Complexation of the basic amino acids lysine and arginine by three sulfonatocalix[n]arenes (n = 4, 6 and 8) in water: microcalorimetric determination of the Gibbs energies, enthalpies and entropies of complexation

Abstract

The complexes formed between three p-sulfonatocalix[n]arenes (n = 4, 6 and 8) and the amino acids lysine and arginine in water have been studied by microcalorimetry, at 298.15 K. For each system, both the apparent association constant and enthalpy of reaction have been extracted from the calorimetric data. The Gibbs energies, enthalpies and entropies of complexation have been determined both in acidic medium (pH 1) and in slightly basic medium (pH 8). The thermodynamic parameters for the complexation of arginine markedly differ from those for the complexation of lysine. The three hosts show very different thermodynamic behaviours. Our results are consistent with the formation of 1∶1 complexes with the calix[4]arenesulfonate and the calix[6]arenesulfonate and with the formation of 1∶1 and 1∶2 complexes with the calix[8]arenesulfonate. Whereas the calix[4]arenesulfonate forms relatively strong complexes, the calix[6]arenesulfonate and the calix[8]arenesulfonate form only weak complexes. In all cases, the complexation is driven by a favourable enthalpy change. The enthalpies and entropies of complexing of arginine by the calix[6]arenesulfonate are remarkably negative. The enthalpies and entropies of complexation of the two amino acids by the cyclic tetramer and by the cyclic hexamer become more negative when the pH is changed from 8 to 1; the same effect is observed upon binding of the cyclic octamer with the first guest whereas the opposite effect is observed upon addition of the second guest.