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 seriesof p-sulfonated calix[4]arenes, were determined via both 1H NMR and calorimetric titrations inbuffered aqueous solution at 25 ◦C. The calorimetric data show that inclusion is enthalpically driven inall cases, regardless of flexibility of the host and the nature of the guest. The most efficient receptor isthe calix[4]arene tetrasulfonate 1, which exists in solution at pH 7 in a cone conformation, stiffened byH-bonding at the lower rim.Molecular mechanics data help in the understanding of why some hosts do not form inclusioncomplexes at all. The comparison of our data with literature reports shows that there are dramaticbuffer-dependent changes in the binding affinities.