Treatment of rotational isomers in free energy calculations. II. Molecular dynamics simulation study of 18-crown-6 in aqueous solution as an example of systems with large numbers of rotational isomeric states

The evaluation of free energy differences using the perturbation method or thermodynamic integration method requires special caution if multiple rotational isomeric states may exist in the system under investigation. In this article a recently suggested procedure to properly treat rotational isomeric states is illustrated with a molecular dynamics simulation of an aqueous solution of uncomplexed 18-crown-6 crown ether, as an example of a system in which large numbers of isomeric states may exist. By using very long molecular dynamics simulations, thermodynamic perturbation methods and symmetry arguments, the free energy of host organization into the conformation required to form the complex with K⁺ is estimated to be 2.6 kJ mol^-1. It has also been found that the lowest energy conformations of 18-crown-6 in vacuo do not necessarily correspond to the most populated conformations in aqueous solution.