Cooperativity and complexity in the binding of anions and cations to a tetratopic ion-pair host.
Cooperative interactions play a very important role in both natural and synthetic supramolecular systems. We report here on the cooperative binding properties of a tetratopic ion-pair host 1. This host combines two isophthalamide anion recognition sites with two unusual "half-crown/two carbonyl" cation recognition sites as revealed by the combination of single-crystal X-ray analysis of the free host and the 1:2 host:calcium cation complex, together with two-dimensional NMR and computational studies. By systematically comparing all of the binding data to several possible binding models and focusing on four different variants of the 1:2 binding model, it was in most cases possible to quantify these complex cooperative interactions. The data showed strong negative cooperativity (α = 0.01-0.05) of 1 toward chloride and acetate anions, while for cations the results were more variable. Interestingly, in the competitive (CDCl3/CD3OD (9:1, v/v)) solvent, the addition of calcium cations to the tetratopic ion-pair host 1 allosterically switched "on" chloride binding that is otherwise not present in this solvent system. The insight into the complexity of cooperative interactions revealed in this study of the tetratopic ion-pair host 1 can be used to design better cooperative supramolecular systems for information transfer and catalysis.
School of Chemistry, The University of New South Wales , Sydney, NSW 2052, Australia.
Ethan N W
EN
Howe
Mohan
M
Bhadbhade
Pall
P
Thordarson
D000577:Amides / Q000737:chemistry*; D000838:Anions / Q000737:chemistry; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*
D000577:Amides; D000838:Anions; D002412:Cations; D009942:Organometallic Compounds; D002118:Calcium