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Cyclization & Cyclic Forms – Simple Carbohydrates

by Kevin Ahern, PhD
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    00:02 You may remember from organic chemistry that single bonds have ability to twist and turn some geometries associated with them. When that happens in a molecule like a sugar, atoms that are within four or five distances of each other can in fact be brought into close proximity in three-dimensional space by appropriate twists and turns. In glucose this means that the hydroxides on carbons four and five, can actually be brought together into close proximity.

    00:28 And when that happens, a reaction occurs between the hydroxide in the glucose as you can see here. That reaction causes a ring structure to be formed and in the process of doing that, a new asymmetric carbon is created in the process. Whereas glucose before had four asymmetric carbons, glucose now in the ring structure, has five asymmetric carbons. The new asymmetric carbon has a name and we call it the anomeric carbon.

    00:55 This structure depicts the ring structure of glucose as a result of the cyclization that I just described. We see this looking at the far right of each molecule, we see the hydroxide that's linked to the asymmetric anomeric carbon. The hydroxide can exist in two different positions because again, the carbon is asymmetric at this point.

    01:16 When the hydroxide is located in the upper position in the ring structure as shown here, it's in what we call the beta position or forming what we describe as beta-D-glucose on the left molecule. When the hydroxide is located in the down position on the asymmetric anomeric carbon, then that is known as the alpha position and we've created a ring structure called alpha-D-glucose. These two molecules, with a different anomeric structures differing only in the anomeric carbon, have a name of their own and these are called anomers.

    01:49 Now there are a variety of ways of drawing the ring structure that I've drawn for glucose.

    01:53 You've already seen the structure on the left which people refer to as the Fischer structure.

    01:58 The structure that I've just shown you is the one that's second and it's known as the Haworth projection. The other two structures don't really have names, but they are other ways of depicting the same molecule, so you may see when looking at sugars different ways of seeing the same information. Now another interesting thing that happens with sugars, is that the bonds between the carbons which I've already described as being able to be twisted to be brought into close proximity, can also be twisted in such a way that the carbons position relative to each other actually changes. Now this is depicted in the slide on the screen here. In both cases what we're looking at is a molecule of beta-D-glucose, like we just made described earlier. You'll notice that the confirmations of these two sugars, however is different. The form on the left we refer to as the chair because I like to think of it is looking sort of like a chaise lounge, where the farthest portion on the left is the back of the chaise lounge and the farthest portion of the right are the portion where the legs would sit. By contrast the boat form of beta-D-glucose, shown in the figure on the right, resembles that of a canoe, where each of the ends of the canoe tip upwards like this. Now as you might imagine, the boat form brings together hydroxides into closer proximity with each other as you can see in the depiction on the screen. And again we've seen over and over that spatial hindrance or steric hindrance can arise as a result of close interactions. So one might predict therefore that the boat form would be less stable and therefore less likely to form than the chair form of beta-D-glucose and that is exactly correct.

    03:43 Now another thing that can happen to rings is that rings can form in different ways.

    03:48 You may remember I said that glucose had carbon number one that can be brought into close proximity of the hydroxide on carbon number four, or that carbon number five. When it comes into proximity of carbon number five, you get the molecule that I described earlier and that's shown on the right of this figure. This form of a sugar in general gets the name pyranose, relating to a molecule called pyran that has six membered ring with oxygen in it.

    04:14 On the other hand, if the glucose combines with the hydroxide on carbon number four, we can create a five membered ring as seen here, and that five membered ring gets a general name of furanose. Now many sugars go in pyranose form, many sugars go in furanose form, some go in both forms as we can see.

    04:38 Here is an example of some common furanoses. So for example fructose is very commonly found in the furanose form. Fructose is important to remember has as its anomeric carbon, carbon number two, not carbon number one. In each case, the anomeric carbon of a sugar is the part that had the carbonyl group, that is the carbon double bonded to oxygen, in glucose's case that was carbon number one. In fructose's case, because it's a ketose, that's carbon number two. So carbon number two is the anomeric carbon of fructose. In fructose's case, carbon number two is brought into close proximity of the hydroxide on carbon number five.

    05:18 That creates a ring structure that has five members. The five members of the ring include carbons number two, three, four and five moving clockwise away from the oxygen. We see carbon number one in the down position and we see number six in the up position. Now this fructofuranose is given the beta designation, because the beta relates to the position of the hydroxide on the anomeric carbon. In this case the hydroxide is up, so we call this beta-D-fructofuranose.

    05:51 If we had the alpha form, the hydroxide would be on the bottom and the CH2OH would be on the top. Another common furanose that we see is that of ribose. In this case, ribose which only has five carbons, forms a five member ring, because carbon number one combines with the hydroxide on carbon number five to make beta-D-ribofuranose. Ribose is a very important sugar with respect to making nucleotides and when we examine the structure of nucleotides, we see that it is made with riboses that are always in the beta configuration, because the hydroxide in the beta configuration is replaced by a base to make the nucleotide.


    About the Lecture

    The lecture Cyclization & Cyclic Forms – Simple Carbohydrates by Kevin Ahern, PhD is from the course Biochemistry: Basics.


    Included Quiz Questions

    1. Boat and chair forms
    2. Epimers
    3. Anomers
    4. Diastereomers
    1. Exists in a five membered ring
    2. Has five carbons
    3. Exists in a six-member ring
    4. Cannot exist in a ring at all
    1. β-D-ribofuranose
    2. α-D-ribofuranose
    3. β-L-ribofuranose
    4. α-L-ribofuranose
    5. α-D-fructofuranose

    Author of lecture Cyclization & Cyclic Forms – Simple Carbohydrates

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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