MOLECULAR RECOGNITION IN AQUEOUS-MEDIA - NEW BINDING-STUDIES PROVIDE FURTHER INSIGHTS INTO THE CATION-PI INTERACTION AND RELATED PHENOMENA

We describe a large number of binding studies in aqueous media designe d to provide new insights into noncovalent binding interactions, espec ially the cation-pi interaction. The studies include 7 different hosts , over 70 guests, and over 150 new binding constants. In addition to t he now standard NMR methods, circular dichroism has proven to be an es pecially useful tool for determining aqueous binding constants. We hav e found that, in addition to the alkyliminium and tetraalkylammonium g uests we have studied previously, sulfonium and guanidinium guests als o show substantial cation-pi effects. Bromination of the host greatly enhances its binding ability in a general fashion, primarily as a resu lt of hydrophobic interactions. Addition of methoxy groups did not enh ance binding, apparently as a result of a collapse of the host into a conformation that is not suitable for binding. Replacement of two benz ene rings of the host by furans or thiophenes also did not enhance bin ding. Ab initio calculations provide a rationalization for this effect and suggest a clearer model for the cation-pi interaction.