The determination of the crystal structure of hydroxy-L-proline, based on nearly complete data accessible with Cu Kα radiation, confirms the chemical assignment of the relative configurations about the two asymmetric carbon atoms, and leads to the following intramolecular bond distances: carboxyl group: C₁-O₁ = 1•25, C₁-O₂ = 1•27, Cr₁-C₂ = 1•52 Å; pyrrolidine ring: C₂-C₃ = 1•53, C₃-C₄ = 1•50, C₄-C₅ = 1•52, C₅-N = 1.48, N-C₂ = 1•50 Å; hydroxyl group : C₄-O₃ = 1•46 Å. The crystal is held together by a system of hydrogen bonds, with N...O₂ = 2•69, N...O'₂ = 3•17, O₃...O₁ = 2•80 Å. The molecule is in the Zwitterion form. These and various other structural features are discussed
