Unexpected similarity of the structures of the weakly toxic amanitin (S)-sulfoxide and the highly toxic (R)-sulfoxide and sulfone as revealed by proton nuclear magnetic resonance and x-ray analysis

Abstract

The 3-dimensional structures of the slightly toxic diastereomeric (S)-sulfoxide of 6''-O-methyl-.alpha.-amanitine [6''-O-Me-.alpha.-ama (S)-sulfoxide, 4] and of the corresponding highly toxic sulfone 5 were determined by X-ray diffraction analysis. The same derivatives along with 6''-O-methyl-.alpha.-amanitine [O-Me-.alpha.-ama (R)-sulfoxide, 3] and the corresponding thioether (O-Me-.alpha.-ama sulfide, 6) were investigated in dimethyl sulfoxide solutions by 360-MHz 1H NMR spectroscopy including nuclear Overhauser effects (NOE). .alpha.-Amanitin (2) was reinvestigated by this high-resolution method involving the identification of the ABMX systems of the Trp, Cys and Asn and discrimination between the Gly residues. The structures of compounds 2-6 were compared with the structure of .beta.-amanitine which was solved previously by X-ray structure analysis. The structures in the crystalline state of (S)-sulfoxide 4 and sulfone 5 were practically identical and in dimethyl sulfoxide solution the structures of compounds 4 and 5 were likewise identical with each other and with those of the (R)-sulfoxide 3 and the thioether 6. The general structure of the peptide backbone of the .alpha.-amanitine derivatives almost corresponded to that of .beta.-amanitine (1), the main difference being a rotated plane of the peptide bond between the Asn and Cys residue. The lack of high toxicity in the (S)-sulfoxide 4 was probably due to an alternative H bonding of a donor from the protein or displacement of the R O to the S O of a H bond donor. This alternative bonding or displacement might not have occurred in the sulfoxide 4. Other explanations which included local conformational changes in the inhibitors or a difference between the SO and SO2 local dipoles were also possible.