Cooperative ion pair recognition by multitopic l-ornithine based salt receptors

Abstract

The L-ornithine scaffold was used to develop molecular receptors with improved efficacy in ion pair binding. With two appropriately oriented strong anion binding domains (urea and (thio)urea groups) and one cation binding group (crown ether moiety), these receptors exhibit effective association with the sodium salts of selected anions. We show that the simultaneous action of the two anion binding domains, reinforced by cation coordination, is responsible for the binding strength of receptors 1 and 2. The binding constants for the anions and sodium salt complexes of these receptors were determined using spectrophotometric and ¹H NMR titration measurements. Besides carboxylate ions, in the presence of sodium cations all the selected anions associate with receptors 1 and 2 in a positive cooperative manner. The strongest cooperative binding was observed for the association of sodium chloride with receptor 2, supported by urea and thiourea anion binding domains (K_a = 85 500 M⁻¹). Lacking two strong anion binding domains, receptors 4 and 5 can only interact with sodium chloride much more weakly (K_a = 5100 and 8900 M⁻¹, respectively).