Complexation of biologically active amines by a water-soluble calix[5]arene. A nanocalorimetric investigation

Host–guest complexes of an amphiphilic water-soluble p-tert-butylcalix[5]arene bearing 4-sulphonatobutoxy groups at the narrow rim with trace amines (2-phenylethylamine and tyramine) and a neurotransmitter (dopamine), originally investigated via 1H NMR, have been re-examined via ITC in order to double check the reliability of the values obtained through a van’t Hoff analysis of the NMR data. The calorimetrically determined data confirm the existence of a 1:1 host–guest species; however, there are inconsistencies between the van’t Hoff derived values and the values determined via direct calorimetry. These discrepancies do not result from proton displacement due to inclusion, but rather result from the temperature dependence of the van’t Hoff enthalpies.