N-Acyl derivatives of piperazic acids display an unusual degree of conformational rigidity. As a consequence, alpha-N-coupling of a piperazic acid of given configuration (D or L) with an alpha-aminoacyl unit of opposite configuration (L or D) produces a "heterochiral" dipeptide that exists in a conformation conducive to the formation of a peptide turn. Piperazic acids and related compounds may thus be regarded as conformationally rigid analogues of proline, and because they are readily available in either antipodal form by synthesis, they should become of great interest in the study of peptide turns.