Final answer:
The letter 'd' indicates a specific stereochemical configuration of a chiral molecule, such as the arrangement of atoms around the chiral center in glucose or the orientation of functional groups in amino acids. This designation, especially in monosaccharides and amino acids, identifies the spatial arrangement but not necessarily the optical activity of the molecule.
Step-by-step explanation:
The molecule indicated by the letter d often refers to a particular form of a chiral molecule in the context of stereochemistry. In the case of monosaccharides like glucose, the d or D configuration is defined by the position of the hydroxyl (-OH) group on the last chiral carbon atom when the molecule is drawn in a Fisher projection. For amino acids, the -NHâ‚‚ group's position determines whether the molecule is designated as D or L.
When discussing glucose, if the -OH group is on the right side of the Fisher projection at the last chiral center (furthest from the functional group, such as the aldehydic group in glucose), the glucose is designated as D-glucose. This designation does not necessarily indicate optical activity, which is the ability of a molecule to rotate plane-polarized light. A dextrorotatory D-glucose molecule is labeled as D-(+)-glucose, and a levorotatory D-glucose molecule as D-(-)-glucose.
It's important to distinguish that uppercase D and L do not always correlate with how a molecule bends light, while lowercase d and l always correspond to the dextrorotatory and levorotatory nature, respectively. In terms of amino acids, cells typically use L amino acids, which are actually dextrorotatory, making them d-amino acids, for building polypeptides and proteins.