images
    4. Enantiomers

    Author of lecture Common Sugars: Nomenclature & Structure – Simple Carbohydrates

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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