On the interaction between ions and complex aromatic systems: amino acid side chains, ion channels, and buckybowls

Abstract

Intermolecular interactions, though weak compared to covalent forces joining the atoms in a molecule, are responsible for a variety of physical, chemical and biological phenomena. Applying computational methods allows the isolation and identification of the phenomena responsible for a particular interaction. In this thesis we focus on the study of interactions between aromatic species and ions, which are crucial for understanding the behavior of a variety of biological systems and new materials. The interactions between cations and aromatic units present in the amino acid side chains determine the properties and structure of proteins and other systems of interest. Although the general characteristics of such interactions are roughly known there are a number of issues that deserve attention. This thesis starts studying the effects microhydration can have upon the properties of complexes formed by cations and phenol, which provides information on how solvent molecules closest to a cation··· π contact alter its characteristics. Moreover, ternary complexes with another aromatic species participating in the cation··· π contact are considered, in order to determine whether the presence of other adjacent aromatic species that compete for the cation can significantly alter its stability. In recent years the study of interactions between anions and aromatic species has become relevant. A priori, such interactions would be unfavorable, but several works, both theoretical and experimental, show that properly substituting the aromatic species stable complexes can be formed. A recent application of these interactions is the proposal of the first synthetic anion channel based on anion··· π interactions. Although anion··· π interactions are the foundation of this channel, the role of solvent seems important but yet to be determined. In this thesis we present a work in which the role of solvent molecules closest to the anion and facilitating the operation of the anion channel, is estimated. Another interesting aspect of anion··· π interactions is the possibility of using them in order to design new anionic receptors. Of particular interest are receptors based on curved aromatic species derived from fullerenes (buckybowls). In three chapters of this thesis, the effect of substitution in these buckybowls and its feasibility as anion receptors is studied. Variables such as the type of carbon skeleton, different substituents, ion nature, different geometries, and the effect of solvent on the interaction are considered. The results indicate that substituted buckybowls may be valid to selectively encapsulate anions.