Synthesis and cation binding properties of fluorescent calix[4]arene derivatives bearing tryptophan units at the lower rim

The fluorescent peptidocalixarenes, 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(O-methyl-l-tryptophanylcarbonylmethoxy)calix[4]arene (1) and 5,11,17,23-tetra-tert-butyl-25,27-di(O-methyl)-26,28-bis(O-methyl-l-tryptophanylcarbonylmethoxy)calix[4]arene (2), were prepared by introducing tryptophan subunits at a lower calixarene rim. Coordination abilities of 1 and 2 towards Eu(III) and alkali metal cations were studied by spectrophotometric, spectrofluorimetric, conductometric and potentiometric titrations in acetonitrile at 25 degrees C. Rather strong complexation was observed for smaller alkali metal cations Li+ and Na+ (logKLi16, logKLi26, logKNa1=8.25, logKNa2=6.94), and moderate for K+ (logKK1=5.09, logKK2=4.09). Larger Rb+ and Cs+ cations did not fit in the ion binding site of 1 so no complexation was detected, whereas the more flexible ligand 2 accommodated Rb+ cation (logKRb2=3.44). The fluorescence of 1 (ex=280nm, em=340nm) was remarkably quenched by Eu(III). Stability constant of 1:1 (Eu3+:1) complex determined spectrofluorimetrically amounted to logKEu1=6.16.