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.

The three-dimensional structures of the slightly toxic diastereomeric (S)-sulfoxide of 6'-O-methyl-alpha-amanitin [6'-O-Me-alpha-ama (S)-sulfoxide, 4] and of the corresponding highly toxic sulfone 5 have been determined by X-ray diffraction analysis. The same derivatives along with 6'-O-methyl-alpha-amanitin [O-Me-alpha-ama (R)-sulfoxide, 3] and the corresponding thioether (O-Me-alpha-ama sulfide, 6] have been investigated in dimethyl sulfoxide solutions by 360-MHz 1H NMR spectroscopy including nuclear Overhauser effects (NOE). In addition alpha-amanitin (2) has been reinvestigated by this high-resolution method involving the identification of the ABMX systems of the tryptophan, cysteine, and asparagine and discrimination between the glycine residues. The structures of compounds 2-6 are compared with the structure of beta-amanitin which was solved previously by X-ray structure analysis. The results are (1) the structures in the crystalline state of the (S)-sulfoxide 4 and sulfone 5 are practically identical and (2) in dimethyl sulfoxide solution the structures of compounds 4 and 5 are 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-amanitin derivatives investigated here almost corresponds to that of beta-amanitin (1), the main difference being a rotated plane of the peptide bond between the asparagine and cysteine residue. In order to explain the lack of high toxicity in the (S)-sulfoxide 4 we tentatively suggest alternative hydrogen bonding of a donor from the protein, or displacement of the R oxygen to the S oxygen of a hydrogen bond donor. This alternative bonding or displacement might not occur in the sulfoxide 4. Other explanations which include local conformational changes in the inhibitors or a difference between the SO and SO2 local dipoles are also possible.