THE CRYSTAL-STRUCTURE OF Z-LEU-AIB-PRO-LEUOL, THE SYNTHETIC, PROTECTED C-TERMINAL TETRAPEPTIDE OF TRICHOVIRIN

The structure of the synthetic, protected tetrapeptide Z-Leu-Aib-Pro-L euol (C29H46O6N4, M(w) = 546.71) which contains the conformationally c onstrained residue alpha-aminoisobutyric acid (Aib), has been determin ed by X-ray crystallography. The title compound comprises the C-termin al tetrapeptide of trichovirin and has been chosen as a first step in a systematic analysis of the trichovirin structure-sequence relationsh ips. Furthermore, the tetrapeptide has an interesting amino acid seque nce, containing a strong helix former (Aib) and a helix breaker (Pro). The title compound crystallizes in P2(1) with a = 10.523(1) angstrom, b = 23.033(5) angstrom, c = 13.462(4) angstrom, beta = 91.81(1)-degre es, V = 3261(2) angstrom3, Z = 4, D(calc) = 1.135 g cm-3 (R = 0.054 fo r 3572 observed reflections). Both independent molecules in the asymme tric unit form a beta-turn of type I with a 4 --> 1 intramolecular hyd rogen bond; the overall folding of each molecule fits approximately th e pattern of a right-handed 3(10)-helix, although the conformational a ngles phi, psi of the Leu residues deviate significantly from the stan dard 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 parall el to the ac-plane.