Provide an example of an enantiomer and a diastereomer.

An example of an enantiomer is L-alanine and D-alanine, while an example of a diastereomer is glucose and galactose.

Enantiomers are molecules that are mirror images of each other and cannot be superimposed onto each other. They have the same chemical and physical properties, except for their interaction with polarized light. L-alanine and D-alanine are enantiomers of the amino acid alanine. They have the same chemical formula and structure, but their functional groups are arranged differently in space. L-alanine rotates polarized light to the left, while D-alanine rotates it to the right.

Diastereomers, on the other hand, are stereoisomers that are not mirror images of each other and have different chemical and physical properties. They arise from molecules that have more than one chiral centre, and have different configurations at one or more of these centres. Glucose and galactose are diastereomers of each other, as they differ in the configuration of their hydroxyl groups at carbon 4. Glucose has a hydroxyl group pointing up, while galactose has it pointing down.

Enantiomers and diastereomers have important implications in biological systems, as they can have different effects on enzymes, receptors, and other molecules. For example, the drug thalidomide exists as a mixture of enantiomers, and one enantiomer causes birth defects while the other is safe. Understanding the properties and interactions of enantiomers and diastereomers is therefore crucial in drug design and development.