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

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 mi crocalorimetry, at 298.15 K. For each system, both the apparent association constant and enthalpy of reaction have been extracted from the calorimetri c data. The Gibbs energies, enthalpies and entropies of complexation have b een 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 ver y different thermodynamic behaviours. Our results are consistent with the f ormation 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]arenesulfonat e form only weak complexes. In all cases, the complexation is driven by a f avourable enthalpy change. The enthalpies and entropies of complexing of ar ginine by the calix[6]arenesulfonate are remarkably negative, The enthalpie s and entropies of complexation of the two amino acids by the cyclic tetram er and by the cyclic hexamer become more negative when the pH is changed fr om 8 to 1; the same-effect is observed upon binding of the cyclic octamer w ith the first guest whereas the opposite effect is observed upon addition o f the second guest.