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The crystal and molecular structure of the fully protected dipeptide Boc-Val-(S)-alpha-MeSer-OMe has been determined by X-ray diffraction techniques. Crystals grown from ethyl acetate/n-pentane mixtures are tetragonal, space group I4(1), with cell parameters at 295 K of a = 15.307(2), c = 18.937(10)angstrom, V = 4437.1 angstrom3, M.W. = 332.40, Z = 8, D(m) = 0.99 g/cm3 and D(x) = 0. 995 g/cm3. The structure was solved by application of direct methods and refined to an R value of 0.028 for 1773 reflections with I greater-than-or-equal 3sigma(I) collected on a CAD-4 diffractometer. Both chiral centers have the (S) configuration. The dipeptide assumes in the solid state an S shape. The urethane moiety is in the cis conformation, while the amide bond is in the common trans conformation. The conformational angles phi1, psi1 of the Val and phi2, and psi2 of the (S)-alphaMeSer fall in the F region of the phi-psi map. The isopropyl side chain of the Val residue has the (t, g-) conformation, while the Ser side chain has a g+ conformation. The hydrogen bond donor groups are all involved in intermolecular H-bond interactions. Along the quaternary axis the dipeptide molecules are linked to each other with the formation of infinite rows.

The synthesis, physical and analytical characterization, and crystal‐state structural analysis by X‐ray diffraction of three analogues of the Nα‐acylated tripeptide amide tail of oxytocin, each containing a cyclic Cα, α‐ disubstituted glycine at position 2, have been performed. The peptides arc Boc‐L‐Pro‐Ac3c‐Gly‐NH2, Z‐L‐Pro‐Ac5c‐Gly‐NH2 and Z‐L‐Pro‐Ac5c‐Gly‐NH2. While the former is folded in a type‐II β‐turn conformation at the ‐L‐Pro‐Ac3c‐ sequence, the two latter tripeptides form two consecutive (type‐II, type‐I′) β‐turns. The Ac5c‐ and Ac6c‐tripeptides are the first examples of such a highly folded structural combination in a position‐2 analogue of the Nα‐acylated ‐L‐Pro‐L‐Leu‐GIy‐NH2 sequence. Copyright © 1993, Wiley Blackwell. All rights reserved