Solution structure of phalloidin obtained by NMR spectroscopy in [D⁶]DMSO and molecular dynamics calculation in vacuo and in water

Homo‐ and heteronuclear NMR techniques have been used to assign all proton and most of the carbon resonances of phalloidin (1) [cyclic(L‐alanyl‐D‐threonyl‐L‐cysteinyl‐cis‐4‐hydroxy‐L‐prolyl‐L‐alanyl‐L‐mercapto‐L‐tryptophyl‐4,5‐dihydroxy‐L‐leucyl) cyclic (3→6) sulfide], a toxic bicyclic heptapeptide of the poisonous mushroom Amanita phalloides. Intramolecular proton–proton distances derived from rotating‐frame NOE experiments have been used as constraints in energy minimization and molecular dynamics calculations using GROMOS. Charge reduction of solvent‐exposed NH protons has been used to minimize artifacts in the in vacuo calculation. The structure has been proven to be unstable without constraints in vacuo. Hence, a procedure is proposed in which first restrained MD in vacuo is performed and refinement is carried out by restrained or unrestrained MD in water.