Chirality of carbohydrates
Carbohydrates contain several stereogenic (or chiral) centers, that is, a carbon atom bonded to four different groups. For each stereogenic center, there are two possible arrangements in space for those four groups. Glyceraldehyde is the simplest carbohydrate, and it contains one chiral center (indicated with an *):
If we have n chiral centers, the maximum number of possible stereoisomers is 2n. For example in an aldohexose (like glucose), there are four stereogenic centers, and therefore 16 possible stereoisomers, each one with a different three-dimensional configuration.
In this case, using the traditional wedge and hatched line representation around each chiral carbon to draw the stereochemistry of each particle stereoisomer becomes nonpractical.
The problem of drawing three-dimensional configurations on a two-dimensional surface, such as a piece of paper, has been a long-standing concern of chemists. The wedge and hatched line notations we have been using are effective but can be troublesome when applied to compounds having many chiral centers. As part of his Nobel Prize-winning research on carbohydrates, the great German chemist Emil Fischer, devised a simple notation that is still widely used. In a Fischer projection drawing, the four bonds to a chiral carbon make a cross with the carbon atom at the intersection of the horizontal and vertical lines. The two horizontal bonds are directed toward the viewer (forward of the stereogenic carbon). The two vertical bonds are directed behind the central carbon (away from the viewer). Since this is not the usual way in which we have viewed such structures, the following diagram shows how a stereogenic carbon positioned in the common two-bonds-in-a-plane orientation ( x–C–y define the reference plane ) is rotated into the Fischer projection orientation (the far-right formula). When writing Fischer projection formulas it is important to remember these conventions. Since the vertical bonds extend away from the viewer and the horizontal bonds toward the viewer, a Fischer structure may only be turned by 180º within the plane, thus maintaining this relationship. The structure must not be flipped over or rotated by 90º.
In the above diagram, if x = COH, y = CH2OH, a = H & b = OH, the resulting formula describes (R)-(–)-glyceraldehyde. The mirror-image formula, where x = COH, y = CH2OH, a = OH & b = H, would, of course, represent (S)-(+)-glyceraldehyde.
The Fischer Projection consists of both horizontal and vertical lines, where the horizontal lines represent the atoms that are pointed toward the viewer while the vertical line represents atoms that are pointed away from the viewer. The point of intersection between the horizontal and vertical lines represents the central carbon. The main carbon chain (longest) is set as the vertical line.
Using the Fischer projection notation, the stereoisomers of glyceraldehyde are drawn in the following manner:
Images by Master Uegly, Public domain, via Wikimedia Commons
The structure in which the OH groups is on the right is designed as D, while the the structure in which the OH groups is on the left is designed as L. This nomenclature system is known as the D,L-relative configuration.
The usefulness of this notation to Fischer and D,L-relative configuration, in carbohydrate studies, is evident in the following diagram. Since there are three chiral centers in 2,3,4,5-tetrahydroxypentanal (an aldopentose) we should expect a maximum of 23 = 8 stereoisomers. These eight stereoisomers consist of four sets of enantiomers. If the configuration at C-4 is kept constant (R in the examples shown here), the four stereoisomers that result will be diastereomers. Fischer formulas for these isomers, which Fischer designated as the "D"-family, are shown in the diagram. Each of these compounds has an enantiomer, which is a member of the "L"-family so, as expected, there are eight stereoisomers in all.
The aldopentose structures drawn above are all diastereomers. A more selective term, epimer, is used to designate diastereomers that differ in configuration at only one chiral center. Thus, ribose and arabinose are epimers at C-2, and arabinose and lyxose are epimers at C-3. However, arabinose and xylose are not epimers, since their configurations differ at both C-2 and C-3.
How to make Fischer Projections
To make a Fischer Projection, it is easier to show through examples than through words. Let's start with the first example, turning a 3D structure of ethane into a 2D Fischer Projection.
Start by mentally converting a 3D structure into a Dashed-Wedged Line Structure. Remember, the atoms that are pointed toward the viewer would be designated with a wedged lines and the ones pointed away from the viewer are designated with dashed lines.
Notice the red balls (atoms) in Figure A above are pointed away from the screen. These atoms will be designated with dashed lines like those in Figure B by number 2 and 6. The green balls (atoms) are pointed toward the screen. These atoms will be designated with wedged lines like those in Figure B by number 3 and 5. The blue atoms are in the plane of the screen so they are designated with straight lines.
Now that we have our Dashed- Wedged Line Structure, we can convert it to a Fischer Projection. However, before we can convert this Dashed-Wedged Line Structure into a Fischer Projection, we must first convert it to a “flat” Dashed-Wedged Line Structure. Then from there we can draw our Fischer Projection. Lets start with a more simpler example. Instead of using the ethane shown in Figure A and B, we will start with a methane. The reason being is that it allows us to only focus on one central carbon, which make things a little bit easier.
Lets start with this 3D image and work our way to a dashed-wedged image. Start by imagining yourself looking directly at the central carbon from the left side as shown in Figure C. It should look something like Figure D. Now take this Figure D and flatten it out on the surface of the paper and you should get an image of a cross.
As a reminder, the horizontal line represents atoms that are coming out of the paper and the vertical line represents atoms that are going into the paper. The cross image to the right of the arrow is a Fischer projection.