Solid state and solution conformation of [Ala(7)]-phalloidin: A synthetic phallotoxin analogue

Phallotoxins are toxic compounds produced by poisonous mushroom Amanita pha lloides and belong to the class of bicyclic peptides with a transannular th ioether bridge, Their intoxication mechanism in the liver involves a specif ic binding of the toxins to F-actin that, consequently, prevents the depoly merization equilibrium with G-actin. Even though the conformational feature s of phallotoxins have been worked out in solution, the exact mechanism of interaction with F-actin is still unknown. In this study a toxic phalloidin synthetic derivative, bicyclo(Ala(1)-D-Thr(2)- Cys(3)-cis-4-hydroxy-Pro(4) -Ala(5)-2-mercapto-Trp(6)-Ala(7))(S-3 --> 6) has been synthesized. A substi tution at position 7, with an Ala residue replaces the 4,5-dihydroxy-Leu pr esent in the natural phalloidin. This analogue has formed crystals suitable for X-ray analysis, and represents the first case for such a class of comp ounds. The solid-state structure as well as the solution conformation have been evaluated. NMR techniques have been used to extract interproton distan ces as restraints in subsequent molecular dynamics calculations. Finally, a direct comparison between structures in solution and in the solid state is presented.