Now coming back to the issue to assign the relative configurations of a stereocentre, several approaches were used. The first one being the stereochemical descriptor D/L which relies on the chemical correlation of the configuration of the chiral center to D-glyceraldehyde. The compounds which can be correlated without inverting the chiral center are named D, those correlated to its enantiomer are designated as L. It is worth a while to note that though D-glyceraldehyde is dextrorotatory (rotates the plane of polarized light to the right), the compounds correlated to D-glyceraldehyde do not have to be dextrorotatory and vice versa (Figure 4).

Figure 4

This system is not much in use today. However, traditionally it is still used for sugars and amino acids.  In cases of carbohydrates, only the carbon at the end of carbon chain is considered for assigning D/L after writing the molecule in Fischer projection with the anomeric carbon on the top. Now if the carbon at the end of chain (farthest from anomeric carbon) has a hydroxyl group to the left, it is denoted as an L-sugar and it has a hydroxyl group to the right, it is called a D-sugar (Figure 19).

Figure 5

All naturally occurring amino acids are L-amino acids since they can be correlated with L-glyceraldehyde with the most oxidized group at the top and amino group at left in Fischer projection (Figure 6).

Figure 6