General Index
D-Aldoses: Configuration
We define as D- series those monosaccharides having their last asymmetric carbon in the same configuration as D-Glyceraldehyde.
Let's see in the first place the structure and the planar representation of D-Glyceraldehyde, aldose of three carbon atoms (Aldotriose):
Dragging with the pointer position the molecule carbon 1 up and carbon 3 down. Rotate the molecule in such a way that substituents of carbon 2 are directed towards the observer. In this position, the -OH group is directed towards the right. All asymmetric carbons having this configuration are said to be D-series, that is, the same configuration of D-Glyceraldehyde.
By convention, in the planar representation of asymmetric carbons (the Fischer projection), this configuration is represented as
in the following figure, that is, with the -OH directed towards the right.
Let's see now the structure of L-Gliceraldehido:
If we position the molecule in the same way as we did in the case of D-Glyceraldehyde, then the -OH group is directed to the left.
In the Fischer projection, the -OH is represented towards the left:
D-Erythrose is the next aldose in the D-series, with four carbon atoms (aldotetrose)
D-Erythrose has two asymmetric carbons, 2 and 3, both with the same configuration as D-glyceraldehyde. Note the configuration in the model: Placing the model with carbon 1 up and carbon 4 down, and rotating the molecule we can see that both asymmetric carbons have the same configuration of D-Glyceraldehyde. However, note that in the Fischer projection both -OH appear to the right, while in the model these groups appear directed towards opposite directions. Remember that the Fischer projection only indicates the structural similarity to D-Glyceraldehyde, and not the real shape of the molecule, as can be seen in the model.
An aldose of five carbon atoms (aldopentose) is D-Arabinose:
D-Arabinose has three asymmetric carbons: 2, 3 and 4. Placing the molecule on a vertical position, as we did with D-Erythrose, we can see that carbons 3 and 4 have the same configuration as D-Glyceraldehyde; while carbon 2 has that of L-Glyceraldehyde.
D-Glucose is an aldohexose, that is, an aldose with six carbon atoms:
In D-Glucose, carbons 2, 4 and 5 have the configuration of D-Glyceraldehyde, while carbon 2 has that of L-Glyceraldehyde.