Instant Organic Chemistry

The other day I made a post that asked “Is your sugar right or left handed?” It was really just an excerpt from a Wired News article, but someone who preferred to remain anonymous commented as follows:

This article appears to be wrong on a number of accounts.

Unless memory fails me, when I learned this in organic, and biochemistry, the dextro and levo forms of the 6 carbon hydrocarbon chain known as glucose, were called isomers (from 2 Greek words) meaning equal parts. Maybe there’s a new terminology, so I’ll give him that point.

The D- form is known as dextrose, the L- form is known as fructose.

Fructose, a.k.a. fruit sugar, is widely recognized as being both sweeter than its counterpart, but also more readily absorbed by the digestive tract, and metabolized by the body.

Well, one of the things that I have learned in my life is that you forget 90% of what you learn. Your brain, being the creative pattern matching machine that it is, tries to fill in the blanks with stuff that seems to make sense. That is the reason why eye-witness testimony is so notoriously bad. I think that is probably what happened here with our anonymous friend. I, on the other hand, never learned organic chemistry, nor biochemistry for that matter. In fact, I think that in my entire educational career I had only one chemistry class at all. It was a lecture that I shared with several hundred others. I think I slept through most of it. I sure as heck could not hear what the guy was saying or see what he was writing on the board, but that was the point. It was a weeding class… but I digress. The point is this: Having never learned organic chemistry, I cannot have forgotten any of it!

Everything I know about organic chemistry, I learned in the last 24 hours, and now I will impart it unto you.

Thanks to Dr. Atkins, we all know what carbohydrates are. They are horrible awful things that want you to be fat and must be avoided at all cost, but seriously…

There could not be anything more glorious, more natural, more pure than the carbohydrate. Take a look at the word: carbo-hydrate. Hydrated carbon. What could be better.

Sugar is a carbohydrate with equal parts of carbon and water, and since water has two atoms of hydrogen for each atom of oxygen, there is a carbon atom for every oxygen atom, and two hydrogen atoms for each. Beauty in its simplest form.

Some people like to use a shorthand notation to represent the chemical structure of a molecule where they simply count the number of each type of atom. For instance, we said that sugar was equal parts of carbon and water. You probably know that water is written as H₂O, which would make sugar C_n + (H₂O)_n, or C_nH_2nO_n; where n is a whole number. For instance, if n is four, then the formula is C₄H₈O₄, and the sugar is classified as a tetrose: tetra- meaning four, and -ose meaning carbohydrate. A four carbon carbohydrate. Likewise, if n is five, six, or seven, the formula are C₅H₁₀O₅, C₆H₁₂O₆, and C₇H₁₄O₇, and the sugars are classified as pentose, hexose, and heptose. Five, six, and seven carbon carbohydrates, respectively.

These are all saccharides. In fact they are the monosaccharides. If these are combined into molecules composed of two monosaccharides, they are disaccharides. Composed of a several monosaccharides, they are oligosaccharides. Composed of many monosaccharides, they are polysaccharides. We will not discuss these here, but they too have many wondrous properties. They make such things as homemade strawberry jam and crab shells possible.

As our anonymous friend points out, glucose is a six carbon carbohydrate — actually, our commenter said it was a hydrocarbon, which it is not, since they contain only hydrogen and carbon, whereas glucose also contains oxygen, making it a carbohydrate, but let us ignore that for the moment and focus on the hexose sugar category of which glucose and fructose are members.

The shorthand notation we have just introduced can take us only so far. For instance, it is fine for distinguishing between tetrose, pentose, hexose, and heptose, but is of no use in identifying the differences between allose, altrose, fructose, galactose, glucose, gulose, idose, mannose, sorbose, talose, or tagatose, all of which have the basic formula C₆H₁₂O₆. These are all composed of the same elements, in the same proportions, but the individual atoms are arranged differently. This is the textbook definition of an isomer. These are all isomers of each other.

Our anonymous commenter also pointed out that “the dextro and levo forms of the 6 carbon hydrocarbon chain known as glucose, were called isomers.” This is a true statement. Since all of the forms of glucose — dextro and levo being two of these — are made from the same elements in the in the same proportions, they are all isomers. However, the introduction of the dextro and levo is a red herring.

To help distinguish these isomers, let us introduce another shorthand representation:

 H-C=O
   |
 H-C-OH
   |
HO-C-H
   |
 H-C-OH
   |
 H-C-OH
   |
   CH2OH

This represents the linear form of glucose. So that we can discuss this figure, let us introduce a few new terms. The C=O at the top is called a carbonyl group. The equals sign (=) between the letters represents a double bond between the atoms. The various -OH are called hydroxyl groups. The dash (-) between the letters represents a single bond between the atoms. If you are unfamiliar with this terminology, suffice it to say that in the great erector set of life, there are only so many places you can stick an oxygen or a hydrogen cog onto a carbon cog.

   CH2OH
   |
   C=O
   |
HO-C-H
   |
 H-C-OH
   |
 H-C-OH
   |
   CH2OH

This represents the linear form of Fructose.

If you take a careful look at these two representations you can see that they are clearly different, and yet, if you count the number of each type of atom, you will find that there are six carbon atoms, twelve hydrogen atoms, and six oxygen atoms (C₆H₁₂O₆).

Take a look a the location of the carbonyl group — the C=O — on each of these sugars. In glucose, the carbonyl group is at one end of the molecule, while in fructose the carbonyl group is in the interior of the molecule. As you can see, the carbonyl group on the end of the glucose molecule has picked up a hydrogen atom on its remaining bond, forming an aldehyde group (HCO), which makes glucose an aldose. Since we have already seen that it is a hexose, it is specifically an aldohexose. Since there are so many hydroxyl groups with the aldehyde in this molecule, it is sometimes also called a polyhydric aldehyde.

On the other hand we have the interior carbonyl group of fructose. Wedged between the two carbon atoms like that, the fructose carbonyl group forms a ketone. As such, fructose is a ketose. Again, since fructose is a hexose and a ketose, it is a ketohexose.

As we found before, a shorthand notation can be useful, and yet have its failings. While the notation we just introduced has advantages over our original, it still does not capture everything we need to know about the molecule. For instance, the linear form of glucose is really not as “linear” as the figure above would make us think. In reality, it looks more like this:

In this figure, the black circles represent carbon atoms, the red circles represent oxygen atoms, and the gray ones represent hydrogen atoms. The lines represent the bonds between the atoms. If you carefully compare this figure to the previous notation for glucose, you should be able to convince yourself that they match.

All of the talk so far has been about the linear form of these molecules. They were easy to draw and talk about. Now that we have moved on to figures, we can move on to the more interesting forms, including the rings.

The most common form of glucose is called dextrose. Dextrose is the form of sugar found in the blood stream and is commonly called “blood sugar.” It is called dextrose because it is dextrorotary. That means that if you suspend it in solution and shine polarized light through it, the light will rotate to the right as it passes through the sugar solution. Dextrose is also called D-glucose, where the D- indicates dextrorotary. If you look closely, you will see that the ring in the figure has six sides. That makes this molecule a pyranose.

Just as glucose had a linear and a ring form, so does fructose. This figure represents the ring form of fructose. Notice that this ring has only five sides, making this molecule a furanose.

As you can plainly see, the D-fructose — being a five sided ring — cannot be the mirror image of D-glucose — being a six sided ring — and as such fructose cannot possibly be the L- form of glucose, as our anonymous commenter posited. In fact, the mirror image of D-glucose is L-glucose, and they look like this:

So now you are asking yourself, “How do I know that the figure on the left, is not simply the figure on the right as viewed from the back?” To answer that question, you will have to use your imagination because again our notation convention has let us down. I cannot draw the molecules in three dimensions, which is what would be required to prove that the images are indeed different spacial arrangements, and not simply different views of the same molecule.

To get an idea of what I am talking about, imagine tracing the outline of your hand on a piece of paper. If you trace it with the palm facing up, you will get one view. If you trace it with the palm facing down, you will get another. Now trace each hand with the palms face down. How can you tell the first set of drawings from the second. You cannot. They are two dimensional. To tell the difference, you need three dimensions. You need to actually look at your hands — or a sufficiently detailed three dimensional model of your hands — to tell the difference. Now look back at the two mirror image molecules above. D-glucose on the left and L-glucose on the right. Imagine that there are bends, like your knuckles, that push parts of the molecule up out of the plane of the page, and others that push parts into the page. The bends are in the same direction — up and down — for both molecules, but since the two are mirror images of each other, they do not end up being identical. Just like your hands.

There is a whole field of study — stereochemistry — dedicated to the study of the spacial arrangement of atoms in molecules and their effects on the chemical, physical, and biological properties of those molecules. Two molecules that have the same atoms bonded to each other but differ in the way these atoms are arranged in space are called stereoisomers. Stereoisomers where molecules differ only in the spatial arrangement around a single carbon atom are called epimers. Stereoisomers where molecules differ only in the spatial arrangement of atoms or groups in the aldehyde or ketone group are called anomers. Finally, stereoisomers that are mirror images on each other are called enantiomers and are chiral. One of the effects of the spacial arrangement of atoms in L-glucose is that it rotates polarized light to the left. Another is that it is not processed by the digestive system, though it tastes the same as D-glucose, quod erat demonstrandum.

What does this have to do with food? Well, you would not have any — at all — without organic chemistry, and chiral molecules hold great promise in calorie reduction without effecting taste or cooking properties.

Is this important beyond food? You bet it is. One of the great tragedies of the pharmaceutical industry is thalidomide. This was a drug given to pregnant women in the 1950’s to prevent morning sickness. It turns out that thalidomide is a chiral molecule, and that D-thalidomide is in fact an excellent cure for nausea. However, it turns out that L-thalidomide causes severe birth defects, and so probably never should have been given to pregnant women.

How did our anonymous commenter do? Well, I give an “A” for effort — the Blogger comment system is not kind to people who do not already have an account. I also give and “A” for spelling and style. However, I have to give an “F” for content, since almost everything said turned out to be wrong.

Am I deliberately being cruel? No, as I said at the beginning, we forget almost all of what we learn. As you can see, there is much about organic chemistry that is cryptic and confusing, so I would guess that the forgetting process works overtime on it. Actually, it is possible, depending on how long ago it was, that what our commenter learned was thought to be true at the time, but it is never to late to learn something new.