Influence of linker structure on the anion binding affinity of biscyclopeptides

A systematic analysis is presented on the influence of the linking unit between two cyclopeptide rings on the affinity of such biscyclopeptide-based anion receptors in aqueous solvent mixtures. Although the differences in the affinity and selectivity of these receptors towards a given anion are not very pronounced, there are profound differences in the thermodynamics of anion complexation. Enthalpic and entropic contributions both (1) play a role in determining the binding affinity and (2) show significant variation as the linking structure is changed. A decrease in conformational rigidity of the linker improves the entropic advantage for complex formation, but not necessarily the overall complex stability. This effect may be due, in part, to the fact that structural constraints within more rigid linkers might prevent efficient interactions between the host and guest. The optimal linker, which exhibits both favourable enthalpic and entropic contributions, was identified using de novo structure-based design methods as implemented in the HostDesigner software.