Most amino acids have a stereogenic alpha carbon bonded to four different groups: an amino group, a carboxyl group, a hydrogen atom, and a side chain. Alanine is a useful first example because its side chain is a methyl group.
The local animation below is an unchanged archived original used only for this proof of concept. The final hosting and optimization strategy remains a later Stage 5 decision.
L and D are relative configurations
The L/D system compares a molecule with glyceraldehyde in a Fischer projection. It does not directly state whether the molecule rotates plane-polarized light clockwise or counterclockwise.
Connect the naming systems
For most protein-forming L-amino acids, the alpha carbon has the S configuration. Cysteine is the familiar exception because sulfur changes the Cahn–Ingold–Prelog priority order. Glycine is achiral because its alpha carbon has two hydrogen substituents.