Crystal structure of cyclo(2-hydroxy-i-valeryl-isoleucyl-2-hydroxy- i- valeryl-alanyl-threonyl) dihydrate, C23H39N3O8 · 2H2O

Abstract

C23H43N3O10, monoclinic, P\2 (no. 4),a = 7.914(1) A, b= 16.509(3)A, c = 10.522(2)λ,β = 92.484(1)°, V= 1373.4 A, Ζ = 2, R#(F) = 0.025, wRnS(F) = 0.028, Γ = 150 Κ. Source of materia] The strain Trichothecium roseum ATCC 36473 (isolated from an apple) was cultivated by submerged cultivation in 101 fermentor on Czapek Dox medium 7 days at 24 °C. The fermentation broth was Altered and roseotoxins were extracted from medium by dichloromethane. Crude roseotoxins were purified by column chromatography on a silica gel (Merck 60, a stepwise gradient 1-10 % methanol in dichloromethane) and by a novel type of preparative NP-HPLC on an amino modified column (Watrex Biosphere SI N, 5 μτα, 250 χ 12 mm, I.D., isocratic elution with isopropanol/τιhexane, 1:4 v/v, 5 ml/min, detector set at 235 nm). The fraction containing the mixture of roseotoxins A and Ζ (about 2:3 wAv) was crystallized from dichloromethane/methanol providing directly a single crystal of roseotoxin Z. Roseotoxins were further characterized by positive-ion electrospray (ESI) mass spectra acquired on Finnigan LCQ-DECA quadrupole ion trap mass spectrometer (Finnigan MAT, San Jose, CA, USA) [1], Discussion Cyclopentapeptides are studied as interesting conformational templates in the rational design of new pharmaceuticals [2]. About ten structures of conformationally restricted proline* Correspondence author (e-mail: husakm@vscht.cz) containing cyclic peptides can be found in the CCDC, however, structures of a cyclic pentapeptide with alifatic amino acids or a cyclic pentadepsipeptide are apparently entirely missing. Trichothecium roseum has been described as a producer of several cyclic hexadepsipeptides [1], The title structure is related to the cyclic pentadepsipeptide roseotoxin S [3], but exhibits different sequence of building units indicating thus the possibility that the original structure assignment of roseotoxin S need not be correct. Since the original material was not available to us for comparison, the new compound is denominated as roseotoxin Z. Roseotoxin Ζ provided a molecular ion m/z 486 [M+H] corresponding to the composition C23H4oN3oOg. The CID mass spectrum of ion [M+H] provided consecutive elimination of carbon monoxide and water m/z 458 [MH-CO], 440 [MH-C0-H20] , and one dominating ring splitting being represented by fragment ion series 385 (b4), 357 (a*), 314 (t>3), and 214 fa). This ion series is in agreement with sequences either cyc/o(Ile-HiV-HiV-AlaThr) or cyc/o(HiV-Ile-HiV-Ala-Thr), where HiV stands for 2hydroxy-isovaleric acid. The latter structure was confirmed by crystal structure determination providing in addition an assignment of its all trans (£,£,£,£5)-configuration. All constituents possess negative Φ and Ψ values (Φ =—131.2(1), Φ=-128.5(2), Φ 3 = -81.1(2), Φ = -126.8(2), Φ = -128.8(2), Ψ = -25.8(2), Ψ 2 = -85.0(2), Ψ = -24.2(2), Ψ = -58.6(2), Ψ = -19.8(2)). In spite of the peptide backbone conformation together with the lack of intramolecular Η bonds does not correspond to usual predefined structural types, the structure can be tentatively assigned as closely resembling /ft-turn. The presence of two (S)-L-HiV residues is very unusual since apparently only (/?)-D-HiV was so far encountered in various depsipeptides, e.g., in beauvericin [4], enniatins [5], or valinomycin [6]. Most marked feature of the structure is its bi-polar arrangement with all C = 0 groups oriented to one site of the ring structure and both aliphatic chains and all three NH groups oriented out off the opposite site. Surprisingly, any intramolecular Η bond is absent, but there are nine intermolecular Η bonds in the structure. The water oxygen 0201 plays the key role in the Η bond system, since it is involved in overall five hydrogen bonds. Three of them point to all roseotoxin N-H groups, one links the second water molecule, and one is bonded for 0 5 of a symmetry related roseotoxin Ζ molecule. The OlOl water oxygen forms three hydrogen bonds: with water 0201 and two carbonyl oxygens O8 and Ol of symmetry related roseotoxin Ζ molecule. In addition to water based hydrogen bonds, threonine OH group forms Η bond with 0 4 from symmetry related roseotoxin Ζ molecule.