Penicillamine is a trifunctional organic compound, consisting of a thiol, an amine, and a carboxylic acid. It is an amino acid structurally similar to cysteine, but with geminal dimethyl substituents α to the thiol. Like most amino acids, it is a colorless solid that exists in the zwitterionic form at physiological pH.
Penicillamine is a chiral drug with one stereogenic center and exist as a pair of enantiomers. Refer the image for the structure of penicillamine enantiomers. The (S)-enantiomer, the eutomer, is antiarthritic while the distomer (R)-penicillamine is extremely toxic. Of its two enantiomers, -penicillamine (having R absolute configuration) is toxic because it inhibits the action of pyridoxine (also known as vitamin B). That enantiomer is a metabolite of penicillin but has no antibiotic properties itself. A variety of penicillamine–copper complex structures are known.