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 Å3 and Z= 8, Dc= 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 βγT (P ∼ 183˚, τ 33 for A and B, t∼18 for C) and molecule D between βγT 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.