MOLECULAR-DYNAMICS SIMULATION STUDY OF 18-CROWN-6 IN AQUEOUS-SOLUTION .1. STRUCTURE AND DYNAMICS OF THE HYDRATION SHELL

Using molecular dynamics simulations crown ether 18C6 as well as the complex 18C6/K+ is examined in aqueous solution. As the most noticeable feature of the D-3d crown's hydration shell, on both sides of the crown's plane a distinct water molecule is translationally fixed by preferably two H-bonds. The close proximity of three equivalent hydrogen-bond acceptor sites for each of these two water molecules produces enhanced and strongly anisotropic rotational mobility, permitting even coverage of all three crown oxygens on each side of the ether. The third crown oxygen, in this way unsaturated at a given moment, is loosely coordinated by a singly bound and rapidly exchanged water molecule. Structural as well as dynamical properties of the hydration shell allow a clear distinction between hydrophilic and hydrophobic regions. A complexed K+ ion stays about 1 Angstrom outside the crown's center and can be regarded as replacing one of the two ''complexed'' water molecules. During the simulation run of 262 ps K+ is oscillating several times between the two equivalent sites on both sides of the ring.