Final answer:
Glucose enantiomers can be in the D- or L-configuration based on the orientation of the -OH group on the penultimate carbon in the Fisher projection.
Step-by-step explanation:
Glucose enantiomers are referred to as D and L configurations. The D or L configuration is determined based on the orientation of the -OH group on the penultimate carbon in the Fisher projection of a monosaccharide. If the -OH group is on the right side, it is assigned a D-configuration, and if it is on the left side, it is assigned an L-configuration.
For example, D-glucose has the -OH group on the right side of the penultimate carbon, so it is in the D-configuration. L-glucose has the -OH group on the left side of the penultimate carbon, so it is in the L-configuration.
To determine whether monosaccharides are in the D- or L-configuration, you need to refer to the Fischer projection of the sugar molecule. The key is to look at the second-last carbon atom, also known as the penultimate carbon. If the hydroxyl group (-OH) on this carbon is on the right side, the sugar is in the D-configuration.
If the -OH group is on the left side, it is in the L-configuration. The most common sugars found in nature, such as D-glucose, are in the D-configuration. D/L designation does not necessarily correlate with the optical activity, so a D-sugar can be dextrorotatory (+) or levorotatory (-), and the same applies for L-sugars.