PSC 3110 Monosaccharides

Pharmaceutical Sciences 3110

Monosaccharides

Notice!!This tutorial is not a substitute for all the lecture notes. I have tried to provide explanations, information, and practice questions to "assist" in the understanding of the lecture and reading material.

I. The Basic Structure of Monosaccharides

The basic feature that makes an aldose different from a ketose is the position of the carbonyl group of the monosaccharide. As seen below, an aldose has the carbonyl group at Carbon #1 (affording an aldehyde) while a ketose has the carbonyl group at Carbon #2 (affording a ketone).

Hmmm. simple enough but, what does the (HCOH)n in the middle of the structure mean and where are you coming up with the numbering system?

Well, the numbering system is simple. For an aldose the carbonyl carbon is always carbon-1. When the aldose is drawn in a fisher projection (as above) you would then continue to number the carbons as you go down the chain. For a ketose the numbering rules are similar. You draw the fisher projection of the ketose, placing the ketone towards the top (fewer carbons above the carbonyl than below it). The top carbon is carbon-1 and you number as you go down the structure. Therefore, the ketose above has the ketone at carbon-2, as will any ketoses that you need to worry about for this class.

The (HCOH)n simply represents more hydroxyl substituted carbons in the monosaccharide structure. If you look below, you will see how "n" correlates to the overall size of the monosaccharide. You will also see that the total number of carbons can be used to classify the aldoses and ketoses by using Aldo- or Keto- followed by numerical prefixes attached to an -ose. (Yes, it's easier to see than say)

Hey, why are some of the groups colored blue? As you may recall, monosaccharides can have either the "D" or "L" configuration. The above sugars are all "D". Therefore, the hydroxyl group on the asymmetric carbon furthest from the carbonyl group is to the 'right' in the fisher projection. An "L" sugar would have the opposite stereochemistry at that carbon (the blue OH group on the 'left' in the fisher projection.

O.K........But, in class you talked about 'enantiomers' and 'epimers', What's the difference?

Well, look below and see for yourself.

That's all fine and dandy but it seems to me that sugars often form rings, how does that all come about?

Well, now that we have some of the basics down regarding the monosaccharides and their Fisher projections lets see how their ring forms come about.

Below you will see how an alcohol and an aldehyde combine to form a hemiacetal. In a similar fashion, this reaction occurs with the sugars to give the ring forms.

So, from a fisher projection of a monosaccharide we can draw two possible cyclic forms (Haworth Projections), the alpha and the beta. These two differ only by the stereochemistry of the hydroxyl group at the anomeric carbon.

A similar strategy is also used for ketoses and the formation of cyclic ketoses. As you will see below, the alcohol and the ketone combine to form a hemiketal. In the case of the ketohexose, we form a hemiketal at the anomeric carbon affording a five member called a ketofuranose.

Hey, that's not so bad. Lets see some more!

Wow! I'm glad you are so interested.

Well, not every monosaccharide is simply a bunch of hydroxyl groups attached to carbon atoms. In fact, the monosaccharides can be modified in many ways to give of a variety of structural derivatives. Shown below is a figure that displays a number of the more common modifications made to the monosaccharides.

Gee Dr. Kerns, that's alot of information. What would you say are the specific points that one should take from such a figure?

One important point is that you should simply recognize that the monosaccharides (and therefore larger oligosaccharides and polysaccharides) can have a variety different substituents in place of hydroxyl groups.

These added functional groups may also serve specific functions such as: alter the sugars physical characteristics, play a role in how the sugar interacts with other biomolecules, or provide a reactive functional group on the sugar for further modification in the body.

From this figure you should be able to recognize the different modified monosaccharides. For example, you now know the difference between an alditol and an aldonic acid. Using your previous knowledge of Haworth projections and epimers, you could now draw the Haworth projection of the C-3 epimer of 2-deoxy-D-galactose if given the fisher projection of D-galactose.

You should also notice that, in most cases, you could still convert the above fisher projections into the cyclic Haworth projections (as shown for D-Glucuronic acid). However, some of the derivatives such as the alditol and the aldonic acid can not cyclize to form a hemiacetal since they no longer have a C-1 aldehyde.

One final important feature that we discussed in class was the oxidation of D-glucose to D-gluconic acid. This oxidation reaction is used to monitor blood glucose levels and is displayed below. Glucose oxidase converts oxygen and glucose to hydrogen peroxide and gluconic acid. The hydrogen peroxide then reacts with a dye in the test to form a colored dye that can be quantitated which tells you how much glucose is in the blood.

You may remember from class that we discussed a chemical test for reducing sugars. This test also relies on the formation of aldonic acids from an aldose as shown below.