A new receptor molecule for lysine and histidine in water: Strong binding of basic amino acid esters by a macrocyclic host

[GRAPHICS] We present the new host molecule 1 which binds basic amino acid esters in water. It recognizes both positively charged groups of the amino acid esters by electrostatic and hydrogen bond interactions with its four strategically placed phosphonate anions, Selectivity for lysine is achieved by the correct distance between both bisphosphonate pairs. By contrast, the smaller amino acid esters arginine, ornithine, and histidine form 2:1 complexes with 1, In methanol, a double chelate assembly enforced by pi -cation interactions with the imidazolium cation leads to a very high association constant for the 1:histidine complex of 3 x 10(4) M-1.