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Components of Cells
The Macromolecules
Stereoisomers

Stereoisomers

Glucose is a good example of an organic molecule that has one, simple, molecular formula (C6H12O6) but can be shown to have multiple different structural formulae.

This was first demonstrated in the pioneering work of the 19th century German chemist Emil Fischer. He showed that a molecule such as glucose could have the same atoms connected with the same bonds, to the same neighboring atoms, and yet have very different physical, biological and chemical properties depending on how the bonds and atoms were arranged in three-dimensional space.

These different versions of the same molecule were called stereoisomers. A good example of a simple organic molecule that shows this property is glyceraldehyde; a molecule in which a central carbon atom has four different functional groups attached to it's four covalent bonds, thus:

As can be seen, the molecule can be symbolically represented in a variety of ways, including an attempt to represent the physical positions of all the atoms in three dimensional space. But these kinds of representations are difficult to draw, and, when the molecule is large, confusing to interpret.

Fischer Projection Formulae

A different kind of representation is the projection formula devised by Fischer. These simple drawings try to get round the problem of presenting a three dimensional object (a carbon containing molecule) on a two dimensional surface (the paper or screen). Using Fischer's system, the four covalent bonds of a single carbon atom are drawn in the form of a cross, each bond at a 90o angle to its neighbor.

The viewer then has to imagine that the horizontal bonds (the "left' and "right" bonds) are actually sticking out of the surface and towards the viewer, while the two vertical bonds (the "up" and the "down" bonds) are actually directed into the surface away from the viewer.

In the case of the glyceraldehyde molecule, the single carbon at the center of the molecule is drawn in the middle of the figure, and the four functional groups are arranged around the four covalent bonds, thus:

D and L forms

The glyceraldehyde molecule, however, can now be drawn with two different structural arrangements. Following a convention (first used in 1906) the drawing that shows the alcohol (hydroxyl, -OH) group directed to the right side of the Fischer projection figure is termed the D-configuration (from the Latin word dextro - meaning "right"), and the version of the molecule in which the hydroxyl group points to the left side of the Fischer projection figure is termed the L-configuration (from the Latin word levo - meaning "left").

[Note: the terms or configural notation "D" and "L" should not be confused with the very similar notation used for the property of optical rotation of polarized light by these, and other, molecules].

The two different structural versions of glyceraldehyde (as seen in the projection fomulae) are, in fact, mirror images of each other. Just as the image of your right hand in a mirror is exactly the same image of your left hand (not seen in the mirror!), so these two isomers of glyceraldehyde can be thought of as the "left handed version" and "right handed version" of the same molecule. The term enantiomorphism is often used for this phenomenon.


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© 2004, Professor John Blamire