THE COMPLEXATION OF AMINO-ACIDS BY CROWN-ETHERS AND CRYPTANDS IN METHANOL

Complex formation between several crown ethers and the cryptand (222) and alpha-amino acids in methanol was studied by calorimetric titratio n. The ligand structure and the donor atoms of the ligands play an imp ortant role in determining the measured values of the reaction enthalp ies and entropies. However, with the exception of the diaza crown ethe r (22) all stability constants are of the same order of magnitude. The enthalpic and entropic contributions to the stabilities of the comple xes formed compensate each other. In methanolic solution the amino aci ds exist in their zwitterionic form, This equilibrium can be influence d. Under acidic, neutral or basic conditions different values of the r eaction enthalpies are measured for the complexation of some amino aci ds with 18-crown-6. These results demonstrate that the concentration o f the zwitterionic form of the alpha-amino acids can be influenced. Th us the reaction between macrocyclic and macrobicyclic ligands and amin o acids should be described by at least two different reaction schemes .