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

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Abstract
The structure of a new crystalline form of tBoc-Pro-Pro-OH (C15 H24 N2 O5) has been determined. The crystals were monoclinic, P2(1), a = 14.667(5), b = 16.600(4), c = 15.502(3) A, beta = 117.84(2) degrees, V = 3337.2 A3 and Z = 8, Dc = 1.24 g/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 beta gamma T (P approximately 183 degrees, tau approximately 33 for A and B, tau approximately 18 for C) and molecule D between gamma beta T and gamma E (P = 10 degrees, tau approximately 38). The second residue of all four molecules has an envelope conformation. Molecules A and B display an alpha E conformation (P approximately 126 degrees, tau approximately 25) and molecules C and D display a beta E conformation (P approximately 168 degrees, tau approximately 37). The molecules are hydrogen-bonded (O...OH), forming helical channels along the alpha-axis.