Non-directional forces drive guest affinity and discrimination in a calix[5]arene-based receptor

In this paper we report the binding behavior of a receptor based on a calix[5]arene possessing two convergent benzoic acids that serve to bind a guest. The receptor recognizes diketopiperazines (DKP) having a variety of alkyl substituents to form a 1:1 host–guest complex. We demonstrate that the receptor shows selectivity for the DKPs based on the size of the alkyl substituent. As a result, the selectivity arises from non-directional forces between the π-basic aromatic ring of the calixarene and the alkyl group of the guest.