Synthetic amatoxin analogues: II. A proton n.m.r. study of S-deoxo-Ile3-(l)Ala5 and S-deoxo-Ile3-(d)Ala5-amaninamide

Amatoxin analogues with d and l-Ala substitutions in position 5 have been studied by means of 1- and 2-dimensional n.m.r. spectroscopy at 500 MHz. The assignment of all resonances for both analogues has been carried out mostly with the use of COSY and NOESY type experiments. Temperature coefficients for the amide NH protons have been measured and the data compared to known amatoxin structures. The results obtained demonstrate that the rigidity of the bicyclic amatoxin framework is preserved in the d and l-Ala5 analogues, although the temperature coefficients point to intramolecular hydrogen bonds stronger in the case of the l-Ala analogue. The 10-fold decrease of biological activity is discussed in terms of structural features involving also the Trp4 indole accessibility.