Conformational isomerism of chlorophenylthioureas and intramolecular NH … Cl hydrogen bonds: IR spectra of NH stretching vibrations

Insight into the Structure of Victorin, the Host-Selective Toxin from the Oat Pathogen Cochliobolus victoriae. Studies of the Unique Dehydroamino Acid β-Chlorodehydroalanine

Victorins, a family of peptide toxins, produced by the fungal pathogen Cochliobolus victoriae and responsible for disease of some oat varieties, contain a β-chlorodehydroalanine residue, ΔAla(βCl). To determine the conformational properties of this unique dehydroamino acid, a series of model compounds was studied using X-ray, NMR, and FT-IR methods, supported by theoretical calculations. The ΔAla(βCl) geometrical isomers differ in conformational profile. The isomer Z prefers the helical conformation α (φ, ψ = -61°, -24°), PPII type conformation β (φ, ψ = -47°, 136°), and semiextended conformation β2 (φ, ψ = -116°, 9°) in weakly and more polar solutions. The isomer E prefers mainly the extended conformation C5 (φ, ψ = -177°, 160°), but with an increase of the environment polarity also conformations β (φ, ψ = -44°, 132°) and α (φ, ψ = -53°, -39°). In the most stable conformations the N-H···Cl hydrogen bond (5^γ) occurs, created between the chlorine atom of the side chain and the N-H donor of the flanking amide group. The method of synthesis of the β-chlorodehydroalanine residue is proposed, by chlorination of dehydroalanine and then the photoisomerization from the isomer Z to E. The presented results indicate that the assignment of the geometrical isomer of the ΔAla(βCl) residue in naturally occurring victorins still remains an open question, despite being crucial for biological activity.

Conformation of some biologically active aromatic ureas

Experimental IR spectroscopic data for the N-H stretching mode frequencies for several types of tri-substituted ureas containing benzyl and/or phenyl substituents as well as theoretical results from B3LYP/6-31G(d,p) computations on selected compounds provide sufficient evidence to determine the conformational state of these molecules. Two types of N-H bands may be found the spectra: (a) A type band due to a classical trans conformation (trans I) of the CONH structure; (b) B type band arising from an alternative trans form (trans II), in which the N-H band is involved in a hydrogen bond like interaction with the aromatic ring at the neighbouring nitrogen atom (benzyl or phenyl substituents). The N-H band of trans ICONH structure is observed at frequencies higher than 3460 cm-1, the actual position depending on weather the non-substituted N-H group is linked to aryl or alkyl substituents. The N-H band of the trans II rotameric structure is observed at 3430-3420 cm-1.