Molecular dynamics simulations on the protonated 222. H+ and 222.2H+ cryptands in water:Endo versusexo conformations

MD simulations on the 222 cryptand, monoprotonated 222.H⁺ and diprotonated 222.2H⁺ in theendo-endo andexo-exo conformations have been performedin vacuo and in a bath of water molecules. It is found that intrinsicallyendo protonation is favoured overexo protonation due to internal N-H⁺ ... O hydrogen bonding which makes the cage more rigid. On the other hand,endo protonated forms display hydrophobic hydration compared toexo forms. For the monoprotonated 222. H⁺endo conformer, one water molecule is hydrogen bonded inside the cage thereby forming a water cryptate. From the hydration pattern found previously for the neutral 222 cryptand and for its cation complexes, we suggest mechanisms (not involvingexo toendo conversions) for the protonation of 222 and for acid catalysed decomplexation of cryptates in theendo-endo form.