Structural characteristics of diproline: a new crystal form of tBoc‐Pro‐Pro‐OH

The structure of a new crystalline form of tBoc‐Pro‐Pro‐OH (C₁₅ H₂₄ N₂ O₅) has been determined. The crystals were monoclinic, P2₁ a = 14.667(5), b = 16.600(4), c = 15.502(3) Å, β= 117.84(2)˚, V= 3337.2 ų and Z= 8, D_c= 1.24g/cm³. There are four molecules in the asymmetric unit, each displaying polyproline‐type structure but differing in the proline pucker. All four molecules display a twist conformation in the first proline ring, with molecules A, B and C being ^β_γT (P ∼ 183˚, τ 33 for A and B, t∼18 for C) and molecule D between ^β_γT and _γE (P= 10°, τ38). The second residue of all four molecules has an envelope conformation. Molecules A and B display an ^αE conformation (P∼126˚, t∼25) and molecules C and D display a ^βE conformation (P∼168˚, τ37). The molecules are hydrogen‐bonded (O…OH), forming helical channels along the a‐axis.