AbstractThe amide bond in L,L‐ and L,D‐α‐chloropropionylalanine methyl ester is shown to be trans by molar polarization and infrared spectroscopy. In these dipeptide diastereoisomer analogues, therefore, differences in physical properties, i.e., melting points, crystalline forms, gas chromatographic mobilities, etc., depend on preferred molecular conformations and not peptide bond configuration. Nuclear magnetic resonance spectra of both compounds were identical, indicating that no major chemical environment differences exist, which might have resulted from dissimilar side group interactions. Based on the data reported here and those of others, most dipeptide conformations can be eliminated because of contradiction with limits set by experimental or theoretical considerations. Of the remaining conformational possibilities, a single pair accounts for observed physical differences in dipeptide diastereoisomers, free or blocked. The preferred form contains α‐hydrogens trans to each other and in the plane of the peptide bond. In this conformation, R1–R2 and amino–carboxyl distances are minimal in L,D diastereomers and maximal in L,L forms.