Synthetic water soluble di-/tritopic molecular receptors exhibiting Ca/Mg exchange.

Structural integration of two synthetic water soluble receptors for Ca and Mg, namely 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and o-aminophenol-N,N,O-triacetic acid (APTRA), respectively, gave novel di- and tritopic ionophores (1 and 2). As Mg and Ca cannot be simultaneously complexed by the receptors, allosteric control of complexation results. Potentiometric measurements established stepwise protonation constants and showed high affinity for Ca (log K = 6.08 and 8.70 for 1 and 2, respectively) and an excellent selectivity over Mg (log K = 3.70 and 5.60 for 1 and 2, respectively), which is compatible with magnesium-calcium ion exchange. While ion-exchange of a single Mg for a single Ca is possible in both 1 and 2, the simultaneous binding of two Mg by 2 appears prohibitive for replacement of these two ions by a single Ca. Ion-binding and exchange was further rationalized by DFT calculations.