Inclusion of naturally occurring amino acids in water soluble calix[4]arenes:: a microcalorimetric and 1H NMR investigation supported by molecular modeling

The thermodynamic parameters for the inclusion of some naturally occurring amino acids into a series of p-sulfonated calix[4]arenes, were determined via both H-1 NMR and calorimetric titrations in buffered aqueous solution at 25 degrees C. The calorimetric data show that inclusion is enthalpically driven in all cases, regardless of flexibility of the host and the nature of the guest. The most efficient receptor is the calix[4]arene tetrasulfonate 1, which exists in solution at pH 7 in a cone conformation, stiffened by H-bonding at the lower rim. Molecular mechanics data help in the understanding of why some hosts do not form inclusion complexes at all. The comparison of our data with literature reports shows that there are dramatic buffer-dependent changes in the binding affinities.