| Abstract
| - NMR studies have revealed that the conformation of the monocyclic viroisin is dissimilar tothat of the corresponding monocyclic derivative of phalloidin, dethiophalloidin, but has much similaritywith the conformation of the bicyclic phalloidin. Obviously, one of three structural features foundexclusively in the virotoxins is able to compensate for the conformational strain that in the bicyclicphallotoxins maintains the toxic conformation. Synthetic work on virotoxin analogues has shown thatboth the additional hydroxy group in allo-hydroxyproline and the methylsulfonyl moiety in the 2‘-positionof tryptophan are unlikely to represent the structural element in question, leaving the d-serine moiety asthe supposed key element. In this study we asked whether it is the hydroxy group of this amino acid orits d-configuration that is responsible for the effect. We synthesized four viroisin analogues and submittedthem to conformational analysis by NMR as well as to an actin binding assay. While the rotating-framenuclear Overhauser effect (ROESY) spectra of the analogues with l-configured amino acids showed severalsets of signals, indicating the existence of conformers interconverting more slowly than the NMR timescale, the spectra of the analogues with d-configured amino acids showed only one set of signals.Remarkably, the two viroisin analogues with d-serine and d-alanine also had distinctly higher affinitiesfor filamentous actin than their l-configured counterparts, suggesting that the high biological activitymay be correlated with the absence of multiple and slowly interconverting conformers. Anyhow,d-configuration of serine is the structural element that maintains the phalloidin-like structure, while thehydroxy group does not contribute to conformational stability but is likely to be in contact with the actinsurface.
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