MOLECULAR-DYNAMICS STUDY OF P-TERT-BUTYLCALIX[4]ARENETETRAAMIDE AND ITS COMPLEXES WITH NEUTRAL AND CATIONIC GUESTS - INFLUENCE OF SOLVATION ON STRUCTURES AND STABILITIES

A series of molecular dynamics simulations have been performed on tert-butylcalix [4]arenetetraamide ligand L, with neutral or anionic guests inside the cone (MeOH, MeCN, H2O, SCN-) or with cations (Li+-Cs+, Eu3+) in the pseudocavity at the lower rim. The uncomplexed ligand L is found to be not preorganized in vacuo or in water to complex cations at the lower rim. The structure and dynamics of LM(n+) complexes differ from the solid-state picture of the LK+ complex. They depend on the size and charge of the cation and on solvation effects. In aqueous solution, the M(n+)/ligand attractions compete with the hydration of the partially encapsulated ion and the carbonyl binding sites, leading to an equilibrium of conformers with more or less converging binding sites. A detailed analysis of the hydration pattern and dynamics features of the apolar cone and of the hydrophilic moiety is presented. A guest-dependent dynamic coupling between the motions of the cone and of the lower-rim oxygens is demonstrated. In the LEu3+ complex, one water molecule is co-complexed with Eu3+. In the gas phase, a binding selectivity is predicted for Li+ over other alkali cations. In water, no firm conclusion can be drawn due to the flexibility of the lower-rim binding sites and related sampling problems.