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 w ater. It recognizes both positively charged groups of the amino acid esters by electrostatic and hydrogen bond interactions with its four strategicall y placed phosphonate anions, Selectivity for lysine is achieved by the corr ect distance between both bisphosphonate pairs. By contrast, the smaller am ino acid esters arginine, ornithine, and histidine form 2:1 complexes with 1, In methanol, a double chelate assembly enforced by pi -cation interactio ns with the imidazolium cation leads to a very high association constant fo r the 1:histidine complex of 3 x 10(4) M-1.