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

A series of molecular dynamics simulations have been performed on tert -butylcalix [4]arenetetraamide ligand L, with neutral or anionic guest s inside the cone (MeOH, MeCN, H2O, SCN-) or with cations (Li+-Cs+, Eu 3+) 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 a t 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 s ize and charge of the cation and on solvation effects. In aqueous solu tion, the M(n+)/ligand attractions compete with the hydration of the p artially encapsulated ion and the carbonyl binding sites, leading to a n equilibrium of conformers with more or less converging binding sites . A detailed analysis of the hydration pattern and dynamics features o f 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 mol ecule is co-complexed with Eu3+. In the gas phase, a binding selectivi ty is predicted for Li+ over other alkali cations. In water, no firm c onclusion can be drawn due to the flexibility of the lower-rim binding sites and related sampling problems.