Investigation of synthetic hosts that model cation-pi sites found at protein binding domains

Small cyclophanes containing aromatic groups and dialkyl ammonium ions were created as model systems of the cation-pi complexes found at some protein binding domains. The hosts had different shapes in order to investigate the effect the arrangement of ammonium ions to aromatic surfaces has on their reactivity. pK(a) values of the hosts were substantially different in DMSO or (95/5) DMSO/D(2)O solutions, which showed that the ions existed in different environments of the hosts. Electrostatic charges, as determined by density functional calculations, revealed that the magnitude of a cationic charge depends on its position relative to an aromatic ring. Association constants of the hosts bound to the sodium salt of N-acetyl phenylalanine in d(6)-DMSO and in (95/5) d(6)-DMSO/D(2)O solutions were inversely proportional to the magnitude of the hosts' acidity constants. These results suggest that the magnitude of the positive charge for cationic groups of cation-pi complexes is reduced by being associated with electron-rich faces of aromatic rings. The aromatic rings, however, lessen the desolvation penalty that must be overcome for ligand binding, giving an overall more favorable association.