The structure of the synthetic, protected tetrapeptide Z-Leu-Aib-Pro-Leuol(C29H46O6N4, Mw = 546.71) which contains the conformationally constrained residue α-aminoisobutyric acid (Aib), has been determined by X-ray crystallography. The title compound comprises the C-terminal tetrapeptide of trichovirin and has been chosen as a first step in a systematic analysis of the trichovirin structure-sequence relationships. Furthermore, the tetrapeptide has an interesting amino acid sequence, containing a strong helix former (Aib) and a helix breaker (Pro). The title compound crystallizes in P21 with a = 10.523(1) Å, b = 23.033(5) Å, c = 13.462(4) Å, β = 91.81(1)°, V = 3261(2) Å3, Z = 4, Dcalc = 1.135 g cm−3 (R = 0.054 for 3572 observed reflections). Both independent molecules in the asymmetric unit form a β-turn of type I with a 4 → 1 intramolecular hydrogen bond; the overall folding of each molecule fits approximately the pattern of a right-handed 310-helix, although the conformational angles φ, ψ of the Leu residues deviate significantly from the standard helical values. In the crystal the molecules are hydrogen-bonded head-to-tail forming infinitely long, helical columns along the a-axis; these columns are associated by hydrogen bonds forming layers parallel to the ac-plane.
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