The crystal structure of S-deoxo[Ile^3]amaninamide, a nontoxic synthetic derivative of the Amanita phalloides mushroom (amatoxins), has been determined by single-crystal X-ray diffraction. The crystals are monoclinic, space group P2_1, 2 formula units per unit cell. Cell dimensions are a = 12.147 Å, b = 11.250 Å, c = 19.267 Å, and (β = 92.41°. The was determined by molecular replacement methods and refined by least-squares techniques to a final R value of 0.065 for 3894 independent observations. Six intramolecular hydrogen bonds hold the bicyclic octapeptide in a compact , which is very similar to the conformation of the naturally occurring (and toxic) amatoxin, (β-amanitin. The demonstrates that the 30-fold reduction in binding affinity to RNA polymerase B of the title amatoxin, and probably most of the other amatoxin analogues with altered side chain 3, is not due to alteration of backbone conformation. The water molecules and two ethanol molecules, crystallized with the amatoxin, form a strong and extensive intermolecular -bonding system.
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Shoham et al. (1984) studied this question.