We have studied the reaction of bothD- andL-penicillamine with pyridoxal hydrochloride and examined the products by single crystal X-ray diffraction. The structure of the title compound,A, formed by the reaction ofD-penicillamine and pyridoxal was determined. Crystals are orthorhombic,P212121, with cell dimensionsa = 9.507(5),b = 19.185(5),c = 7.766(2) Å andZ = 4. The structure was solved by standard methods and refined toR = 0.088,Rw = 0.079 for 2433 independent reflections.Aexists as a zwitterion, and bond lengths and angles are normal. In the solid state,Aand the corresponding product obtained fromL-penicillamine,B, have identical geometrical structure but are of opposite chirality; that is,D-penicillamine produces the 2R,4Sdiastereomer andL-penicillamine produces the 2S,4Rdiastereomer (with noS,SandR,Rcomponents). In solution, however, NMR spectra show the presence of both pairs of diastereomers (2R,4Sand 2S,4S; 2S,4Rand 2R,4R). In neutral or alkaline solution there appears to be a rapid epimerization at the thiazolidine carbon atom attached to the pyridoxal moiety. Features of the mass,1H NMR, vibrational, and electronic spectra are also discussed.Keywords:D-penicillamine, vitamin B6, pyridoxal hydrochloride, therapeutic uses, bischemical reactions, structures.