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

Victorins, a familyof peptide toxins, produced by the fungal pathogen Cochliobolusvictoriae and responsible for disease of someoat varieties, contain a & beta;-chlorodehydroalanine residue, & UDelta;Ala(& beta;Cl).To determine the conformational properties of this unique dehydroaminoacid, a series of model compounds was studied using X-ray, NMR, andFT-IR methods, supported by theoretical calculations. The & UDelta;Ala(& beta;Cl)geometrical isomers differ in conformational profile. The isomer Z prefers the helical conformation & alpha; (& phi;, & psi;= -61 & DEG;, -24 & DEG;), PPII type conformation & beta;(& phi;, & psi; = -47 & DEG;, 136 & DEG;), and semiextendedconformation & beta;2 (& phi;, & psi; = -116 & DEG;, 9 & DEG;)in weakly and more polar solutions. The isomer E prefersmainly the extended conformation C5 (& phi;, & psi; = -177 & DEG;,160 & DEG;), but with an increase of the environment polarity alsoconformations & beta; (& phi;, & psi; = -44 & DEG;, 132 & DEG;)and & alpha; (& phi;, & psi; = -53 & DEG;, -39 & DEG;).In the most stable conformations the N-H & BULL;& BULL;& BULL;Cl hydrogenbond (5(& gamma;)) occurs, created between the chlorine atomof the side chain and the N-H donor of the flanking amide group. Themethod of synthesis of the & beta;-chlorodehydroalanine residue isproposed, by chlorination of dehydroalanine and then the photoisomerizationfrom the isomer Z to E. The presentedresults indicate that the assignment of the geometrical isomer ofthe & UDelta;Ala(& beta;Cl) residue in naturally occurring victorinsstill remains an open question, despite being crucial for biologicalactivity.