L-serine and D-serine are stereoisomers. Specifically, they are enantiomers, which are a type of stereoisomer.
Stereoisomers are molecules that have the same chemical formula and connectivity of atoms but differ in their spatial arrangement. In the case of L-serine and D-serine, both molecules have the same atoms bonded to each other in the same order. However, they differ in the arrangement of their three-dimensional structure around a central carbon atom, known as the chiral carbon.
Chiral molecules are those that have a non-superimposable mirror image. This means that L-serine and D-serine are mirror images of each other, but they cannot be perfectly overlapped by rotation or translation.
The terms L and D refer to the configuration of the chiral carbon based on the molecule's relationship to glyceraldehyde, a reference compound. L-serine has its functional groups oriented in the same way as L-glyceraldehyde, while D-serine has its functional groups oriented in the same way as D-glyceraldehyde.
To summarize, L-serine and D-serine are enantiomers, which are a type of stereoisomer. They are chiral isomers, differing in their three-dimensional arrangement around the chiral carbon.