Phalloidin synthetic analogues: Structural requirements in the interaction with F-actin

Synthetic derivatives of phalloidin have been investigated in solution by circular dichroism (CD) and NMR spectroscopy. They differ from natural phalloidin (PHD), bicyclo(Ala(1)-D-Thr(2)-Cys(3)-cis-4-hydroxy-Pro(4)-Ala(5)-2-mereap-to-Trp(6)-(OH)(2)Leu(7))(S-3 --> 6), in that they are modified at positions 2, 3, and 7. Among these synthetic analogues, structural differences and varying degrees of atropisomerism are found. By comparing the respective molecular models obtained by restrained molecular dynamics (RMD) simulations based on experimental NMR data, structural features that may be responsible for the different biological behavior become apparent. Our results indicate that the structural changes that result from an inversion of chirality of residue 3 lead to a complete loss of toxicity. Conversely, toxicity is less affected by the structural changes that stem from an inversion of chirality of residue 2. Moreover, unlike the other phallotoxins, when the thioether unit bridges to the opposite face of the main peptide ring, in contrast to the situation in other phallotoxins, large structural changes are observed as well as a total loss of activity. Molecular models of the synthetic phalloidin analogues have been used to investigate the necessary structural requirements for the interaction with F-actin. To this end, the F-actin/PHD model of M. Lorenz et al. was employed; docking experiments of our molecular models in the PHD binding site are presented.